Monogamy is the predominant breeding system of most of the Laniidae. Nevertheless, under certain environmental pressures, such as biased sex ratios wherein females outnumber males, or when a marked difference in quality among males is observed during the pre-breeding season by females (see Morphological Aspects), several species may sometimes practise polygyny. Polygyny has been recorded for Southern Grey Shrikes in Israel, for Great Grey Shrikes in Austria and Poland, for Red-backed Shrikes in Poland and Sweden, and for Loggerhead Shrikes in North America. Polygynous pairs of Southern Grey Shrikes in Israel had the choice of two different breeding strategies. Males with large food caches used a different strategy from that adopted by conspecifics with small caches. Pairs with large caches used a parallel strategy: the females laid a second clutch soon after the first had hatched, and males cared for the first brood. Those pairs with small caches employed a serial strategy: both parents cared for the young until the latter fledged, the male doing all of the provisioning, and the females laid the next clutch after the first brood had fledged. Pairs which adopted the parallel strategy were significantly more productive, having more breeding attempts, laying many more eggs and rearing many more young in the season than pairs which adopted the serial strategy.
Extra-pair copulations have been recorded for at least five members of the Laniidae. They can be costly for both sexes, especially if they are detected by a social partner. The threat of extra-pair fertilization of the partner could result in reduced parental investment or divorce. In order to reduce the costs of extra-pair copulations, natural selection may favour behaviour that reduces the likelihood of its being detected by a partner. Moreover, habitat structure may influence the efficiency of a male’s mate-guarding behaviour and, therefore, the possibilities of occurrence of extra-pair copulations. In studies of the Great Grey Shrike, a socially monogamous passerine of semi-open habitats, it was found that individuals of both sexes selected more secluded sites for extra-pair copulations than for within-pair copulations. For females, the costs of participating in extra-pair copulations are an interesting topic. An obvious potential cost to females, one proposed in theory, is that of physical sanctions by the male partner, although retaliation and punishment by male partners have not been experimentally demonstrated. A team of European researchers, combining field observations and a field experiment, found that, with the Lesser Grey Shrike, there was a high rate of intrusion by extra males during the female’s fertile period, and that extra-pair copulations occurred in the population; when females were “detained” during the fertile phase, males retaliated physically against their partners, thereby increasing the costs related to female extra-pair behaviour, but there were no obvious costs to males of “punishing” the mate. Interestingly, DNA-“fingerprinting” revealed that, in this Lesser Grey Shrike population, extra-pair paternity was rare or non-existent, although it could not be proven that this apparent monogamy at the genetic level was the result of male retaliation. The researchers suggested, however, that in future studies the males’ strategies should be considered when attempting to explain interspecific variation in extra-pair paternity.
Further, the aggressiveness of male shrikes during the fertile period of their females may be a method of preventing extra-pair copulations. Hence, the level of aggression of Red-backed Shrikes was tested experimentally, by placing a stuffed male at a distance of, in most cases, 10–20 m from active nests. At each stage of the breeding process some males ignored the dummy or did not more than approach it, while others attacked it weakly or fiercely. With two exceptions, the breeding females took no notice of the stuffed male. The reactions during the nest-building and laying periods were, on average, stronger than those during incubation, diminishing further during the nestling stage.
In the USA, a genetic study of Loggerhead Shrikes in Oklahoma used six nuclear microsatellites in order to assess the extent of intraspecific brood parasitism and extra-pair paternity among 218 offspring from 44 broods. In this, the first genetic assessment of parentage in a wild population of Loggerhead Shrikes, it was apparent that no offspring were the result of intraspecific brood parasitism, but eight young from five families, representing 4% of all offspring, had been sired by extra-pair fertilization.
Overlap among the home ranges of breeding male Red-backed Shrikes in a valley in the Lombardy Alps, in Italy, was correlated with the results of DNA-fingerprinting for six family groups. One out of 19 young had paternity different from that assumed on the basis of behavioural observations. In addition, after the young shrikes had fledged, several recorded instances of the care of juveniles being shared by more than one pair of adults were observed. Occasionally, unmated individuals of this species help in the task of brood-rearing, at times to the virtual exclusion of the breeding pair. Although co-operative breeding has been described for a few tropical members of the Laniidae, this seems to be the only example documented outside Africa.
