Re-examination of the capital and income dichotomy in breeding birds


As part of a general investigation into the energy and nutrient requirements of female birds during egg-laying, this study reviewed the evidence from previous published studies on the effects of supplementary feeding on the laying date, clutch size and egg size of breeding birds.

A review was conducted of published studies of supplementary feeding experiments in birds. Studies were classified according to the timing of food supplementation (i.e. whether started before or during the breeding season) and the length of time for which food was provided. The effect of supplementary food provision on the timing of egg-laying, clutch size and weight of eggs was summarised for each study.

In total, 36 published studies documenting the effects of supplementary feeding in 24 bird species were reviewed. Of the 20 species that were fed from at least two weeks before the start of egg-laying (which the authors considered to be the minimum amount of time to detect an effect on laying date), 15 species showed a significant advance in the onset of laying (osprey Pandion haliaetus, common kestrel Falco tinnunculus, American coot Fulica americana, Tengmalm's owl Aegolius funereus, dunnock Prunella modularis, great tit Parus major, blue tit Parus caeruleus, crested tit Parus cristatus, willow tit Parus montanus, black-billed magpie Pica pica, Eurasian jackdaw Corvus monedula, carrion crow Corvus corone, common starling Sturnus vulgaris, song sparrow Melospiza melodia and red-winged blackbird Agelaius phoeniceus). Of the five species that did not, three were fed for only two weeks (Eurasian sparrowhawk Accipiter nisus, American kestrel Falco sparverius and yellow-headed blackbird Xanthocephalus xanthocephalus), and two were colonially breeding species (lesser black-backed gull Larus fuscus and glaucous-winged gull Larus glaucescens).
A significant increase in clutch size following supplementary feeding was found in only seven species (American coot, lesser black-backed gull, Tengmalm’s owl, marsh tit Parus palustris, red-backed shrike Lanius collurio, jackdaw, song sparrow), although in four of these a second study found no effect. Only a very few studies showed a significant increase in egg weight with supplementary feeding.
Note: If using or referring to this published study, please read and quote the original paper, the abstract of which can be viewed at:

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