| In the eastern tropical Pacific, an expanse of ocean that includes more than 20 million km{dollar}sp2,{dollar} there exists a speciose group of seabirds that feeds in flocks (averaging 36 individuals and 2.2 species per flock) and is composed of a total of 90 species from 27 genera. This group of seabirds is largely dependent for successful feeding upon schools of subsurface, predatory dolphins and tunas, which trap prey fishes at the surface. Within this guild are three distinct flock types which differ significantly in species composition, size, diversity and distribution. The distribution of each flock type corresponds to a gradient in thermocline depth, which itself reflects surface water productivity.; I hypothesize that productivity of the water over which seabirds forage may affect species composition of feeding flocks through the energetic cost of locomotion. Species with a high cost of transport will be restricted to areas of high productivity, where prey patches are abundant, while species with a lower cost of transport will be able to forage in areas of lower productivity where prey patches may be less abundant or farther apart. This hypothesis predicts a number of things about the energetics of flight, wing morphology and flight behavior, which are tested for one of the most abundant flocking species, the Red-footed Booby.; The power required for flight for an adult Red-footed Booby with a mass of 1014 g was 68.1 kJ h{dollar}sp{lcub}-1{rcub},{dollar} more that 3 times lower than the power predicted from equations based on aerodynamic theory. Cost of transport averaged 1.5 kJ km{dollar}sp{lcub}-1{rcub}{dollar}, a value 36% of that predicted. These differences are likely due to the fact that Red-footed Boobies use the wind as an energy source during flight, apparently resulting in energy savings over continuous flapping flight that are striking, and may also result from subtle characteristics of wing morphology.; On a global scale, seabird communities show regular and predictable changes in species composition which depend greatly on productivity of the ecosystem and on flight capabilities of the birds. This research indicates that energetics may be an important factor in species composition and distribution of seabirds within a single guild, in a single ocean system. |
