Western Olivaceous Warbler Iduna opaca Scientific name definitions

13·5–15 cm; 9–17 g. A pale Hippolais with fairly long tail, which often looks rather full and with rounded sides when folded; strong legs; very long, strong and broad bill with straight or convex sides (seen from below), and with comparatively thick and bluntly pointed tip. Has short and poorly marked whitish supercilium and whitish eyering, pale lores; pale greyish-brown or pale brown above , in certain lights appearing to have faint olive tinge on head and mantle ; wing feathers rather uniformly brown, fringes only marginally paler (no obvious pale secondary panel), tail with diffusely paler edges and tips of outer feathers; off-white below , faint greyish-buff hue on breast side and flanks; iris dark brown; bill dark, lower mandible all pale pinkish-yellow; legs greyish-pink. Differs from I. pallida mainly in somewhat larger size, browner tone above, heavier bill with less acutely pointed tip; also in habit of keeping tail still (no “dipping” down when feeding and moving in canopy). Sexes alike. Immature is similar to adult.

E & S Spain, also NW Africa in Western Sahara, much of Morocco, and E along coastal range and N mountains in Algeria and Tunisia (S to S slopes of Atlas) apparently to coastal NW Libya; winters in W Africa from Senegambia E to W Chad.

Migratory. Non-breeding quarters in W Africa, mainly in Sahel zone from S Mauritania and Senegal E to Niger and W Chad, S to S Mali, Nigeria and N Cameroon. Departure in autumn late Jul to Oct; arrives back on breeding grounds in NW African in late Mar and Apr, in Spain generally not until mid-May.

Song  rather well articulated, a mixture of hard, soft, nasal and scratchy notes, phrases vary between c. 15 seconds and 30 seconds, at times longer, each phrase commonly starts with repeated “chek” call notes; delivered at slightly slower pace than that of I. pallida, and more varied and pleasing, lacking monotonous and cyclic repetition of latter’s; some resemblance to song of Acrocephalus scirpaceus. Call  a throaty tongue-clicking “chek” with nasal undertone, sometimes prolonged to slurred trills, “cher’r’r’r”; calls very similar to those of I. pallida, possibly inseparable, but perhaps on average stronger.

Season late Apr to Jun. Nest a well-built cup of plant stems and soft twigs placed in fork of branch, often at 1–4m. Clutch usually 3–4 eggs; incubation by female, period 11–13 days; nestlings fed by both parents, fledge after 11–15 days.