Hitting the buffers: conspecific aggression undermines benefits of colonial breeding under adverse conditions

Colonial breeding in birds is widely considered
to benefit individuals through enhanced protection
against predators or transfer of information
about foraging sites. This view, however, is
largely based on studies of seabirds carried out
under favourable conditions. Recent breeding
failures at many seabird colonies in the UK
provide an opportunity to re-examine costs and
benefits of coloniality under adverse conditions.
Common guillemots Uria aalge are highly colonial
cliff-nesting seabirds with very flexible
parental care. Although the single chick is
normally never left alone, more than 50 per cent
of offspring were left unattended at a North Sea
colony in 2007, apparently because poor conditions
forced both parents to forage simultaneously.
Contrary to expectation, unattended
chicks were not killed by avian predators.
Rather, although non-breeders and failed breeders
sometimes provided alloparental care, unattended
chicks were frequently attacked by
breeding guillemots at neighbouring sites, often
with fatal consequences. These results highlight
a previously unsuspected trade-off between provisioning
chicks and avoiding conspecific
attacks, and indicate that understanding how
environmental conditions affect social dynamics
is crucial to interpreting costs and benefits of
colonial breeding.