S07.Summary: Energetics and the ecology of bird species

1Department of Biology, University of California, Los Angeles, California, 90095 1606, USA, fax 1 310  825 9433, e-mail kennagy@biology.ucla.edu; ²Department of Vertebrate Zoology and General Ecology, Moscow State University, Moscow 119889, Russia, e-mail valery@VGavrilov.home.bio.msu.ru

Correlations, of course, may be indications of cause and effect, but by themselves, they do not establish cause and effect. For example, assume that one studies a species of seagull, and finds that maximum aerobic metabolic rate increases linearly with increasing clutch size in females. Does this mean that higher maximum metabolic rates cause or allow the female to produce more eggs per clutch? Or, does this mean that females that lay more eggs develop higher maximum metabolic rates as a result? Without more information, it is not possible to determine if one property determines the other, or if both are determined by a third, unmeasured variable. However, it is possible to do experimental manipulations in the field that can tease out cause from effect. The larger goal is to uncover evolutionary pathways that have occurred among birds’ physiologies and their ecologies.

The paper by Valery Gavrilov is a summary of many years of thought and research on energy budgets of passerine and non-passerine birds. Maximum and minimum metabolic rates differ in a predictable fashion between these taxonomic groups, with passerines processing energy about 30% faster than do similar-sized non-passerine birds. Thus, the energy available for thermoregulation, reproduction and other functions is greater in the passerine, when compared at a given body mass. These differences may explain differences in latitudinal distribution, reproductive parameters, moulting events, diet and habitat selection. When these energetic differences are analyzed in relation to variation in body mass between species, possible explanations emerge for differences in reproductive and other parameters between small and large birds. The argument here is that energetic parameters determine ecological properties, and that evolutionary processes have operated on the basis of these parameters to result in the species distributions seen today. Experimental tests of these hypotheses remain to be done.

In the second paper, one facet of Gavrilov’s energetics model is given closer scrutiny. Anvar Kerimov and Elena Ivankina looked for a correlation between an energetic parameter (BMR) and important performance parameters (social and reproductive status) in order to evaluate the relationships between energetics and fitness. They studied two small forest-dwelling birds near Moscow, and determined their territoriality and reproductive status in relation to age and experience, and measured BMRs overnight in birds held captive temporarily. For Great Tits, newly resident males that obtained territories had higher BMRs than those that did not get territories, especially among young males. It is suggested, but not demonstrated yet, that BMR may determine the degree of territorial and reproductive attributes in this species.

The third paper includes tests for correlations between FMR, BMR and reproductive behaviour in a small passerine bird, and reports the first field measurements of reproductive effort for male birds. Ken Nagy joined Valery Gavrilov, Anvar Kerimov and Elena Ivankina at their study site near Moscow to add doubly labelled water measurements of FMR to their ongoing studies of Great Tits. FMR correlated positively with territorial status in males, indicating that the cost of territoriality increased FMR by about 45%. In this small sample, BMR did not correlate with residency times, but the FMR/BMR ratio did, such that FMR increased from 2.6 to 3.7 times BMR in non-territorial vs. territorial males. Cause and effect was not established in this study.

In a field study of reproduction in Yellow-eyed Juncos, Kim Sullivan, Jay Roper and Wes Weathers looked for relationships among aspects of nest building, nest success, fledgling and adult survival, and FMR of females. They found similar results in their females as did Kerimov and Ivankina in their male Great Tits: experience makes a big difference. Experienced female Juncos (those that bred in multiple years) renested faster, started more nests each year, and produced more surviving fledglings than did inexperienced females. Moreover, experienced females did all this with lower FMRs than inexperienced females. Experience may not be the direct result of age, but may reflect variation in resource availability or use by individual females, or inherent genetic variation among individuals. The source of variation between females, and the cause and effect sequence of events were not determined in this study, but suggestions regarding the ecological benefits of variation are given.

Finally, David Bryant offers an essay on the connection between energy metabolism and longevity in birds. His eight reasons that energy metabolism should influence longevity all suggest that high rates of energy expenditure should lead to (cause) shorter lives. A variety of causes of increased FMR are discussed, including higher activity costs due to greater predation or competition, extra costs of thermoregulation, more rapid growth, or combating diseases or parasites. Supporting evidence is clearly separated into correlative and experimental evidence. Several correlative studies support the hypothesis that energy expenditure and life span are inversely related. Two experimental studies offer good experimental support, and other studies involving experimental manipulations in the field illustrate ways that cause and effect relationships may be established. A number of exciting approaches to answering cause and effect questions are described, so the young ornithologist should find rewarding ideas for future research.