Co-operative breeding, involving a breeding pair and a variable number of helpers, is practised by both of the Corvinella shrikes and both Eurocephalus species, as well as by at least one African species of Lanius, the Grey-backed Fiscal. Two sympatric species of Lanius were studied near Lake Naivasha, in Kenya, in an effort to understand the ecological factors favouring the evolution and maintenance of co-operative breeding. The co-operatively breeding Grey-backed Fiscal occupied territories having significantly greater tree and shrub cover than those occupied by the Common Fiscal, which is not a co-operative breeder; in addition, areas with greater vegetation cover held significantly more insects in the dry months of the year than did relatively open sites. Possibly associated with these differences in habitat and resources was a significant difference between the two species in the rate at which they disappeared during the study, population turnover among Grey-backed Fiscals being about half that of Common Fiscals. Similarly, territorial stability, as measured by the percentage of territories continuously occupied during the 18-month study, was nearly twice as high for the Grey-backed Fiscal. Within the restricted acacia (Acacia) woodland Grey-backed Fiscals were dominant over Common Fiscals, and in four instances the latter were observed to be driven out of their own territories by the Grey-backed Fiscals. It was suggested that co-operative breeding by Grey-backed Fiscals is related to occupancy of a stable but spatially restricted habitat of high quality; this may lead to relatively higher survival rates among Grey-backed Fiscals and, as a consequence, the habitat becomes “saturated” by the species. In such circumstances, juveniles have only limited opportunities for dispersal, and one evolutionary solution is that of group-living.
In the same Kenyan study, the roles of social and ecological factors in the maintenance of group-living were examined. Grey-backed Fiscals occurred in groups of 2–9 or more individuals, with only one breeding pair per group, the “supernumeraries” acting as helpers. The amount of perennial shrub cover in the habitat varied, and in the dry months areas with high cover contained significantly more prey items than did low-cover areas. Group size was positively correlated with shrub cover on three of the four occasions during the three-year study when it was tested, and the average group size over an 18-month period was likewise correlated with shrub cover. Both individual survival and the number of young produced increased with increasing cover. Moreover, group size was correlated with these two factors of survival and productivity. Pairs or small groups of Grey-backed Fiscals occasionally colonized areas with poor vegetation cover, but, typically, these disappeared without having bred successfully. In contrast, high-cover areas harboured a constant number of breeding pairs and a varying number of supernumeraries. The latter appeared to have a choice between dispersal and remaining on the natal territory, the second option offering the probability that they would ultimately attain breeding status within or near the natal territory.
Male shrikes, in order to drive away rivals and to court females, sometimes execute a display in which they mimic the action of impaling prey. This is followed by courtship displays in which the male feeds the female and performs a bowing dance, as well as song and sometimes flight displays, about 7 m from the female. The female will accept prey from the male when she is ready to copulate, holding the food and lowering her upper body. Groups of neighbouring shrikes sometimes gather at the edges of territories and call at each other or perform dancing displays, behaviour which is thought to help in the establishing of pairs, as well as in reducing aggression and defining territories, thereby also indicating the location of vacant territories. Individuals may fight others in order to establish territories, but the marking of boundaries is usually done solely with vocalizations. Encounters with intruders involve bowing and fluttering displays, in which the body is held horizontal, the wings drooped and fluttered rapidly, the back feathers fluffed up, the tail spread and the head lowered, the shrike then pecking at the ground while uttering harsh calls. The rivals will face away from each other, and then whirl around to face each other and start the bowing again. If one does not retreat, a brief fight may ensue involving loud rasping calls, foot-grappling, and occasionally pecking.
Courtship feeding has been documented for a number of shrike species, and is probably a regular feature of the family. The provision of nutrients by a male can increase the female’s reproductive success, and males can, therefore, use such gifts to influence female choice. Polish researchers investigated courtship feeding by Great Grey Shrikes. It was found that the males offered food both to their mate and to extra-pair females, and that food offered to the latter had a significantly higher energy value than did that offered to their own partners. There are, however, very few definite examples of courtship feeding prior to extra-pair copulations. The size of the prey being offered to a female may enable her to assess a male’s quality. The observations revealed that a larger gift resulted in a higher chance of copulation for the male, whether with its own mate or with an extra-pair female, and it was concluded that the energy value of additional food received through courtship feeding by males could explain why females of some species engage in extra-pair copulations.
Ornithologists studying Red-backed Shrikes in Sweden found that courtship feeding had an influence on clutch size. Males, through courtship feeding, directly influenced the number of eggs the females laid, but the mechanism behind the female response is at present unknown. In Japan, however, the rate of courtship feeding by Bull-headed Shrikes was not associated with copulation, but was most frequent in the cold season, during the critical stages of egg-laying and incubation. This suggests that it is important for the early breeders in that it helps the female to build up reserves while the ambient temperature is low and food is scarce.
Most shrike species hold all-inclusive and non-overlapping breeding territories within which mating, foraging and brood-rearing take place. Territory size varies according to the species, and also in relation to such factors as population density, habitat characteristics and resource availability. Males attract mates through song and through the maintenance of large food caches (see Food and Feeding). These stores often include colourful inedible items, presumably to increase their attractiveness to possible mates. Males with more prey in their caches attract more females, and their mates generally lay more eggs and fledge more young than those paired to males with smaller larders.
