| The requirement of an organism to simultaneously balance energy and material flux limits how it can interact with its environment. Most studies of physiological constraints on an organism's ecology have concentrated on the importance of energy limitations, while ignoring the interacting balances of water, ions, and a variety of nutrients. In this study I chose to evaluate the importance of the interacting balances of energy, water, and salt during the stressful reproductive period of marine birds. To demonstrate the general importance of this approach, I studied chicks of Leach's storm-petrels, Oceanodroma leucorhoa, a temperate species, and adults of several tropical species. Storm-petrel chicks had extremely low rates of water influx because they only received water in the lipid rich meals delivered by their parents. These meals were also low in salt and hence meal supplements of an isotonic salt solution increased the growth rate of chicks while solutions either high or low in salt decreased their growth.;Another water balance difficulty is faced by adult brown noddies, Anous stolidus, sooty terns, Sterna fuscata, and wedge-tailed shearwaters, Puffinus pacificus, incubating eggs in the hot microclimates of the Hawaiian Islands. Nesting success of both tern species was high at unshaded sites, but all unshaded shearwater nests failed. Terns were able to maintain energy and water balance by elevating their bodies into a faster air stream, making short trips to drink seawater, and perhaps by beginning their incubation shifts with more water in their stomachs. Shearwaters could not stand above their eggs, because of the anatomical position of their legs, did not make frequent trips for water, and probably had less water in their stomachs due to stomach oil formation. Because 25-30% of the shearwater population nests at sites where no offspring are produced, the growth of the population is significantly reduced. To produce a compact energy rich meal, adult storm-petrels and shearwaters reduce the salt and water present in their stomach contents. This may lead to water and salt limitations for chicks of storm-petrels, and water shortages for shearwaters incubating in hot microclimates. |
