Bird Densities At

Attempts to understand the relationship between oceanic birds and their prey at sea have been hastened by the need for information on human krill harvesting

(Knox, 1984) and possible oil and mineral exploitation in the Ross Sea and other basins adjacent to Antarctica. One facet of this endeavour is to obtain an index of seabird biomass.

With hindsight, it was inevitable that field observers might adopt their own approach to counting birds at sea. This made comparative analyses extremely difficult, if not impossible, to achieve with any degree of reliability. Tasker et al. (1984) have been singularly helpful to future studies by reviewing the various methods used to count seabirds from ships and proposing a standardized approach.

Problems in assessing bird numbers at sea result from several factors. Some species are attracted to ships, others avoid them. Rafts of birds resting or feeding from the surface can be invisible in a moderate swell to observers even close-by. The greatest problem, however, is surely the patchy nature of bird distributions at sea. In early March and April, one can search for days in the southeastern Pacific Ocean without seeing a single bird (P.C. Harper, pers. observ.).

Birds are highly mobile, with energy requirements related to their flying efficiency and body size. Abrams (1985) estimated that to sustain continual flight, albatrosses require between 85 g and 160 g of squid daily. Hence, with a meal of 650 g (Prince, 1980) a relatively small albatross could forage for six days, and a Wandering Albatross for 3-4 days before losing body mass. Prions, on the other hand, use 67 g of food per day for flight alone, which is over 50% of their daily energy expenditure. At 12 g per meal, they require 5-6 meals per day, or a meal every 4-5 hours, just for flight.

Such information suggests that smaller species of procellariids need to be skilled at finding plankton swarms, especially at night when the vertical migrations of krill bring these organisms to the sea surface. It follows that a ship might fortuitously steam through most of the plankton swarms with their attendant birds, or miss them entirely. Either way, the resultant data will give an erroneous picture of the biomass of birds in any particular area. Certainly, convergence zones and places of regular upwelling such as continental margin waters may seasonally generate reliable plankton stocks which may in turn routinely attract birds. However, it is our belief that, because the Antarctic Sector of the Pacific is a deep, remote ocean where there are few foci on which productive upwelling waters can develop with any predictability, any such foci are probably ephemeral and are not the consistent food resources that birds require. In consequence, Pacific birds must range widely over their ocean in search of food and this may explain why large flocks of birds, so common in the Scotia Sea, are comparatively rare in the Pacific.

0 0

Post a comment