Albatross foraging behaviour: no evidence for dual foraging, and limited support for anticipatory regulation of provisioning at South Georgia

Abstract

Many pelagic seabirds are thought to regulate reproductive effort by adopting a dual foraging strategy, alternating or mixing short foraging trips over local shelf waters (maximising provi- sioning rates) with longer trips over distant oceanic water (allowing restoration of lost condition). Many species also respond to chick condition, decreasing food supply to over-fed, and sometimes increasing it to under-fed chicks. Analysis of tracking data from 4 albatross species breeding at South Georgia provided evidence that adults responded to prevailing environmental conditions, but did not provide evidence for a dual foraging strategy. Trip durations and maximum foraging ranges tended to follow a positively skewed, unimodal distribution, with the exception of the light-mantled albatross for which no significant modes were apparent. Individual distributions deviated from this, but none were strongly bimodal or showed regular alternation of trip lengths, trip distance or predominant bathy- metric regime. There were significant relationships between meal mass and trip duration, time since the last feed and chick condition on return, reflecting responses to current rather than predicted chick needs. On average, adults returned with smaller meals after 1 to 2 d trips, but otherwise stayed away until a threshold payload was obtained; consequently, provisioning rate (g d -1 ) was much greater after shorter trips. Lack of dual foraging may reflect the diversity of foraging zones available in this highly productive region. By inference, this would mean that adoption of dual foraging elsewhere is a con- sequence of greater heterogeneity in resource availability in waters surrounding those colonies.