Food resource use and responses to changes in prey availability of common and thick-billed murres (Uria aalgae and U. lomvia) breeding at the Gannet Islands, Labrador

Bryant, Rachel L. (1998) Food resource use and responses to changes in prey availability of common and thick-billed murres (Uria aalgae and U. lomvia) breeding at the Gannet Islands, Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

This work focused on two sets of comparisons: the first was an inter year comparison of the chick diets and related breeding parameters of Common and Thick-billed Murres (Una aalge and U. lomvia) breeding at the Gannet Islands, Labrador before and after a local decline in capelin abundance. The second was an interspecies comparison of these parameters. In 1996 and 1997, years of locally low capelin abundance, I measured murre chick diet composition , feeding rates, breeding success, chick growth, adult masses, maximum dive depths and off-duty pair members' time spent at site. I then compared these data with those gathered by previous researchers in years when capelin were more abundant (1981, 1982 and 1983). After the decline, murres fed their chicks up to 75 percent less capelin and up to 65 per cent more daubed shannies than they had fifteen years previously. Feeding rates of both species varied by up to 250 per cent among the five years, but this variation was not concomitant with changes in the proportion of capelin in the chicks' diets. The murres' dietary shift did not seem to affect their colony attendance adversely; counts of both murres on some plots increased, while on others they did not differ significantly. Neither murre species had significantly different breeding success among years. Chicks of both species grew well following the decline in capelin availability. Masses of adult murres were not significantly different after capelin became scarce. No time-at-site data were taken before the decline in capelin abundance, but after the decline, members of chick rearing Common and Thick-billed Murre pairs both spent a mean of ten minutes together at their sites per feeding visit. Results from the interspecies comparison suggest that chick food resource partitioning might have been negligible between Common and Thick-billed Murres at the Gannet Islands in 1996 and 1997. In both years, the murres' chick rearing periods overlapped almost exactly. In the year they were both measured, the sizes of the principal item in their chicks' diets did not differ significantly. During one of two aU- day feeding watches, the murres’ chick feeding peaks were concurrent, but during the other they were not. In the year dive depths of both species were measured, Common and Thick-billed Murres dove to similar maximum depths.

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