Revaluating brood reduction in cormorants and shags: only facultative brood reducers?

SVAGELJ, W. S.

Barcelona, España

Congreso; 31st Annual Meeting of the Waterbird Society; 2007

Waterbird Society, Universidad de Barcelona

Cormorants and shags (Phalacrocoracidae) are aquatic birds usually laying 3-4 eggs clutches but routinely producing broods of at least one-less chick compared to clutch size. For this family, Drummond (1987) proposed that facultative brood reduction must be common as an adaptation to uncertainty over hatching success, prey availability and individual reproductive skills, with non-aggressive feeding competition between siblings being the mechanism reducing broods. Although cormorants and shags are currently accepted as straight facultative brood reducers, detailed analysis concerning brood reduction in this group is lack. Using published and own data, I reviewed the existence and magnitude of brood reduction for 39 species encompassing Phalacrocoracidae family, assessing factors influencing siblings survival, and analyzing different hypotheses accounting for the adaptive value of brood reduction. When data exists, hatching asynchrony was the main factor favoring brood reduction. Last-hatched chicks were usually excluded from food access by elder siblings (with no aggression) when all eggs hatched. Although the reviewed information supported the widespread brood reduction strategy within the family, it also highlighted for the first time, the existence of three species exhibiting obligate brood reduction. Flightless Cormorant, Crozet Shag and Imperial Shag fulfilled the requirements to be considered as obligate brood reducers. Brood reductions degree in cormorants and shags appears to be a continuous from species exhibiting slight facultative systems to others showing rough obligate brood reduction. The insurance egg hypothesis (with last-laid eggs providing insurance against early failures of core eggs) seems the more plausible explanation of supernumerary clutch sizes in this group.