In studies of Loggerhead Shrikes carried out in Canada, the average size of territories in south-east Alberta, at the northern limit of the species’ range, was 8·5 ha. Although there were small differences from one year to another, territory sizes did not differ between the incubation and nestling periods. Territories were larger than those in more central parts of the range, probably because the region is more arid and prey fewer in number.
An important characteristic discovered in recent years is that shrikes appear to “clump” their nests in certain areas, as was found, for example, with Loggerhead Shrikes in Oklahoma. A statistical analysis of Loggerhead Shrike territory distribution produced some interesting findings. When the data were examined without regard paid to the dates of nest establishment, the results were equivocal. When the times at which nests were established were taken into account, however, the later ones were far more likely to be closer than expected to pre-existing nests of conspecifics. This was true even when the distribution of resources such as suitable nest-sites was included in the analysis. Similarly, in Idaho, Loggerhead Shrikes usually nested in proximity to others of the species, even in large areas of sagebrush, and this was thought possibly to offer additional awareness of nearby predators and/or to increase the ability of individual shrikes to find mates. These findings support the hypothesis that Loggerhead Shrikes, when looking for suitable breeding habitat, are guided by and base their selection on the distribution of breeding conspecifics. This clumping of nests has been reported also for the Red-backed Shrike across much of its distribution, the Brown Shrike and Bull-headed Shrike in Japan, and the Common Fiscal in South Africa.
Predation is possibly an important factor affecting an individual’s behaviour and general life, but few studies have focused on the question of whether predators affect the prey species’ selection of breeding habitat. A study in Sweden tested whether breeding-habitat selection and reproduction by the Red-backed Shrike were linked to the presence of breeding pairs of the shrike’s potential nest predators, which were the Eurasian Magpie (Pica pica), the Hooded Crow (Corvus cornix) and the Western Jackdaw (Corvus monedula). An artificial experiment with nests mimicking those of Red-backed Shrikes indicated that only Eurasian Magpie and Hooded Crow territories were associated with an elevated risk of predation. Among Red-backed Shrike nests in the wild, the predation risk was likewise greater for those close to nests of these two predator species than it was for nests elsewhere in the landscape. Occupation frequency of known Red-backed Shrike territores during the study increased with increasing mean distance to the nearest magpie nest. In addition, changes in the spatial distribution of corvids affected the spatial distribution of the shrikes. It was found that vacant Red-backed Shrike territory sites were more likely to become occupied in the following year when Eurasian Magpies and Hooded Crows had moved away, and, conversely, occupied sites were more likely to be abandoned in the following year when crows had moved closer. The evidence indicates that the breeding territories of nest predators may, indeed, have an influence on the breeding-habitat selection of their prey.
Intensity of nest defence increases as the breeding cycle progresses. An example is that of the Red-backed Shrike, which protects its nest aggressively, attacking predators and potential predators which approach the site. In one study, this shrike, in response to the presence of a human observer during the nesting period, became increasingly bold its its defence. The level of aggression was not, however, influenced by the number of offspring present and the time of the season, and, contrary to previous predictions, the researchers did not find any differences between the sexes in nest defence. More aggressive parents, both females and males, had significantly better breeding success than did calmer individuals.
Nests of shrikes are generally large, untidy, uncamouflaged structures, normally incorporating a variety of materials, including man-made ones, found in the breeding pair’s territory. The two Eurocephalus species and Souza’s Shrike (Lanius souzae) are exceptions in that their nests are small, neat, well-moulded and camouflaged, the construction and general positioning being much more reminiscent of nests of the malaconotid genus Prionops than of those of the Lanius species. Laniid nests are almost always built in thorny bushes or small trees, in hedgerows or in the outer parts of tall trees. There are reports also of nests being placed in strongly entangled barbed-wire rolls, and one nest of a Lesser Grey Shrike was situated at the bottom of a cavity 38 cm deep. Nests are often built one or two metres above the ground, and it seems that the exact site is selected on the basis of structural aspects of the vegetation cover, and not the plant species themselves.
Both sexes gather nest material, but it is generally the female that does the construction work, taking 6–12 days to complete the task. The size of the finished nest is variable, but nests are roughly 15 cm in outside diameter, 10 cm in internal diameter, and 7·5 cm deep. In general, they are tightly woven, bulky, open cups lined with soft material. The nests usually consists of large pieces of vegetation, such as twigs, forbs and bark, with rootlets added; the lining is highly variable, including such items as flowers, lichens, grasses, moss, feathers, fur, cattle hair, string, cloth and the like. Even so, shrikes are able to alter their nest-building behaviour to suit the immediate surroundings. Typically, the nest is very well insulated, with a low level of thermal loss, this being a possible adaptation to cool, wet weather.
