Does food supplementation really enhance productivity of breeding birds?
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Food availability influences multiple stages of the breeding cycle of birds, and supplementary feeding has helped in its understanding. Most supplementation studies have reported advancements of laying, whilst others, albeit less numerous, have also demonstrated fitness benefits such as larger clutches, shorter incubation periods, and greater hatching success. Relatively few studies, however, have investigated the effects of supplementary feeding for protracted periods across multiple stages of the breeding cycle. These effects are important to understand since long-term food supplementation of birds is recommended in urban habitats and is used as a tool to increase reproductive output in endangered species. Here, we compare the breeding phenology and productivity of blue tits Cyanistes caeruleus and great tits Parus major breeding in food-supplemented and non-supplemented blocks in a broadleaf woodland in central England over three seasons (2006–2008). Supplementation was provided continuously from several weeks pre-laying until hatching, and had multiple significant effects. Most notably, supplementation reduced brood size significantly in both species, by half a chick or more at hatching (after controlling for year and hatching date). Reduced brood sizes in supplemented pairs were driven by significantly smaller clutches in both species and, in blue tits, significantly lower hatching success. These are novel and concerning findings of food supplementation. As expected, supplementary feeding advanced laying and shortened incubation periods significantly in both species. We discuss the striking parallels between our findings and patterns in blue and great tit reproduction in urban habitats, and conclude that supplementary feeding may not always enhance the breeding productivity of birds.
KeywordsClutch size Lay date Supplementary feeding Tit Urban
This research was funded by the Natural Environment Research Council (NERC) through a studentship to TJEH, and we are grateful to CJ Wildlife Ltd. (especially Peter Deans and Chris Whittles) for providing equipment, consumables and financial support. The Worcestershire Wildlife Trust generously allowed the use of Chaddesley Woods National Nature Reserve, and we also thank Mervyn and Rose Needham for their help in facilitating this research. Numerous people provided considerable help with field, laboratory and office work, especially Munhazaya Battsengel, Ivana Budinski, Louise Gilmour, Nardie Hanson, Chan Heu, Elizabeth Kingston, Ewa Kos, Marija Majer, and Vladimir Rakić. We are grateful to Phillip Cassey, Simon Harold, and Kate Lessells for statistical advice, and to Steve Schoech and an anonymous reviewer for helpful comments on an earlier version of this paper. We declare that this research was conducted lawfully within the United Kingdom.
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