Disentangling brood reduction in the Imperial Shag (Phalacrocórax atriceps)

SVAGELJ, W.; QUINTANA, F.

Veracruz, Mexico.

Congreso; IV North American Ornithological Congress; 2006

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.