Nests may be reused from year to year, but it is more common for the old nest to be dismantled and the materials used for a new one nearby. When renesting occurs in the same season, as may occur, for example, after a failed attempt, shrikes generally tend to nest higher above the ground on their second attempt. A study in Germany looked for possible reasons why Red-backed Shrikes did not conceal their nests better. Altogether 296 nests were categorized for degree of visibility from distances of 1 m and 2 m from above and on all four sides of the nesting bush or tree. Almost half of the nests, 47%, were exposed and only 16% were well concealed, and the number of exposed nests increased as the breeding season progressed. The nests are easily detectable by predators, whether foraging actively or by accident, and concealed nests indeed had higher breeding success. The authors of the study considered the findings to reflect the lack of better alternatives in the changing habitat and limited potential nesting sites. For Red-backed Shrikes in Germany, a precondition for the establishment of a territory was the existence of vegetation suitable for nest-building. Shrikes preferred to build nests at heights of 0·8–1·6 m. The importance of the quality of the nesting bush or tree is underlined by the fact that, besides weather and predation, losses occur owing to inadequate attachment of the nest or weakness of the supporting branches, resulting in 7·5% of the nests slipping or tipping over.
In Estonia, nest-card data for the years 1942–2001 revealed that Red-backed Shrike nests had been found in 41 different plant species, but 41% of all the nests were in spruces (Picea). The average height of the nest tree was 2·8 m and the average height of the nest placement was 1·2 m; there was no difference in the average nest height between successful and unsuccessful pairs. On Hokkaido, in north Japan, Bull-headed Shrikes nested in dwarf bamboo and vine bushes in the early part of the breeding season, and, as the season progressed, moved their nest-sites to a variety of deciduous shrubs as the foliage load of these plants increased. Nest height increased gradually throughout the breeding season, an observation that was attributed to seasonal changes in the use of vegetation types. Thus, Bull-headed Shrikes exhibited a seasonal change in nest-sites with the progress of plant phenology, and this reduced the risk of nest predation.
In North America, the Loggerhead Shrike is a widely distributed member of Idaho’s sagebrush-rangeland avifauna. In a study of its breeding ecology and nesting locations in this semi-arid habitat, most nests, 65% of those found, were constructed in sagebrush. The height of the nest shrubs was 0·89–2·97 m, the average being 1·62 m, and the mean height of nests was 0·79 m, with the range 0·33–1·60 m. The nest variables measured did not differentiate successful nests from unsuccessful ones. Nests constructed in dense greasewood shrubs contained fewer medium-sized and large sticks, typically used in construction of the nest substructure, than did nests built in relatively open sagebrush; in contrast, there was no difference in the quantity of lining fibres, twigs and small sticks used in nests constructed in these shrubs. These findings could be due to the different nest-stabilization requirements imposed on the shrikes by structurally dissimilar nesting substrates, and suggest a degree of plasticity in the nest-building behaviour of the shrikes. In another part of the Loggerhead Shrike’s range, in central Missouri, success was highest for nests in deciduous trees and lowest for those in multiflora rose (Rosa multiflora), possibly because rose bushes are not so structurally sound nor so thorny as those deciduous trees used for nesting. In south-central Washington nests having better concealment produced more fledged young, and in Minnesota nesting success was positively correlated with the percentage cover of grassland and fledging success with percentage cover of herbaceous vegetation and of grassland. In Manitoba, nest-sites with lower amounts of understorey (ground cover and vegetation height) were more successful, and nests in pasture were more productive than were those in cropland or in mixed habitat types.
In Idaho, the breeding density was one pair per 8·9 ha in an isolated 89-ha stand of sagebrush and bitterbrush, with an average nearest-neighbour distance of 203 m; in the same general area, it was one pair per 25 ha in a rugged 475-ha bowl of sagebrush and the average distance to the nearest neighbour was greater, at 328 m. The disparity was attributed to habitat characteristics, as the average shrub height was greater at the site with smaller territories.
Clutch size varies with latitude. Large clutches, containing 7–9 eggs, are laid at high latitudes and small clutches, of 2–3 eggs, are normal in the tropics. If the first clutch is lost, however, a comparatively smaller replacement is laid. From historical records, the average reported clutch size for Loggerhead Shrikes is 5·4 eggs, with a range of 1–9 eggs; 36% of clutches contained six eggs and 34·2% had five eggs, and at the two extremes only 0·4% were of one egg and 0·1% of nine eggs.
Eggs are laid usually in the early hours of the morning, one per day over the course of a week or so. Shrike eggs tend to be variable in colour and patterm. For example, those of the Red-backed Shrike are smooth and oval, with a ground colour varying from pale green to pinkish, buff or creamy white, and with a band of light brown, olive, brownish-red, grey or purple speckles and small blotches near the broad end or, sometimes, with the markings scattered over the whole surface or even, unusually, present only at the narrow end. The eggs of Souza’s Shrike are pale green-grey, with grey, brown and purple freckles around the broad end; they seem to lack the blotching typical of other Lanius species.
