A curate's egg: feeding birds during reproduction is 'good in parts'. A study of blue tits Cyanistes caeruleus and great tits Parus major

Abstract

Food supplementation of birds in gardens is widespread and UK householders have recently been advised to supplement birds throughout the spring and summer. This coincides with reproduction of many avian species and supplementation with specific foods (e.g. live invertebrates) is encouraged to support breeding attempts in gardens. To investigate this further I mimicked food supplementation in gardens by providing two commercial bird foods (peanut cake and mealworms \(Tenebrio\) \( molitor\) to blue tits \(Cyanistes\) \(caeruleus\) and great tits \(Parus\) \(major\) breeding in woodland in central England from 2006 to 2008. Supplementation advanced laying and reduced the number of young fledged significantly in both species, but provisioning with mealworms during the nestling phase increased apparent survival of fledglings. Intriguingly, however, stable isotope analysis revealed that supplement use was insubstantial and similar between birds on supplemented and non-supplemented territories. Analyses of data from the British Trust for Ornithology’s Nest Record Scheme demonstrated strong parallels between findings of my field study and patterns of reproduction of blue and great tits in urban habitats across the UK. I discuss the implications of my findings, including the use of food supplementation in avian conservation and in pure scientific research, and I outline exciting future directions.