Disentangling brood reduction in the Imperial Shag (Phalacrocorax atriceps)

SVAGELJ, W. S.; QUINTANA, F.

Veracruz, México

Congreso; 4th North American Ornithological Congress; 2006

Ornithological Societies of North America

Brood reduction usually occurs when parents lay a larger number of eggs than offspring they can actually raise. Applying survival time analysis, we analyzed breeding biology, chick survival and brood reduction on 329 nests of the Imperial Shag Phalacrocorax atriceps at the Punta León colony, Patagonia, Argentina, during 2004 and 2005 breeding seasons. Mean clutch size was 2,82 ± 0,02 eggs, with three eggs being the modal clutch size (81% of nests). Hatching order, hatching delay, laying date, and egg weight affected nestling survival time, while number of hatchlings in the nest and parental condition did not. Resource-tracking Hypothesis was not supported because Imperial Shags were obligate brood reducers. None of 128 nests with three hatchlings fledge three chicks. Death of third-chicks (C) occurs mainly during the first five days after hatching due to starvation, C-chick survival only occurring when one of the elder siblings died. Only 11% of C-eggs laid became fledglings. Offspring Facilitation Hypothesis was not supported, because presence of C-chick in the nest did not improve either first-chick (A) or second-chick (B) survival. However, our findings supported the Insurance Egg Hypothesis because three-egg nests produced more fledglings than those nests with only two eggs.