Avian Life History Research Articles

Evolution of avian life histories - does nest predation explain it all?

Evolution of avian life histories - does nest predation explain it all?

The Evolution of Avian Senescence Patterns: Implications for Understanding Primary Aging Processes

SYNOPSIS. The long life spans of birds relative to those of mammals are intriguing to biogerontologists, particularly in light of birds' high body temperatures, high blood glucose levels, and high metabolic rates—all of which should theoretically increase their biochemical liability for rapid aging. The comparative longevity of birds and other flying homeotherms is consistent with evolutionary senescence theory, which posits that species with low mortality rates from predation or accident will be released from selection for rapid maturity and early reproduction, and will exhibit retarded aging. Comparative analyses of avian life history parameters to date, although not as extensive as those for mammals, broadly support an association between low mortality rates, slow reproduction, and long lifespan. The diversity of bird life histories suggests the importance of developing a diversity of avian models for studies of aging mechanisms, both proximate and ultimate, and for using wild as well as domestic representatives. Birds studied in the laboratory thus far show many of the same manifestations of aging as mammals, including humans, and many ornithologists are beginning to document actuarial evidence consistent with aging in their study populations. We encourage greater communication and collaboration among comparative gerontologists and ornithologists, in the hope that the study of aging in birds will lead to an integrated understanding of physiological aging processes well grounded in an evolutionary paradigm.

Avian Life History Evolution in Relation to Nest Sites, Nest Predation, and Food

Food limitation is generally thought to underlie much of the variation in life history traits of birds. I examined variation and covariation of life history traits of 123 North American Passeriformes and Piciformes in relation to nest sites, nest predation, and foraging sites to examine the possible roles of these ecological factors in life history evolution of birds. Annual fecundity was strongly inversely related to adult survival, even when phylogenetic effects were controlled. Only a little of the variation in fecundity and survival was related to foraging sites, whereas these traits varied strongly among nest sites. Interspecific differences in nest predation were correlated with much of the variation in life history traits among nest sites, although energy trade—offs with covarying traits also may account for some variation. For example, increased nest predation is associated with a shortened nestling period and both are associated with more broods per year, but number of broods is inversely correlated with clutch size, possibly due to an energy trade—off. Number of broods was much more strongly correlated with annual fecundity and adult survival among species than was clutch size, suggesting that clutch size may not be the primary fecundity trait on which selection is acting. Ultimately, food limitation may cause trade—offs between annual fecundity and adult survival, but differences among species in fecundity and adult survival may not be explained by differences in food abundance and instead represent differing tactics for partitioning similar levels of food limitation. Variation in fecundity and adult survival is more clearly organized by nest sites and more closely correlated with nest predation; species that use nest sites with greater nest predation have shorter nestling periods and more broods,yielding higher fecundity, which in turn is associated with reduced adult survival. Fecundity also varied with migratory tendencies; short—distance migrants had more broods and greater fecundity than did neotropical migrants and residents using similar nest sites. However, migratory tendencies and habitat use were confounded, making separation of these two effects difficult. Nonetheless, the conventional view that neotropical migrants have fewer broods than residents was not supported when nest site effects were controlled.

Do Variable Environments Significantly Influence Optimal Reproductive Effort in Birds?

It is often assumed that environmental variability strongly influences the evolution of life-history strategies, principally by determining the optimal level of reproductive effort, and that differences in life histories between populations and species represent fundamental differences in reproductive effort. In this paper, we use computer simulations and simple analytical approaches to demonstrate that even large amounts of temporal variation in either fecundity or survival have relatively little influence on optimal reproductive effort for generalized avian life histories. We suggest that demographic differences among populations or closely related species are not likely to result from differences in the optimization of reproductive effort to environments with different levels of variability alone, but may reflect instead differences in average values of fecundity and survival

Testosterone and Avian Life Histories: Effects of Experimentally Elevated Testosterone on Behavior and Correlates of Fitness in the Dark-Eyed Junco (Junco hyemalis)

Hormones influence many aspects of organismal behavior, physiology, and morphol- ogy, and thus hormones may lie at the root of many life-history trade-offs. By manipulating hormones we can create novel phenotypes (i.e., perform phenotypic engineering) and attempt to relate phenotypic variation to fitness. We report the effect of testosterone treatment on parental behavior and vocal behavior of adult male dark-eyed juncos. Testosterone partially suppressed paternal behavior and increased the frequency of song. When we compared treated males and controls for nine potential correlates of fitness (offspring growth and survival to the age of 10 d, condition of females, length of the interval between consecutive nestings, size of subsequent clutches and broods, mate retention within and between breeding seasons, and survival rate), we found no statistical differences. In some measures treated males outperformed controls, but in most the reverse was true. The power of some of our tests was not great enough to detect small differences. At this stage of our investigation, three interpretations of our results seem almost equally probable: (1) a broad range of behavioral phenotypes is selectively neutral in the junco, (2) male parental behavior is beneficial to males only in some years or habitats, or (3) we have yet to measure the correlates of fitness that are most strongly affected by the behavioral changes induced by elevated testosterone.

