The effects of over-winter dietary provisioning on health and productivity of garden birds

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The effects of over-winter dietary provisioning on health and productivity of garden birds

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Title: The effects of over-winter dietary provisioning on health and productivity of garden birds
Author: Plummer, Kate Elizabeth
Advisor: Blount, JonathanBearhop, Stuart
Publisher: University of Exeter
Date Issued: 2011-06-17
Abstract: Food supply plays a crucial role in regulating bird populations. For many small passerines, both in the UK and globally, winter food availability is substantially increased through the provision of supplementary food. Garden bird feeding is a popular and growing phenomenon. Yet there remains a distinct lack of understanding of the ecological impacts this may be having on wild bird populations. Using a three year landscape-scale study I have investigated the carry-over effects of winter supplementary feeding on the health and productivity of resident blue tit populations (Cyanistes caeruleus) during the breeding season. Replicating the diffuse nature by which food is provisioned in gardens, I have examined the importance of energy (fat) and antioxidants (vitamin E) as carry-over effect mediators. Females showed greater resource allocation through a proportional increase in yolk mass, whilst males exhibited an improved oxidative status during the brood-rearing period in response to vitamin E provisioning. But in addition, significantly lower feather carotenoid concentrations were seen in individuals from vitamin E fed woodlands, suggesting that birds of poorer condition prior to feeding were able to survive winter and recruit into breeding populations as a result of antioxidant provisioning. This indicates that winter supplementary feeding has the capacity to perturb natural selection and alter the phenotypic quality of breeding populations. Furthermore, over-winter provisioning led to a reduction in fledging success across both treatments, which suggests that it may give birds false cues as to natural food availability and encourage them to make an unsustainable investment in nestling numbers, thereby acting as an ecological trap. With garden bird feeding promoted as a method for conserving declining wild bird populations, these new insights suggest much more needs to be done to fully understand its impacts.
Type: Thesis or dissertation
Funders/Sponsor: Natural Environment Research Council (NERC) with CASE partner the British Trust for Ornithology (BTO)

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