David W. Winkler

A GENERAL MODEL FOR PARENTAL CARE

This model provides a general framework for analyzing the effects of a relatively large number of variables on quantitative variation in parental care. On the assumption that natural selection favors parents that maximize their reproductive value in each of many time intervals throughout a breeding attempt, the model predicts optimal parental responses to variation in age and number of offspring, time since the beginning of the breeding season, effort of the mate, and parental condition. This list of independent variables can easily be expanded or restricted within the framework of the model. Although the model is developed here for biparental monogamous species, it serves as the foundation for applications to other mating systems and for the addition or subtraction of variables to which the parents respond.

Climate change has affected the breeding date of tree swallows throughout North America

Increasing evidence suggests that climate change has affected the breeding and distribution of wildlife. If such changes are due to global warming, then we should expect to see large‐scale effects. To explore for such effects on avian reproduction, we examined 3450 nest records of tree swallows from across North America. The egg‐laying date in tree swallows advanced by up to nine days during 1959 to 1991. This advance in phenology was associated with increasing surface air temperatures at the time of breeding. Our analysis controlled for several potentially confounding variables such as latitude, longitude, breeding density and elevation.We conclude that tree swallows across North America are breeding earlier and that the most likely cause is a long‐term increase in spring temperature

Offspring Size and Number: A Life History Model Linking Effort Per Offspring and Total Effort

We develop an extension of Schaffers (1972) model for a simple life history with constant annual adult and juvenile survival rates. In contrast with previous models, we distinguish between total effort and effort per offspring. We assume that offspring production is the product of the juvenile survival rate and brood size, that the juvenile survival rate is a function of effort per offspring, and that brood size is a function of effort per offspring and total effort. We further assume that the adult survival rate is a function of total effort and that optimal combinations of effort per offspring and total effort are those resulting in the highest population growth rate. Optimal effort per offspring depends only on the minimal effort per offspring and the rate at which the juvenile survival rate increases with increasing effort per offspring. Optimal total effort depends on effort per offspring, the ratio between the realized juvenile survival rate and the maximal adult survival rate, and the threshold at which increasing total effort leads to sharp declines in the adult survival rate. The model and its predictions justify the inclusion of effort per offspring and total effort in descriptions of life history tactics and facilitate the formulation and testing of more-precise life history hypotheses. We suggest several experiments. The model predicts that the rate of increase in optimal total effort with increasing brood size decreases as brood size increases. The relative numbers of variables on which optimal total effort and effort per offspring depend suggest that current variability in effort per offspring should be less than variability in total effort.

Predicting the effects of climate change on avian life-history traits

Across North America, tree swallows have advanced their mean date of clutch initiation (lay date) by ≈9 days over the past 30 years, apparently in response to climate change. In a sample of 2,881 nest records collected by the lay public from 1959 to 1991, we examined whether clutch size has also responded to climate change. We found that clutch size is strongly related to lay date, both within and among years, and there has been no significant temporal variation in the slopes or intercepts of the clutch-size/lay-date regressions. As a consequence, we expected increases in clutch size with advancement in lay date; however, we detected no such trend over time. The distributions of egg-laying dates were more constricted in the warmest (and earliest) years, suggesting that changes in mean clutch size might be constrained by changes in the distribution of laying dates. If spring temperatures continue to increase, we predict further reductions of variance in laying dates and relatively small increases in clutch size. Such constraints on life-history variation probably are common and need to be considered when modeling the effects of climate change on reproduction in natural populations. Predicting the long-term effects of constraints and interpreting changes in life-history traits require a better understanding of both adaptive and demographic effects of climate change.

The Seasonal Decline in Tree Swallow Clutch Size: Physiological Constraint or Strategic Adjustment?

The seasonal decline in clutch size has been explained as being due to either: (1) a constraining effect of female condition on both laying date and clutch size; or (2) a seasonal decline in the prospects of chick recruitment, leading to a strategic decrease in clutch size with laying date. In an effort to shed light on this area of disagreement, we analyzed the physiological condition (as measured by body mass, breast muscle thickness, and fat stores) and body size (as measured by wing and head lengths) of 184 female Tree Swallows (Tachycineta bicolor) nesting near Ithaca, New York in 1993, 1994, and 1995, and related their condition to their laying date and clutch size. Through multiple regressions, we found that female age and wing length were the only significant predictors of laying date and that female age and laying date were the only significant predictors of clutch size. Thus, when the effects of laying date were held constant, there was no detectable effect of condition on clutch size in this species. This result is inconsistent with the constraining- condition hypothesis, and it suggests that the seasonal decline of clutch size in Tree Swal- lows is most appropriately seen as a strategic adjustment by the female to varying prospects for her offspring. The lack of evolution in laying date in at least the Tree Swallow remains a paradox.

