THE INFLUENCE OF BROOD SIZE ON THE ENERGY METABOLISM AND WATER LOSS OF NESTLING GREAT TITS PARUS MAJOR MAJOR

At normal outdoor temperatures there is a distinct influence of brood size on the heat production of ten-day-old Great Tits. One ten-day-old nestling proved unable to maintain its body temperature at 12^°C. Two ten-day-old tits together in one nestbox at 12^Á°C were able to elevate the air temperature sufficiently to maintain homoiothermia. The same of course holds for tits in larger broods.
At an air temperature of 18^°C, six or seven ten-day-old tits placed in a nestbox elevated the air temperature to a level at which they almost reached a state of hyperthermia: their metabolism was at the basal level. The basal metabolism of a ten-day-old tit was found to be slightly more than 0–1800 kcal/h. The metabolism intensity of 12 tits in a nestbox at 12^°C was of the same order of magnitude.
Tits in broods comprising more than 12 or 13 nestlings at normal outdoor temperatures probably develop hyperthermia, which is unfavourable both for their energy and for their water balance.     

FACTORS GOVERNING FEEDING RATE, FOOD REQUIREMENT AND BROOD SIZE OF NESTLING GREAT TITS PARUS MAJOR

SUMMARY Observations were made on feeding rates and food-consumption of nestling Great Tits Parus major mainly in Larch plantations at lake Yamanaka, Japan. Feeding frequencies were recorded by an automatic recorder. There were marked differences between early and late broods; the feeding frequencies were twice as great in early than in late broods of the same size. No clear tendency was observed in the variations of feeding frequencies in relation to brood size. There was, however, a clear inverse relationship between the frequencies and the average size of food brought to the nests. The males' share in terms of feeding frequencies is described. These figures, however, did not follow the males' contribution in terms of weight of food, which was nearly always higher than the females'. It is pointed out that feeding frequencies are far too variable to be used as a true index of food consumption by nestlings, and are not reliable. Attempts were made to measure the weight of food; the method is described. The average weight of food brought by males was lighter in early than in later broods. The total weight of food was estimated. The trend of daily food consumption per chick was similar to that of the chick's growth curve. It was found that up to about the tenth day of the nestling period daily food-intake per chick increased linearly as body weight increased. At some nests, rate of defaecation was observed. This was at first low, but it increased steeply on the third day, with a steady increase thereafter. By comparing the rates of food intake, faeces output, and weight increment of a chick, it was found that only 20–30% of digested matter (the difference between food-intake and faeces-output was used up daily (for body temperature regulation various external effort, etc.). The factors responsible for this high efficiency of growth in nestlings are discussed. There was a clear inverse relationship between the total weight of food brought per chick per day and the brood size. This is largely because the heat-loss is greater in small than in large broods, so that a chick from a small brood in fact needs more energy to maintain its body temperature after a certain age than one from a large brood. This is discussed in detail. Factors which caused variations in size of food are discussed in relation to feeding frequencies. It is pointed out that, because of the inverse relationship between energy requirement by each chick and brood size, the total food requirement by a brood as a whole did not vary directly in proportion to the brood size. An estimation showed that a b/3 still required about 75% of the total food required by a b/8. A smaller brood is less advantageous than expected to parents feeding nestlings when they encounter adverse conditions, e.g. food shortage in the habitat, or a lack of help by their mates, etc. On the other hand, it is suggested that once they have left the nest, the food-demand by a brood of fledglings the parents have to feed, so that, in the fledging period, in times of food shortage it would certainly be advantageous to have fewer young. It is suggested that, although fledglings may consume three to four times as much food as nestlings, the parents, in providing this food, would not work proportionately harder, since the parents' efficiency of providing food could be higher in feeding the fledglings, which always follow the parents as they are hunting, than in feeding the nestlings to which food has to be brought. On this basis, the adaptive significance of the length of the nestling period in nidicolous species is discussed in relation to clutch size, brood size and food requirement.     

BODY SIZE, ITS HERITABILITY AND INFLUENCE ON JUVENILE SURVIVAL AMONG GREAT TITS, PARUS MAJOR

This paper reports on part of a three-year study into the biological significance of variation in body size of individuals from a single population of Great Tits in Wytham Woods near Oxford, from 1973 to 1976. It is a reasonable hypothesis that the body size of an individual is controlled partly by heredity and partly by environmental factors. Genetical factors by themselves are shown to account for nearly three-quarters of the total observed phenotypic variance of body size. Of the likely environmental factors considered, only the date of hatching (in 1973) and an annual difference in body size are shown to have significant effects.
Previous workers have shown that heavier fledglings have a better chance of surviving the immediate post-fledging period than lighter fledglings (e.g., Perrins 1965). It had been postulated that this differential survival rate might be due to variations in the fat reserves of fledglings. However, the data presented here suggest that this hypothesis is unlikely, and that the body size of fledglings has a more significant influence. The aggressive behaviour of fledgling Great Tits towards one another may be the mechanism responsible for this since it is shown that larger fledglings tend to dominate smaller fledglings.     

THE SELECTION OF TITS PARUS SPP. BY SPARROWHAWKS ACCIPITER NISUS

Selection of the Wytham Wood population of tits by Sparrowhawks during the tit post-fledging period was studied over three years. Within three weeks of the median date of tit fledging, juvenile Blue Tits were selected more frequently than juvenile Great Tits but thereafter juvenile Great Tits were selected more frequently; it is suggested that the former was due to greater vulnerability of Blue Tits in the two to three weeks after fledging, and the latter due to changes in availability due to immigration/emigration of juvenile tits. When juvenile tits killed by hawks were compared on the basis of brood and physical characteristics with their cohorts in the entire population and in those surviving to the following year, only one factor, fledging date, was found to have affected selection by hawks. It is believed that this was because hawks selected late fledged young still in family parties in preference to early fledged young which had already become independent of their parents and were foraging in the better cover of the forest canopy. When adults killed by hawks were compared with all adults available for selection on the basis of species, sex, timing of nesting and whether or not they had been born in the wood, the only selection trend found was that male Great Tits were taken more often than females due, possibly, to greater risk of exposure for males while foraging or during territorial behaviour. When compared with availability, juveniles were selected more frequently than adults in only one year, possibly a result of yearly differences in the number of prey available per hawk, the rate of non-predator related juvenile mortality or unknown hawk hunting strategies. The overall finding of relative non-selectivity by hawks was attributed to the surprise factor associated with the hunting methods used by Sparrowhawks in woodland.