Thursday, October 08, 2009

Albatrosses Following an Orca

Via the Round Robin blog, I learned of this interesting study on Black-browed Albatrosses just published in PLoS ONE. Researchers mounted small cameras and sensors on the backs of four Black-browed Albatrosses (Thalassarche melanophrys) from Bird Island in South Georgia. The cameras recorded time-lapse still images that documented the birds' behavior as they foraged while raising their chicks, while sensors logged information about temperature and depth.

The cameras recorded 28,725 images, of which about half had a clear view of what the birds were doing. The most interesting showed several albatrosses following an orca (a.k.a., killer whale) that had just breached the surface (frame C in the composite image below).

Figure 1. Digital still images obtained from three cameras mounted on black-browed albatrosses. A: a ‘featureless’ sea, B: an iceberg encountered, C: a killer whale breaking the ocean surface, apparent from its dorsal fin (white arrow) and three black-browed albatrosses attracted to the whale, D: two albatrosses flying in association with the camera-mounted bird, E: a fisheries vessel in the distance (white arrow) with an aggregation of birds, F: a bright light source during the night, possibly a vessel or the moon. Image source: PLoS ONE.

The paper's authors explain the interaction as follows:
Black-browed albatrosses feed mainly on squid, fish and krill (reviewed in Xavier et al.), but the deep-water toothfish constitutes an important component of their diet in some breeding localities. Patagonian toothfish or other deep-water fish that occur in their diet could be available to shallow-diving black-browed albatrosses only through an interaction with deep-diving predators (from their food scraps) or with commercial fisheries (from offal or bycatch items). When killer whales feed on fish, fragments of prey are often left near the sea surface. These prey fragments could be an important food resource for albatrosses.
So it seems that albatrosses follow orcas for the same reason they follow ships. Birders who have attended or read about pelagic birding should be familiar with the practice of chumming – spreading food scraps in the water to attract birds and other creatures in search of an easy meal. Picking up the scraps left by orcas ought to be an easier feeding method than hunting live prey, especially when those hunts involve diving under water.

The PLoS ONE blog highlights two other bird-related papers published in the journal this week.