Despite numerous studies of bird eggs, the function of eggshell patterning remains largely unknown. Possible functions include the signalling of condition, and mimicry. It has been suggested that eggshell patterning, as well as egg size and shape, might be linked to natural selection. Description of egg variability and its causes among Loggerhead Shrikes could be particularly important, because of this species’ wide distribution in North America and the uncertainty over the reasons for its population decline.
It has been postulated that egg size could be affected by climate change. During the period from 1971 to 2002, in a long-term study of Red-backed Shrikes in Poland, it was evident that egg volume decreased significantly, the shrikes arrived at the breeding sites significantly earlier, and arrival dates were correlated with the earliest first-egg dates. It was tentatively suggested that the causes of these changes might include, among other factors, changes in temperature and also in food supply, although further studies are required.
Although the mean volume of Bull-headed Shrike eggs in Japan did not change during the course of a season, the variation in volume in six-egg clutches increased in 1994 and 1995 but did not change in 1996. Peaks in arthropod biomass occurred early in the breeding seasons in the first two of these years, but late in the season in 1996. Increased food availability was associated with reduced variation in egg volume within a clutch. A significant difference in egg size was found within six-egg clutches, the first egg being the smallest. Nevertheless, chicks that hatched from small eggs early in the hatching sequence suffered lower mortality rates than did those from large eggs laid late in the order. It was suggested that egg-size variation within a clutch was perhaps caused by a trade-off between the time required to develop an egg and the volume of the first egg within a clutch. Because eggs hatching first in sequence do not have to be large for the hatchling to survive, the Bull-headed Shrikes may start to lay the clutch earlier at the cost of reducing the volume of the first egg.
Incubation normally starts from either the last egg or the penultimate one. Females incubate while the males forage, but females generally lose weight during the period of incubation, which lasts for about 15–20 days. During this period the females turn the eggs 6–10 times daily, and often more frequently on hot days.
Hatching is typically synchronous, except in cases when incubation was started before the full clutch had been laid. The sequence in which the eggs in a clutch hatch matches the order in which they were laid, the first-laid also being the first to hatch. Hatchlings are altricial, being blind and helpless and usually naked. They vary somewhat according to species, but in very general terms they have pinkish-orange skin and a yellow or orange-yellow mouth lacking spots. On the first day their weight is in the approximate range 3–7 g, and they gain some 3 g or more daily until the twelfth day; growth then slows until the time of leaving the nest, generally between days 17 and 21, when they weigh roughly 40 g. The wings have developed fully by 15 days. The rate at which the parents deliver prey to the nestlings increases from day 6 to day 12, thereafter decreasing until just before the young leave the nest, when it reaches a peak. In the first 4–5 days, the female feeds the young with food presented to her by the male, but after this she normally spends more time in foraging and less time in brooding, while her mate feeds her less and starts to provision the chicks directly.
Many aspects of the begging behaviour of nestlings relate to sibling competition for food within the nest, and one could expect the chicks to exhibit some evidence of adaptive learning and behavioural adjustment in response to experience of the competitive environment. Researchers conducted trials involving the hand-feeding of Southern Grey Shrike nestlings in an artificial nest, in which they created zones of differing profitability by adjusting either the prey size or the number of food items delivered. The chicks did, indeed, detect the differences and positioned themselves accordingly; by the end of both the prey-size experiment and the delivery-rate trial, the nestlings had increased the amount of time that they spent in the “high-quality zone”.
In a study of Bull-headed Shrikes in Ishikari, in northern Japan, seasonal changes in the relationship between brood size, the body mass of nestlings and the body mass of parents were investigated. When broods were twelve days old, the body mass of the heaviest nestling in a brood did not differ with brood size, or throughout the season. The body mass of the lightest nestlings, however, varied according to brood size, and the body mass of the lightest nestling in broods of five and six chicks decreased throughout the season. The lightest nestling in four-chick broods and the lightest and second lightest in five-chick broods had weights significantly lower than that of the heaviest nestling in broods of these sizes. It is likely that shrike pairs with six 12-day-old chicks in the nest can feed at least five of these sufficiently to ensure their survival. The standardized body mass of the parents, defined as the mass divided by the length of the tarsus, did not vary with brood size or with time of the season. The researchers concluded that this constancy, coupled with the seasonal decline in the body mass of nestlings, might indicate that Bull-headed Shrikes have a limit to their parental efforts.
