Several studies have suggested dietary segregation between nestling and adult birds resulting from both dietary requirements of offspring and distance to the foraging sites. We examined the diet in terms of composition and dimension, as the weight, habitat and taxonomy of prey in nestling and adult male and female Bluethroats Luscinia svecica spp. cyanecula at a recently colonized area in a mosaic of wetland (with sewage water) and terrestrial habitats in south-west Poland. On the basis of faecal samples collected between 2009–2012 from several broods and 94 adults, we found that nestlings had significantly lower diet diversity, consisting of heavier prey items. Comparing the proportion of seven major food types (order of insects/invertebrate class) we showed that the diet composition of adult and nestling Bluethroats differed significantly. The diet of nestlings contained significantly more soft-bodied prey items, namely Diptera and Lepidopteran larvae, and significantly fewer Coleoptera and Hemiptera. Furthermore, since adult showed marked decrease (contrary to young Bluethroats) of diet diversity and number of prey in the progress of the breeding season, hence our entire findings can imply that nestlings are fed in a selective manner receiving more profitable (soft-bodied and terrestrial) prey, and adults adjust their diet consuming less profitable (more chitinized and smaller) prey. This ultimately suggests the partial dietary segregation between nestling and adult birds, both in the term of size (biomass) and composition of prey. The use of a broad spectrum of food items and various prey types, and particularly the low dependence of nestlings on aquatic insects, suggests that Bluethroats have very plastic dietary requirements, which is most likely a factor facilitating the recent population recovery and adaptation to ecotonal zones between moist/wetland and terrestrial habitats with abundant moist soil, in newly settled areas of Europe.
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