Foraging, Diet, and Habitat

4 Foraging, diet, and habitat use A pair of Seychelles Warblers foraging. Insects are gleaned from the undersides of the leaves. 72 When searching for food birds must constantly make decisions about what they should eat, where they should look for it, and which part of their habitat is likely to be the most profi table source. When members of the family Acrocephalidae forage they are constantly on the move; none of the species uses a ‘passive’ sit- and-wait technique with long periods of immobility, as would be typical for fl ycatchers or shrikes. Although the spectrum of their search-and-capture feeding method shows few modifi cations, it is very surprising just how much the techniques used by Moustached Warblers and Eurasian Reed Warblers differ in the same habitat. In an old undisturbed reedbed a Moustached Warbler utilizes the lowest layer, the tangled jam of broken reed stems, to search for tiny, immobile, hidden invertebrates, while higher up a Eurasian Reed Warbler actively hops from stem to stem in the rapid pursuit of larger, mobile prey, which it seizes in skilful ‘leap-catches’. 1 1 A variety of ‘search-and-capture’ tactics standing position. Leap and aerial manoeuvres merge The reedswamp-dwelling Acrocephalus species, the more into one another in these techniques. During ‘leap-catch- bush-loving Iduna, and the arboreal Hippolais warblers ing’, brief fl utters of the wings are employed to snatch a all feed opportunistically on arthropods gleaned from fl ying insect or one just in the act of taking off in escape. leaves, stems, and twigs or snatched in fl ight. Some typi- In ‘fl ycatching’, airborne insects are taken following a cal photographs (photos 4.1 - 4.17) graphically demon- brief fl ight, accompanied in some species by an audible strate the spectrum of capture techniques in the group. snap of the bill. Flycatching techniques include the ‘sal- Such differences are often only fi ne, as are the corre- ly’, in which the bird fl ies from a perch in pursuit of an sponding variations in morphology (chapter 5), where the insect and then returns to a perch – a manoeuvre occa- bill form can indicate such species-specifi c differences sionally executed by tree warblers (Hippolais and Iduna) (Glutz & Bauer 1991, Cramp 1992). as well as by small plain reed warblers – and the ‘hover’, generally in the form of ‘hover-gleaning’ by an airborne The most widespread foraging tactics are the so-called bird seizing a stationary prey item (Glutz & Bauer 1991, ‘near-perch’ manoeuvres, when a target food item can be Cramp 1992). This manoeuvre is frequently performed in reached without the bird moving from its perch (Remsen evergreen tamarisk by Eastern Olivaceous Warblers of & Robinson 1990). These include various forms of glean- the north African subspecies reiseri, for example, and is ing, including picking prey from a surface while remain- very similar to that employed by Coal Tits in conifers. ing at rest (‘stand-picking’). A number of species feed methodically on the ground (e.g. some island reed war- When ‘pouncing’, a bird fl ies to the ground for either blers, Upcher’s Warblers), or on mats of fl oating vegeta- stationary or mobile prey. Acrocephalid substrate tion where they hop about, reminiscent of chats manoeuvres consist of ‘probing’ and ‘fl aking’ of mate- (Moustached Warblers). Also common are ‘reaches’, rial, where loose substrate is pushed or fl icked away with where the legs or neck are completely extended upwards, a sideways motion of the bill in order to reach hidden outwards, or downwards, sometimes accompanied by prey, as sometimes seen in Moustached Warblers. Other fl uttering of the wings. In this way prey animals are more substrate-oriented and more complex extractive reached from a perching position by a rapid stretching of techniques have been developed by insular species in the body, or by a reaching upwards to glean them from particular (chapter 12). These include leaf-turning and the underside of a leaf, or by hanging downwards as ‘prying or gaping’, whereby the bill is opened while it is exemplifi ed by Seychelles Warblers (introductory draw- inserted in substrate to expose prey. Use of the feet in ing). treatment of prey is also known from some island reed warblers and Australian Reed Warblers have also been In addition, the plain-coloured reed warblers and most observed to use their feet as though to disturb prey (Hig- Hippolais species will seize passing aerial prey from a gins et al. 2006a). Tree warblers (genus Hippolais) use 73.

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