The diets of all members of the family consist primarily of insects, other arthropods, and especially spiders. Other prey items include crustaceans, snails, and harvestmen (Opiliones). Sap (golden-crowned kinglet) and nectar (several African and Asian species, especially Prima) are occasionally consumed. Prima hodgsonii and Orthotomus sutorius are known to carry pollen attached to the feathers of the throat and forehead; it and other nectar-feeding species may be important pollinators in the tropics. Some of the larger reed-warblers (including Acrocephalus arundinaceus, A. rufesecens, and A. stentoreus) occasionally take small frogs and fish. Young sylviids are fed almost exclusively arthropods, usually soft-bodied larvae and small insects, but in some cases receive berries as well. Variation in prey size and type is found among sympatric foraging guilds. During the pre-migratory period of Palearctic Sylvia warblers, individuals shift their diet from largely insects to largely berries and fruits in order to accumulate fat for migration.
Foraging strategies among the Old World warblers are diverse. Some species forage singly or in pairs, while others forage in groups of several family members or other conspecifics. Still others readily join mixed-species foraging parties, especially in the nonbreeding season. Cisticola nigriloris forages in groups of 5-8 birds, apparently keeping in contact by group-singing. Many African and nontropical species, join foraging parties in the nonbreeding season.
The typical foraging mode is perched gleaning (also known as standpicking) from the ground or from vegetation. Sallying and hawking of aerial insects is also used by many species, including the flycatcher warblers (genus Seicercus). Bradypterus sylvaticus forages on the ground by scratching up litter and humus with its feet, or by disturbing the humus with its wings and tail, in a motion reminiscent of dust-bathing. Tesias, Macrosphenus kretschmeri, and other terrestrial warblers feed in a similar manner. Even some Phylloscopus warblers, members of a genus composed mostly of arboreal species, have adapted to ground-feeding. Kemp's longbill Macrosphenus kempi climbs about on undergrowth, probing its bill into dead wood.
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