Testosterone and Avian Life Histories: Effects of Experimentally Elevated Testosterone on Prebasic Molt and Survival in Male Dark-Eyed Juncos

Male Dark-eyed Juncos (Junco hyemalis) that breed in Virginia begin their prebasic molt after breeding has ended, usually in August. Almost all males caught in late October have completed the molt. In 1989, we obtained anecdotal evidence that males whose testosterone (T) we maintained at artificially elevated levels beyond the end of the breeding season postponed or suppressed prebasic molt. To test the effect of T experimentally, in spring 1990 we implanted some males (T-males) with testosterone and others (Cmales) with empty implants, and we released both groups to breed. We caught some members of both treatment groups in October and removed their implants. The T-males had delayed their prebasic molt, while the C-males were molting on schedule. Other implanted Tand C-males were not caught; these carried their implants into winter. Next spring we examined surviving males whose implants we had removed in October as well as males whose Tand C-implants had not been removed. T-males whose implants we had removed had molted completely, despite their delayed start, whereas T-males whose implants we had not removed had not molted. Still-implanted C-males had molted. We compared the minimum overwinter survival (i.e., return rates in spring) of the treatment groups. Tand C-males whose implants we had removed in October returned at the same rate, but among males whose implants we had not removed, significantly fewer T-males than C-males returned. The transition between reproduction and molt of male juncos apparently can be blocked by preventing the normal seasonal decline in T. This suggests a physiological basis for a possible trade-off between time allocated to reproduction and time allocated to molt. Our results indicate that males could maintain high T and prolong breeding, possibly into October, and still molt completely with no adverse effects. We consider why such a modification of schedule has not occurred. However, postponement of molt beyond some date in autumn, possibly late October, suppresses it altogether, as indicated by the failure to molt of the returning T-males whose implants we did not remove. This treatment group apparently suffered higher overwinter mortality, and we consider possible reasons.

Testosterone and avian life histories: The effect of experimentally elevated testosterone on corticosterone and body mass in dark-eyed juncos

To assess whether alterations in the normal pattern of testosterone (T) secretion might be beneficial or detrimental, we studied a breeding population of darkeyed juncos in which we elevated T experimentally and measured its effect on potential correlates of fitness. We treated both free-living and captive males with implants that were either empty (C-males, controls) or packed with T (T-males, experimentals). Timing of implant varied and was designed to mimic natural peak breeding levels except that peaks were either prolonged or premature. We bled the birds at recapture and analyzed their plasma, and that of their female mates, for T and corticosterone (B). We also measured body mass and fat score in free-living T- and C-males. In the field, T-implants elevated T and kept it elevated for at least a month. Experimental males also had higher B than controls. In captives, the effect of the implants on plasma T was detectable within 24 hr. B in captive T-males was again higher than in captive C-males. In females, neither T nor B differed between mates of T- and C-males. T-males implanted in early spring lost more mass between implant and recapture in late spring than did controls and also had lower fat scores when recaptured. When implants were inserted in summer, treatment did not influence mass. Elevated T in early spring apparently hastened the transition from the winter to the breeding mode of fat storage. We suggest that prolonged elevation of testosterone might be selected against because of the association between T and B. Premature elevation of T might be costly because of the resultant loss of mass and fat reserves, which could lead to mortality when spring snowstorms prevent access to food.

Nest predators and the evolution of avian reproductive strategies: a comparison of Australian and New Zealand birds.

Nest predation has been considered an important factor in the evolution of avian life histories: smaller clutches and shorter incubation and nestling periods are expected where nest predation has significant effects on reproductive success. Unlike the Australian avifauna, terrestrial New Zealand birds have evolved in the absence of reptilian and mammalian predators. Here we compare the reproductive strategies of terrestrial native New Zealand birds with those of their Australian sister taxa. In 11 of 14 comparisons, New Zealand birds were larger than their Australian relatives, but we did not find any significant differences in reproductive tactics between the two regions, a result inconsistent with the nest predation hypothesis. We discuss several reasons why this may be so. One possibility is that selection imposed on avian life history tactics by mammalian predators following the arrival of humans in New Zealand has led to strategies similar to those adopted in Australia.