In a study of the body constituents of nestling Red-backed Shrikes in Poland, it was found that, as the chicks became older, the percentage content of protein and fat increased and that of water decreased. The bioenergetics of nestlings are of great interest because of the chicks’ fast growth rate, which is dependent on brooding and feeding and may limit brood size. Of 13 broods containing a total of 59 nestling Red-backed Shrikes in Poland, two stages in the nestling period could be distinguished, the first from hatching to 7–8 days of age and the second from 7–8 days to fledging. In the first stage, nestlings have no effective thermoregulation and are brooded by the parent; this allows them to develop under presumably optimal temperatures, and to use most of their assimilated energy for rapid growth. During the second stage, the chicks have effective thermoregulation and brooding is very limited; a large proportion of assimilated energy is then used for heat production, and the rate of growth decreases. The development of plumage, an increase in body size, and huddling with nestmates prevent excessive heat loss. The chicks’ highest energy requirements were in the middle part of the nestling period, and, if conditions were then unfavourable, this period could limit the brood size.
After fledging, the young shrikes remain in the nest tree or bush for a few days before flying to other perches. They perch in heavily foliaged trees or in undergrowth, very close together, and they keep quiet, begging for food only when the parents arrive. About one week after leaving the nest, they are able to fly increasing distances, and they begin to follow the parents in order to learn hunting skills. The young remain dependent on the adults for 3–4 weeks after fledging, and they then begin to forage on their own successfully. They begin to perform impaling movements at 20–25 days of age, and can impale food items successfully at 33–35 days, but it is not until they are 40 days or so old that they have fully developed the ability to attack and kill vertebrate prey.
A notable aspect of parental behaviour observed in the Negev Desert of Israel was that of the mother transferring the nestlings to a different place. This was discovered by accident when, at four nests of Southern Grey Shrikes, the entire broods disappeared immediately after having been colour-ringed at the age of 10–13 days. All of the young were subsequently found at the base of different bushes at distances of 3–55 m from the nesting tree or bush; each brood was clumped together in a cavity excavated on the ground and concealed by dense vegetation. The female was seen to prod each individual nestling out of the nest and on to the ground, and then to coax it to the alternative hiding place. During the process the male remained perched in the vicinity, keeping a watch for potential predators. Similar behaviour was reported for Loggerhead Shrikes in Idaho, where the parents were observed to induce nestlings to leave the nest earlier than they would naturally have done. This appears to be the reaction of the parents to human disturbance or to possible discovery of the nest by a predator, and was substantiated by the fact that it was observed at a nest that was partially preyed on by a diadem snake (Spalerosophis diadema) and, elsewhere, after a Common Kestrel perched on the nest tree. It appears that the translocation can be made only when the nestlings are sufficiently developed that they do not require brooding. The high ratio of such nestling transfers suggests that the strategy has adaptive advantages, and that it allows an increase in fledging success that would not be possible if young were left at the nest once discovered by the predator. This behaviour was observed also for Bull-headed Shrikes in Japan and Red-backed Shrikes in Belgium.
Brood parasitism by cuckoos and others also takes place. Red-backed Shrikes in Hungary were parasitized by the Common Cuckoo (Cuculus canorus) at a low frequency until the late 1960s, but since then there have been no confirmed cases of parasitism in that country. It seems probable that the cuckoo abandoned this host species because it lost the so-called “arms race”, which may be indicated by the shrike’s high level of the egg recognition. Experiments showed conclusively that Red-backed Shrikes are able to recognize their own eggs and reject the parasite’s eggs. Although this shrike was a host of the cuckoo in the past, it learned to identify accurately the parasitic eggs; the cuckoo, rather than evolving perfectly mimetic eggs to counteract the host’s recognition ability, switched host species.
In Germany, no cases of successful nest parasitism were found in a population of Red-backed Shrikes intensively studied for 20 years, during which time more than 1200 nests were monitored. In 1978, three of 22 nests contained young cuckoos, but all three of these broods were unsuccessful because of inclement weather and predation. It was noted that one of the cuckoo nestlings was unable to eject the eggs of the host. In another study, in the eastern Moravian region of the Czech Republic, a total of 45 young Common Cuckoos was found in a total of 2681 Red-backed Shrike nests. Here, the annual proportion of parasitized nests ranged from 0% to 5·5%, the average for the rest of the Czech Republic being 0·5% in 2949 nests. Again, the impression is that the cuckoo appears not to be responsible for any significant reduction in the shrike’s breeding capability. In Japan, the frequency of parasitism of Brown Shrike nests by Common Cuckoos on Hokkaido was very low, and the shrikes either removed the cuckoo egg or deserted the nest containing it.
Little is known about the frequency of brood parasitism by Brown-headed Cowbirds (Molothrus ater) on Loggerhead Shrikes in North America. The cowbirds opportunities to parasitize shrike nests may be limited owing to the Loggerhead Shrike’s aggressive and predatory nature. In fact, only two documented cases of brood parasitism have been recorded: three of 261 nests in south-west Iowa and two of 1661 nests in Manitoba were parasitized. Experimental attempts to parasitize Loggerhead Shrike nests met with limited success; the shrikes rejected Red-winged Blackbird (Agelaius phoeniceus) and Tricoloured Blackbird (Agelaius tricolor) eggs that were placed in shrike nests.
