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Bioenergetics of growth in the Pigeon Guillemot, Cepphus columba Koelink, Anthony Francis


The energetics of growth and feeding of the chicks and their relation to the problem of evolution of brood size were investigated in the alcid Cepphus columba, the Pigeon Guillemot, a colonial species nesting at Mandarte Island, British Columbia. The study involved the naturally occurring broods of one and two chicks, but to elucidate information on the evolution of brood-size, experimentally induced broods of three were created. Following the technique of Royama [1966], film records show an "optimal working capacity" for the parent Pigeon Guillemots at Mandarte Island of a total daily fish ration of 200 grams. In no case were parents observed to bring more than this amount when means were computed over 5-day periods. The resulting limitation expresses itself in a depressed growth rate in artificially induced three-chick broods in all seasons, and in twin broods in certain years. The parent does not endanger its own survival in favour of its brood, but maintains body weight in all circumstances. Each parent requires for itself 90 grams of food daily or 20% of its own weight. The "optimal working capacity" of each parent is about 100 grams of food delivered to the nest or 22% of the parental body weight. Subsequently when feeding the nestlings the parent doubles the rate of fishing. A procedure for whole carcass analysis on 27 Pigeon Guillemot chicks and 2 adults is described to determine; cooling rate for calculation of heat production, surface area to calculate thermal conductance, gross fat, ash, moisture, crude fat and heat of combustion. The lean dry featherless biomass of 8 nestlings of various ages had an average energy content of 4.75 Kcal./gr., and using this figure as well as assuming a combustion value of 9 Kcal./gr. for fat and 5 Kcal./gr. for plumage, the average caloric value for live weight was calculated at 1.92 Kcal./gr. with a range of 1.34 Kcal./gr. at hatching to 2.65 Kcal./gr. at the time of fledging. The adult Pigeon Guillemot had an average caloric density of 2.56 Kcal./gr. live weight. The change in the live weight caloric density is related to the deposition of fat which occurs between the 11th and 21st day following hatching and at fledging is in excess of the fat depot found in adults. Growth in body weight, i.e. in absolute terms as daily increment in grams, reaches a maximum between the 8th and 15th following hatching, whereas the relative rate of growth, i.e. the daily increment with respect to the nestlings weight, reaches a maximum about the 4th day following hatching. The conversion rate for the entire fledging period of chicks in the field is 2400 grams of fish per chick for an increment of 375 grams in body weight or 2800 Kcal. for an increment of 1001 Kcal. body weight gain (=35%). Consumption of fish for the two chicks hand-raised successfully at camp is 3400 grams for a conversion rate of 25%, and compares well with results of similar experiments by other investigators. The maintenance costs of the chick are kept low by keeping energy losses to a minimum so that a large share of the energy input is allowed for growth, a process of overriding importance in the Pigeon Guillemot chick (Figure 19). The findings on fledging success in the Pigeon Guillemot (summarized in Table 3b) are consistent with Lack's hypothesis on the evolution of brood-size in that two-chick broods represent the limit that the parents can raise successfully. Overall success in the artificially induced three-chick broods is poor compared with the normal two-chick broods in that they fledge at an older age (37.3 to 33.9) , their weight at fledging is lower (385 to 407 grams) which may impair their post-fledging survival, and the average number of chicks produced from triplet broods is only marginally greater than from twin broods (1.9 to 1.8 chicks per nest).

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