Growth‐Mortality Trade‐Offs and the Evolution of Juvenile Life Histories in the Alcidae

The juvenile life history of each of the 22 living species in the avian family Alcidae is composed of a period in the nest followed by a move to the ocean (fledging), where development is completed. The age, degree of development, and the mass of the alcid chick at fledging are extraordinarily variable, both between and within species. The available information suggests that nests are safe places (the daily mortality of chicks there is low) relative to the ocean, but that growth in the nest is slow relative to that attainable at sea, because of the parental travel time between the nest and feeding areas. The chick therefore faces a trade—off in deciding when to make the transition from nest to ocean. A dynamic programming model incorporating (1) the differential growth and mortality in the nest and ocean habitats, and (2) assumptions about the relation between size and survival, accurately predicts the fledging age and mass of the Common Murre (the only species for which reasonable estimates are available). The model suggests explanations for some other phenomena associated with the reproduction of alcids and other northern seabirds, namely the seasonal decline in mass at fledging, and the negative relation of mean fledging age and mass observed between colonies in several species. The optimal fledging policy is also considered from the point of view of the parents. There are many indications that visits to the nest endanger the parents' lives, and hence parents should be @`reluctant" provisioners, leading to conflict between parents and their offspring. This evolutionary conflict and its implications forour understanding of avian life histories are discussed.

In Memoriam: Samuel Charles Kendeigh

Ornithology lost an eminent scientist with the passing of Dr. S. Charles Kendeigh, professor emeritus of zoology at the University of Illinois, at Urbana, Illinois, on 13 November 1986. Burial was in South Amherst, Ohio. Dr. Kendeigh was born on 18 December 1904 at South Amherst, Ohio. He attended Oberlin College, where he received B.A. (1926) and M.A. (1927) degrees, and the University of Illinois, where he received a doctorate (1930). He taught at Western Reserve University from 1930 to 1936, when he joined the faculty at the University of Illinois. From 1925 to 1939 he was a research associate for the Baldwin Bird Research Laboratory at Gates Mills, Ohio. He retired from the University of Illinois in 1973. Dr. Kendeigh received the A.O.U. Brewster Award in 1951 and the Ecological Society of America's Eminent Ecologist Award in 1978. He wrote the classic Physiology of the Temperature of and edited Shelford's Ecology of North America. The most recent of Dr. Kendeigh's 90+ publications (1979, Illinois Biol. Monogr. No. 50; 1982, Illinois Biol. Monogr. No. 52) were extensive summaries and analyses of 50 years of data from three Illinois natural areas. He was compiler from 1941 to 1973 of the Champaign Co., Illinois, Christmas Bird Count. His research interests included bird populations in relation to habitat, avian life histories, and physiological responses of birds to various seasonal and climatic regimes. He co-edited the book Productivity, Population Dynamics, and Systematics of Granivorous Birds, which was the proceedings of the First General Meeting of the Working Group on Granivorous Birds of the International Biological Program (1973, Warsaw, Polish Acad. Sci.). He cooperated with

Climographic Studies of Certain Introduced and Migratory Birds

Considering the relation of animals to their environments, one may conclude that where natural factors limit the range or distribution of a species, usually very few complicated factors are involved. The extent to which these factors actually limit in their effect ordinarily involves not the entire life-cycle of the species or the individual, but some particular or critical period of the life of the organism. Of all the periods in this life cycle, the breeding season followed by that in which the young are raised, is undoubtedly the most critical. At such periods the environmental conditions must be as nearly optimum as possible else catastrophe results. By optimum is meant that which is most suited to the physiological requirements of the individual. The toleiance of the adult is greater than that of the young, and if the conditions under which the young are raised are too far from this optimum, the young will perish though the adult may survive, but such a sequence of events quickly results in the extermination of the race. It is not the purpose to review here the many studies of physiological conditions and physical factors which control the movements of an organism. Many studies have been made involving temperature, humidity, raiiifall, and radiation, as individual factors influencing or limiting the distribution of animals, but there are few studies which take into consideration a possible combination of two or more of these factors as indicating the range preference or range limitations of a species. The author has been unable to find any work using two factors as a means of measuring or comparing certain phases of the migration problem of animals, or to explain either the obvious success or equally obvious failure of species introduced into a new environment. Irhe purpose of this study is to present two distinct problems of avian life history by means of duplex data involving both temperature and humidity. These two problems are: (1) the problem of the introduced species: why it is either a success or a failure; and (2) a comparison of the summer and winter ranges of certain migratory birds as shown in terms of these two factors, temperature and humidity. The first involves the question: Can we, in terms of temperature and humidity, show any cause or reason why certain species have been introduced successfully into certain regions and entirely unsuccessfully into others? The second involves the questions: