Abstract
Several studies on birds have proposed that a lack of invertebrate prey in urbanized areas could be the main cause for generally lower levels of breeding success compared to rural habitats. Previous work on house sparrows Passer domesticus found that supplemental feeding in urbanized areas increased breeding success but did not contribute to population growth. Here, we hypothesize that supplementary feeding allows house sparrows to achieve higher breeding success but at the cost of lower nestling quality. As abundant food supplies may permit both high‐ and low‐quality nestlings to survive, we also predict that within‐brood variation in proxies of nestling quality would be larger for supplemental food broods than for unfed broods. As proxies of nestling quality, we considered feather corticosterone (CORT f), body condition (scaled mass index, SMI), and tarsus‐based fluctuating asymmetry (FA). Our hypothesis was only partially supported as we did not find an overall effect of food supplementation on FA or SMI. Rather, food supplementation affected nestling phenotype only early in the breeding season in terms of elevated CORT f levels and a tendency for more variable within‐brood CORT f and FA. Early food supplemented nests therefore seemed to include at least some nestlings that faced increased stressors during development, possibly due to harsher environmental (e.g., related to food and temperature) conditions early in the breeding season that would increase sibling competition, especially in larger broods. The fact that CORT f was positively, rather than inversely, related to nestling SMI further suggests that factors influencing CORT f and SMI are likely operating over different periods or, alternatively, that nestlings in good nutritional condition also invest in high‐quality feathers.
Highlights
Given that our experimental food supplementation resulted in higher fledging production (Table S1 sensu Peach et al, 2014) but not, in a separate study, enhanced population growth (Peach et al, 2015), we hypothesized here that food supplementation allows parents to fulfill the requirements of both low- and high-quality nestlings
These predictions were partially supported by our results, as we did not find an overall effect of food supplementation on CORTf, tarsus-based fluctuating asymmetry (FA) or nestling body condition, while there was a slight, not significant, tendency of higher within-n est variation in CORTf and tarsus-based FA early in the breeding season, but not scaled mass index (SMI)
We found that food supplementation affected nestling phenotype in terms of elevated CORTf levels, but only early in the breeding season
Summary
House sparrows breed from April until August, during which pairs can produce up to four clutches (Summers-Smith, 1988). House sparrows lay clutches of typically 3–5 eggs, with incubation lasting approximately 11 days, and with chicks fledging around 14 days after eggs hatch (Summers-Smith, 1988). Sparrow nestlings are fed almost exclusively on invertebrates such as beetles, caterpillars, and aphids, but as nestlings get older, more vegetable material is added to their diet (Anderson, 2006). We conducted this study on house sparrow nestlings raised in nest boxes in three rural (Houghton-on-the-Hill, Hungarton, and Keyham) and three suburban (Braunstone, Thurmaston, and Western Park) sites in Leicester, UK, in 2008 (Table A1 in Peach et al, 2014). Data for this study were obtained from 29 nests situated in rural areas and from 16 nests in suburban areas
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