Abstract
The composition and quality of food provided to nestling birds influence their growth and development and offers key insight into the ecological requirements of birds. One bird species whose feeding ecology is poorly understood is the Barred Warbler (Sylvia nisoria), which utilizes semi-natural shrubby vegetation in agroecosystems. Because Barred Warbler nestlings vary greatly in body mass we hypothesised that diet and prey properties (size, diversity, taxonomic composition, and chitin content and resulting body hardness and digestibility) would differ as the nestlings aged. We quantified the diet based on faecal analysis, sampling faecal sacs from the nestlings pooled into three age classes: 2-3 days old, 4-6 d old, and 7-9 d old. Nestlings were provided a wide diversity of food and a strong relationship existed between food characteristics and nestling age. The youngest nestlings (2-3 d old) had the lowest values of each dietary characteristic (diversity, number and total biomass of prey, and individual prey weight), that were significantly lower than the oldest nestlings (7-9 d old). Nestlings aged 4-6 d exhibited intermediate dietary characteristics. Differences in dietary composition of the six major food types showed marked differences between the individual broods and age categories. Percentages of the number and biomass of soft-bodied prey were highest in the diet of 2-3 d and 4-6 d old nestlings, and decreased with increasing age, whereas the opposite trend was observed in the percentage of intermediately and heavily chitinised prey. Parent Barred Warblers probably preferentially select soft-bodied prey for the youngest nestlings, and satisfy the greater energy demands of the older ones by providing them with a greater variety of prey containing more chitin, as well as plant food. The provisioning of less-readily digestible prey to older nestlings suggests that as the quality of food decreases the quantity increases, implying that the youngest nestlings may be physiologically limited as regards their ability to digest more heavily chitinised prey.
Highlights
Knowledge of the different food types provided to nestlings offers key insight into the ecological requirements of birds
For each faecal sac we determined three main dietary characteristics and the composition of the diet expressed as the number and biomass of six major food types consumed and three groups of prey with different chitin content
The four main dietary characteristics calculated for individual faecal sacs showed statistically significant differences associated with brood identity and nestling age: diet diversity expressed as the number of prey taxa, the number of prey items, the total biomass of prey (Table 2) and individual prey weight (ANOVA Kruskal-Wallis, H2,661 = 13.0, P = 0.015) (Fig 1)
Summary
Knowledge of the different food types provided to nestlings offers key insight into the ecological requirements of birds. The energy demands of growing nestlings increase considerably, and their growth requires changes in the amount and type of food provisioned by parents. These cost-benefit trade-offs in prey delivery to nestlings are dynamic and depend on both intrinsic and extrinsic factors (after [1]). The age-related increase in prey size implies that the nutritional requirements of the nestlings are a primary determinant of adult foraging strategy [6]. Another potential explanation are the gape-size constraints, which principally limit the size of prey ingested [5,6,7]. The development of digestive abilities reflects the growing tolerance to the chitin content of prey, affecting overall changes in diet characteristics, and possibly prey selection by parent birds [10,11,12]
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