title: Nutritional constraints on egg production in the blue tit
Creators: name: Ramsay, Scot L
Publisher: name: University of Glasgow
Year: 1997
Subjects: subject: Ecology, Blue tit, egg production, food supply, Cyanistes caeruleus.

description: Food supply is known to be a major factor influencing timing and level of investment in avian reproduction and the nature of its influence has been a subject of research for many years. Most previous studies, however, have considered food primarily in terms of energy supply. In this research I have investigated the possibility that other nutrients, particularly protein, may proximately constrain egg production. An experimental approach, involving the provisioning of breeding blue tits with supplementary foods of differing nutritional composition, was combined with observation and analysis of the birds' natural foraging sites and prey availability around the lime of egg formation. Early in the breeding season (15-29th April), before most birds had begun to lay, approx. 76% of all observed foraging occurred in oak, with the remainder observed predominantly in birch. By mid to late May, however, when the majority of birds had begun laying, blue tits were observed to forage almost exclusively in oak (~98% of foraging records). Over the whole observation period around 92% of all arboreal foraging was recorded on twigs, buds and leaves. Systematic sampling of arthropods at the identified foraging locations of breeding blue tits was carried out concurrently in order to examine prey availability. In oak, arthropods were found to be relatively scarce during mid to late April, increasing in abundance through the laying period of the population. The predominant arthropods present were Lepidopteran larvae and small Araneae. Arthropods were found to be extremely scarce in birch during the period when birds were seen to forage there. Amino acid analyses were performed on samples of blue tit eggs, Araneae and Lepidopteran larvae collected at three times through the laying season. These analyses indicated that the sulphur-containing amino acids, methionine and cysteine, would most likely be the most limiting essential amino acid for a laying blue tit consuming an insectivorous diet. In the first supplementary feeding experiment, one group received a pure energy food (animal fat) whilst the other received cooked eggs (on the assumption that this would contain all the necessary nutrients for egg formation). Supplementary lipid and supplementary egg resulted in the same degree of laying enhancement compared with control birds. However, the provision of supplementary egg resulted in a significant increase in egg volume (approx. 7%) whilst supplementary lipid had no significant effect on egg size. The eggs laid early in the laying sequence showed the greatest size increase, with eggs from the egg-fed treatment group being significantly larger than those in both the fat-fed and control groups. I discuss the hypothesis that energy supply may influence the initiation of laying, whilst specific nutrients may proximately constrain egg production. The hypothesis that egg production may be proximately constrained by the availability of high quality protein was examined in a second supplementary feeding experiment. This involved the provision of supplementary foods of identical calorific, 'crude' protein and nonprotein content, but differing amino acid composition. Both supplementary diets resulted in a similar degree of laying advancement compared with control birds; supporting the hypothesis that initiation of laying is influenced by energy supply. Clutch size declined significantly through the season in all three experimental groups and, after controlling for laying date, mean clutch size was found to be significantly larger in the treatment group receiving 'high-quality' protein compared with those receiving 'low-quality' protein or controls; whilst in the treatment group receiving 'low-quality' protein, mean clutch size did not differ significantly from control. The increased clutch size in the 'high-quality' protein treatment group was reflected in a higher mean number of fledglings compared with control, although this difference was not statistically significant. (Abstract shortened by ProQuest.).