The design of artificial nestboxes for the study of secondary hole-nesting birds: a review of methodological inconsistencies and potential biases

Abstract. The widespread use of artificial nestboxes has led to significant advances in our knowledge of the ecology, behaviour and physiology of cavity nesting birds, especially small passerines. Nestboxes have made it easier to perform routine monitoring and experimental manipulation of eggs or nestlings, and also repeatedly to capture, identify and manipulate the parents. However, when comparing results across study sites the use of nestboxes may also introduce a potentially significant confounding variable in the form of differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. However, the use of nestboxes may also introduce an unconsidered and potentially significant confounding variable due to differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. Here we review to what extent the characteristics of artificial nestboxes (e.g. size, shape, construction material, colour) are documented in the ‘methods’ sections of publications involving hole-nesting passerine birds using natural or excavated cavities or artificial nestboxes for reproduction and roosting. Despite explicit previous recommendations that authors describe in detail the characteristics of the nestboxes used, we found that the description of nestbox characteristics in most recent publications remains poor and insufficient. We therefore list the types of descriptive data that should be included in the methods sections of relevant manuscripts and justify this by discussing how variation in nestbox characteristics can affect or confound conclusions from nestbox studies. We also propose several recommendations to improve the reliability and usefulness of research based on long-term studies of any secondary hole-nesting species using artificial nestboxes for breeding or roosting.

Telomeres shorten more slowly in long-lived birds and mammals than in short–lived ones

We know very little about physiological constraints on the evolution of life-history traits in general, and, in particular, about physiological and molecular adjustments that accompany the evolution of variation in lifespan. Identifying mechanisms that underlie adaptive variation in lifespan should provide insight into the evolution of trade–offs between lifespan and other life–history traits. Telomeres, the DNA caps at the ends of linear chromosomes, usually shorten as animals age, but whether telomere rate of change is associated with lifespan is unknown. We measured telomere length in erythrocytes from five bird species with markedly different lifespans. Species with shorter lifespans lost more telomeric repeats with age than species with longer lifespans. A similar correlation is seen in mammals. Furthermore, telomeres did not shorten with age in Leachs storm–petrels, an extremely long–lived bird, but actually lengthened. This novel finding suggests that regulation of telomere length is associated not only with cellular replicative lifespan, but also with organismal lifespan, and that very long–lived organisms have escaped entirely any telomeric constraint on cellular replicative lifespan.

The Determination of Clutch Size in Precocial Birds

The evolution and regulation of clutch size has long been a central issue in ornithology. Early ornithologists realized that females of each species of bird lay a characteristic number of eggs, and we have been trying to determine ever since why this is so. In pursuit of the answer to this seemingly simple question, ornithologists have not only accumulated a wealth of egg data, but also have made important contributions to such diverse topics as life-history strategies, population regulation and group selection. Yet how clutch size is determined remains a controversial issue. The consensus that was once sought in the form of a central theory (Lack, 1968; Cody, 1966; Klomp, 1970; von Haartman, 1971) has disappeared in a sea of specific hypotheses. In this review we attempt to organize and summarize clutch size theories as they emerge in modified form from recent research and evaluate their ability to explain observed patterns in clutch size variation. We concentrate on the literature and concepts published since the review of Klomp (1970), but we incorporate earlier work when necessary.

Integrating concepts and technologies to advance the study of bird migration

Recent technological innovation has opened new avenues in migration research - for instance, by allowing individual migratory animals to be followed over great distances and long periods of time, as well as by recording physiological information. Here, we focus on how technology - specifically applied to bird migration - has advanced our knowledge of migratory connectivity, and the behavior, demography, ecology, and physiology of migrants. Anticipating the invention of new and smaller tracking devices, in addition to the ways that technologies may be combined to measure and record the behavior of migratory animals, we also summarize major conceptual questions that can only be addressed once innovative, cutting-edge instrumentation becomes available.

Longer telomeres associated with higher survival in birds

Differences in individual quality and survival within species are a major focus in evolutionary ecology, but we know very little about the underlying physiological mechanisms that determine these differences. Telomere shortening associated with cellular senescence and ageing may be one such mechanism. To date, however, there is little evidence linking telomere length and survival. Here, we show that tree swallows (Tachycineta bicolor) with relatively short telomeres at the age of 1 year have lower survival than tree swallows of the same age with relatively long telomeres. The survival advantage in the long telomere group continues for at least three breeding seasons. It will be important to identify mechanisms that link telomere length with survival early in life.