Two crucial aspects of fecundity are the age of first breeding and the reproductive output. As with most other songbirds under good conditions, shrikes start to breed in the first spring following the one in which they hatched. Breeding productivity is dependent on a variety of factors, and some examples are given in the paragraphs below.
Food abundance influences various aspects of a bird’s breeding ecology, such as the timing of laying, the clutch size, and the success rate. A team of European researchers examined the effects of a natural superabundance of food, in this case cockchafers (Melolontha melolontha), on the nesting success of a monogamous long-distance migrant, the Lesser Grey Shrike. In years of cockchafer outbreaks, these beetles make up 88% of the adult diet and 48% of the nestling diet of this shrike. The researchers compared the timing of egg-laying, clutch size and fledging success in three years and chick development in two years, incorporating years with cockchafer outbreaks and years without. In “cockchafer years”, laying dates were about one day earlier, clutch size increased by about one egg, and heavier chicks were produced. Fledging success, however, was unaffected, because more eggs failed to hatch during cockchafer years. Increased clutch size in periods of superabundant food did not, therefore, always result in an increase in fledgling production. The team concluded that, in the study population of Lesser Grey Shrikes, limited incubation ability of the adult shrikes or intrinsic problems in physical egg properties, resulting in inefficient incubation, were the most likely explanations for the increase in hatching failure in years with a superabundance of food.
A long-term, 30-year study of the Red-backed Shrike population in south-west Germany found that pairs produce an average of 2·84 fledglings per year, productivity varying between 1·9 and 3·9 fledglings per pair. Years with low breeding success are almost always those with inclement weather, and even brief cold spells, which occur regularly in June, lead to heavy nest losses. In addition to breeding success, the survival of the adults of this migratory species is clearly of critical importance. In the three decades of the study, no change has been detected in the return ratio of adults in consecutive years. In the Karlovy Vary region of the Czech Republic, a comparison of Red-backed Shrikes breeding at two different altitudes, 450 m and 650 m, revealed no difference in the dates of nest initiation and egg-laying, nor in clutch size; breeding success was 77% at lower levels but only 62% at higher-lying sites, because predation was much greater at the higher elevations.
In recent decades, the Brown Shrike has undergone a severe decline on Hokkaido, in north Japan, and a study of the reproductive performance and nestling growth was therefore undertaken. During 1992–1996, 41 active nests were examined, and the mean nesting success was found to be 53·7%. The average clutch size was 5·3, and the average number of eggs hatched per nest was 5·1; successful nests produced an average of 4·4 fledglings each, giving a mean fledging success per successful nest of 90·3%. The main cause of nesting failure was identified as predation, but the identity of the predators was unknown.
Average nest success of Loggerhead Shrikes across North America was reported as 56%. In Oklahoma, the probability of survival from the start of incubation until fledging was 46%, which is low compared with the findings of studies in other areas, but the percentage of Oklahoma nests that produced fledglings was somewhat higher, at 59·5%. Furthermore, nest success at any one site can vary greatly from one year to another, as demonstrated by a study in Missouri, where success decreased from 82·1% in 1980 to 55·5% in 1981. The average number of fledged young per nest varies from less than one to more than four. Mortality in the period from fledging to independence is rather high; for example, 46% of young died during the first week after fledging in Indiana, 33%–53% were lost during the first ten days after fledging in southern Alberta, and mortality of 33% from fledging to independence was recorded in Virginia. In western shrub-steppe communities, average survival decreased from 5·1 young per nest at the time of fledging to just 2·3 two weeks later; similar results were found in Ontario, where the corresponding figures were about four young per nest at fledging and 2·5 when the young became independent.
Most Loggerhead Shrikes of the migratory subspecies have single broods and renest only after failure with the first attempt. The sedentary populations, however, raise up to three broods per year, this depending largely on climate, where warmer and longer growing seasons result in more broods. Further, non-migratory shrikes are more likely to attempt second broods, and high nest success increases the probability of additional nesting attempts.
In Africa, in a study of Common Fiscals, young fledged from only 15% of the nests found. Of the 17 breeding pairs studied, only eight produced any young, at an average of 1·9 fledglings per nest.
Various studies have revealed that productivity and breeding success of Red-backed Shrikes in central Europe are reduced in years with wet and cold weather. The same has been found to be true for Loggerhead Shrikes in North America, Great Grey Shrikes in Poland and Southern Grey Shrikes in Israel. Inclement weather in Japan in 1993 allowed comparison, with 1992, of the effects of weather conditions on aspects of the Bull-headed Shrike’s breeding biology, such as the timing of breeding, nestling growth and overall nesting success. For both years, the season was divided into two periods, early and late. While the probability of nestling survival was almost the same during both periods, the probability of nest survival in the egg stage during the early period was significantly lower than that during the late period. In 1993, nestling survival during the late period was significantly less probable than during the early period; the late period was colder than the same period in 1992, and with larger fluctuations in precipitation. The number of “lost” nestlings was positively correlated with the mean daily precipitation, and most were the lightest in weight in each brood. It was found that the late breeders fledged lighter young than did the early breeders. Although the shrikes adopted hatching asynchrony, the late breeders could not overcome the effects of the unpredictable adverse weather in 1993.
