The effect of egg size and laying date on the fledging mass of Thick—Billed Murre chicks was examined at Cape Hay, Bylot Island, and Coburg Island, Northwest Territories, Canada, to test the idea that late—laying females face a trade—off between the time of egg laying and egg size. There was a seasonal decline in egg volume, and egg volume was correlated with mass and size (wing length) of chicks soon after hatching. Partial correlations showed that larger eggs hatch into heavier, but not larger chicks. Chics hatched early in the season fledged at heaver masses than late—hatched chicks. Hatching mass contributed significantly to this effect, but other factors (e.g., energy intake) were probably also important. A model, based on a multiple regression of the effects of hatching mass and hatching date on fledging mass, shows the possible combinations of egg size and timing of breeding while maintaining a constant fledging mass. Data on the replacement interval and size of replacement eggs suggest that females that lay late may do better (in terms of fledging mass of their chick) to minimize the delay in laying by producing a small egg earlier, rather than delay laying further in order to produce a larger egg. This is because they are unable to accumulate nutrients at a fast enough rate to make a delay worthwhile. This suggests that the seasonal decline in egg size is an adaptive response to a seasonal deterioration in environmental conditions.
The breeding success of puffins (Fratercula arctica) in different habitats was studied on Great Island~ Newfoundland. A . multiple regression analysis showed that puffin burrow density was negatively correlated with distance from the cliff edge~ and positively correlated with angle of slope.The biological significance of these correlations was found to be that close to the cliff edge, where the angle of slope is steep, breeding success is significantly higher than on adjacent level habitat. The reasons for the difference in breeding success are~ firstly, eggs and chicks are more exposed to gull predation on level habitat, and secondly, parents are more vulnerable to robbing by gulls when taking food to chicks on level areas than when taking food to chicks on slope habitat. Thus, during the breeding season, natural selection acts more strongly against birds nesting on level habitat away from the cliff edge than against those nesting close to the cliff edge.In the absence of gull Interference puffins showed higher breeding success. A model is presented which relates the distribution of puffin nests to habitat characteristics in the presence and absence of gulls .
Incidental catches of 12,243 Common Murres (Uria aalge), 875 Atlantic Puffins (Fratercula arctica), 36 Black Guillemots (Cepphus grylle), and 9 Razorbills (Alca torda) were recorded off Newfoundland during the summers of 1980-1982 (26,445 net-days of fishing effort). Most catch occurred in stationary gill nets set on the sea floor at .depths of up to 180 m and revealed that murres, Razorbills, puffins, and guillemots can dive to depths of at least 180, 120, 60, and 50 m, respectively. Diving ability appeared to be directly correlated with body size.
A simultaneous comparison of the foods of Common Murre, Uria aalge, and Thick-billed Murre, Uria lomvia, chicks at the same location showed that the species composition of fish fed to chicks of the two murre species differed significantly. In both years of the study, Common Murre chicks were fed predominantly capelin, Mallotus villosus (78–80% by weight), whereas Thick-billed Murre chicks were fed mainly daubed shanny, Lumpenus maculatus (61–70% by weight). Thick-billed Murres tended to feed their chicks more frequently than Common Murres, probably because in both years the mean caloric value of their prey was lower than that of the Common Murre's. No other statistically significant interspecific differences were consistent between years. Prey lengths and weights overlapped considerably between the murre species, and the caloric intake of chicks showed no consistent interspecific difference. Marked interyear differences in feeding rate, prey size, and caloric intake occurred in both species. In 1982 ice breakup was late, and the murres' breeding seasons were delayed; this appeared to result in poor synchronization between the temporal pattern of food availability and the timing of each species' chick-rearing period, and the caloric intakes of chicks of both species were lower in that year than in 1983.
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