What grounds some birds for life? Movement and diving in the sexually dimorphic galapagos cormorant
Wilson RP., Vargas FH., Steinfurth A., Riordan P., Ropert-Coudert Y., Macdonald DW.
Flightlessness in previously volant birds is taxonomically widespread and thought to occur when the costs of having a functional flight apparatus outweigh the benefits. Loss of the ability to fly relaxes body mass constraints which can be particularly advantageous in divers, because underwater performance correlates with mass. The Galápagos Cormorant Phalacrocorax harrisi is flightless and the largest of its 27-member genus. Here, the loss of flight, and consequent reduced foraging range, could be compensated by enhanced dive performance. Over three years, 46 Galápagos Cormorants were successfully equipped with time-depth-temperature recorders, and 30 birds with GPS recorders during the breeding season. Birds foraged at a mean of 690 m from the nest and just 230 m from the nearest coast, confirming an extremely limited foraging range during the breeding season and corresponding increased potential for intraspecific competition. Although the maximum recorded dive depth of 73 m tallied with the species body mass, >90% of dives were conducted in water <15 m deep. The heavier males foraged in different areas and dived longer and deeper than females, which exposed males to colder water. Consideration of how plumage insulation decreases with depth indicates that diving males should lose 30% more heat than females, although this may be partially compensated by their lower surface area : volume ratio. A simple model highlights how energy expenditure from swimming underwater due to buoyancy and energy lost as heat have opposing trends with increasing depth, leading to the prediction of an optimum foraging depth defined by the volume of plumage air and water temperature. This has ramifications for all diving seabirds. It is proposed that the reduction in wing size, together with energy-expensive flight musculature, allows the Galápagos Cormorant to be more efficient at shallow depths than other seabirds, but only in warm equatorial waters. The high prey density and predictability of benthic prey in defined areas of the Galápagos can be particularly well exploited by this flightless species, with its limited foraging range, but the Galápagos Cormorant is unlikely to be able to accommodate much change in environmental conditions. © 2008 by the Ecological Society of America.