In contrast, predators were the most important factor in determining the breeding success of Woodchat Shrikes in Spain. It was estimated that predation reduced breeding success by an average of 35% annually. In Mediterranean France, this species’ breeding habitats consist of open, dry grassland with a discontinuous short-grass layer and scattered bushes and trees, where about 75% of the Woodchat Shrike’s nests were at heights between 1 m and 3 m. The mean survival rate of nests from laying to fledging was 36%, and there was no correlation between breeding success and the site, height and concealment of nests. The main cause of the low breeding success was, again, a high rate of nest predation. Woodchat Shrikes in the El Kala National Park, in north-east Algeria, nested most frequently in cork oak (Quercus suber), at a mean height of 5 m. First clutches were laid on 7th May, about the same as in south France, and average clutch size was 4·9 eggs, slightly less than in France. Approximately 42% of eggs in Algeria produced fledglings. In this case, fledging success was positively correlated with the height of the nest above the ground, as well as with mean egg size of the clutch. It was suggested that heavy predation and poor food availability were probably the major selective pressures shaping the life history of Woodchat Shrikes in Algeria.
A most interesting phenomenon is that of a breeding association between shrikes and other species. Such associations are know to occur between the Red-backed Shrike and the Barred Warbler (Sylvia nisoria), the Isabelline Shrike and the Barred Warbler, and the Woodchat Shrike and the Orphean Warbler (Sylvia hortensis). In southern France, active nests of both Woodchat Shrikes and Orphean Warblers have been found in the same bush, and this, along with behavioural evidence, suggests that the association goes far beyond being a simple commensalism for the sole benefit of the warbler. It may be considered, instead, an example of mutualism designed to counter the problem of predators, but its precise extent evidently requires further observations. In North America, on the other hand, it was discovered that shrikes could exercise a significant control on the distribution and numbers of passerines in their breeding areas. In one study, Loggerhead Shrikes were observed while feeding on the nestlings of other passerines, resulting in the probable nest loss for the latter. This suggests that, although birds comprise only a small percentage of the shrike’s annual diet, the timing of predation on passerines could result in a marked negative effect on the annual production of the passerine community. One must, however, consider other factors, including the broader interactions of predator–prey relationships, competition and habitat structure. For instance, if short vegetation and a greater density of perches lead to an increase in shrike presence and, at the same time, a decline in the broader bird community is detected, this decline is not necessarily attributable to shrikes; the increased number of perches may attract higher numbers of raptors, which could have a negative impact on the entire bird community, including, ultimately, shrikes, through increased predation and competition. Clearly, a great deal of additional research is required before a proper understanding can be gained of the impact of shrikes on the bird communities of which they are a part.
Conversely, prey populations are known to have a direct influence on shrike populations. As would be expected, decreasing abundance of such prey as insects and small mammals leads to the reduction or even the elimination of local shrike populations. Furthermore, predation by other animals is, for shrikes, often the greatest cause of nest failure, although it is not known if this has serious impacts at the population level. In addition, various other birds attack recently fledged shrikes without killing them. In America, for example, young Loggerhead Shrikes are attacked by, among others, Scissor-tailed Flycatchers (Tyrannus forficatus), American Robins, Eastern Meadowlarks (Sturnella magna), Common Grackles (Quiscalus quiscula) and Brown Thrashers (Toxostoma rufum). In return, shrikes will harass other passerines, especially during territory establishment, when the shrike often chases the other from its own territory, sometimes causing other birds to move out of the immediate area.
Survivorship of adult shrikes is uncertain. In Oklahoma, a Loggerhead Shrike was recaptured after 11 years, in the same general area in which it had been ringed. Other ringed individuals have been documented as living for six years and more, but there are few data on longevity of laniids. It has been suggested that mortality in the non-breeding areas is the reason for observed reductions in abundance of some species, a possibility which is supported by the low return rates of first-year shrikes. Low return rates, however, cannot be taken as evidence of high winter mortality without considering breeding-site fidelity. In all studies of migratory Lanius species, the percentage of juveniles returning to the natal site to breed is very low, which suggests that there is a high rate of exchange of individuals between different populations.
In a colour-ringed population of Common Fiscals, the annual survival was 39%. Since 25% of the adults were known to have resided in two or more territories, however, this is probably an overestimate of mortality because of dispersal out of the study site.