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Influence of brood-size on reproductive success of two species of cormorant, Phalacrocorax auritus & P. pelagicus, and its relation to the problem of clutch-size. Robertson, Ian

Abstract

Brood-rearing capabilities and their relation to the problem of clutch-size were investigated in two cormorant species, Phalacrocorax auritus and P. pelagicus, at Mandarte Island, British Columbia. Extra chicks were added to normal sized broods (1 to 4 chicks) so that the range of brood-sizes studied was 1 to 8 in Double-crests (auritus) and 1 to 7 in Pelagics (pelagicus). Fledging success in Double-crests is generally 90 per cent or higher in broods of up to 6 chicks. In Pelagics fledging success drops sharply below 90 per cent in broods greater than 4. In no Pelagic nest did more than 4 chicks fledge, whereas up to 7 chicks fledged in Double-crest nests. Growth rates in both species are lower in supernormal broods. However, this decline amounts to only 12 per cent in Double-crest broods. It is 35 per cent in Pelagic broods. There is a strong relationship between mortality and the low growth rates of certain Pelagic chicks in supernormal broods. Slower growth in large broods is reflected in a fledging period which is longer by several days in both species. The return to Mandarte Island of yearling Double-crests (in 1970) indicates a post-fledging mortality of approximately 50 per cent in normal broods and 47 per cent in supernormal broods, although the latter sample includes only one brood of 7. This slight survivorship difference is not enough to offset the reproductive advantage of large broods. Observation of feeding rates in both species indicates an increase in feeding trips with increasing brood-size which is less than proportional. As this does not appear to be enough to explain the ability of parent Double-crests to feed supernormal broods it was hypothesized that these parents return with more food. Several other aspects of feeding frequency were examined and found consistent with this hypothesis. Study of the success of individual feedings indicated no tendency for a higher success level in normal broods. The strain on parents raising large broods in both species was shown by the decreasing nest-site attendance as broods get larger and as the chicks grow older. The high brood-rearing capabilities of the Double-crests were discussed and two problems on which there was little or no data were raised. It was concluded that Double-crest parents do not endanger their future survivorship in order to raise supernormal broods, although data is poor on this question. A possible breakdown in the feeding of broods greater than 5 may occur under certain conditions. The differing brood-rearing capabilities of the two species were tentatively explained in terms of their population status, i.e., growing versus stable, with the Double-crests the growing population. This idea corresponds with one raised by Lack (1965) who has suggested a correlation between higher than normal brood-rearing capabilities and a growing population. As such, the findings are consistent with Lack's clutch-size hypothesis. The occurrence of most of the nestling mortality in the first two weeks of the nestling period suggest factors other than food may limit brood-rearing success.

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