No abstract

Increasing evidence suggests that climate change has a¡ected the breeding and distribution of wildlife. If such changes are due to global warming, then we should expect to see large-scale e¡ects. To explore for such e¡ects 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 1959^1991. 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.

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 Leach's 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 distributions of animal populations change and evolve through time. Migratory species exploit different habitats at different times of the year. Biotic and abiotic features that determine where a species lives vary due to natural and anthropogenic factors. This spatiotemporal variation needs to be accounted for in any modeling of species' distributions. In this paper we introduce a semiparametric model that provides a flexible framework for analyzing dynamic patterns of species occurrence and abundance from broad-scale survey data. The spatiotemporal exploratory model (STEM) adds essential spatiotemporal structure to existing techniques for developing species distribution models through a simple parametric structure without requiring a detailed understanding of the underlying dynamic processes. STEMs use a multi-scale strategy to differentiate between local and global-scale spatiotemporal structure. A user-specified species distribution model accounts for spatial and temporal patterning at the local level. These local patterns are then allowed to "scale up" via ensemble averaging to larger scales. This makes STEMs especially well suited for exploring distributional dynamics arising from a variety of processes. Using data from eBird, an online citizen science bird-monitoring project, we demonstrate that monthly changes in distribution of a migratory species, the Tree Swallow (Tachycineta bicolor), can be more accurately described with a STEM than a conventional bagged decision tree model in which spatiotemporal structure has not been imposed. We also demonstrate that there is no loss of model predictive power when a STEM is used to describe a spatiotemporal distribution with very little spatiotemporal variation; the distribution of a nonmigratory species, the Northern Cardinal (Cardinalis cardinalis).

The design of artificial nestboxes for the study of secondary hole-nesting birds: a review of methodological inconsistencies and potential biases. Acta Ornithol. 45: 1-26.

No abstract

Once-abundant aerial insectivores, such as the Tree Swallow (Tachycineta bicolor), have declined steadily in the past several decades, making it imperative to understand all aspects of their ecology. Aerial insectivores forage on a mixture of aquatic and terrestrial insects that differ in fatty acid composition, specifically long-chain omega-3 polyunsaturated fatty acid (LCPUFA) content. Aquatic insects contain high levels of both LCPUFA and their precursor omega-3 PUFA, alpha-linolenic acid (ALA), whereas terrestrial insects contain much lower levels of both. We manipulated both the quantity and quality of food for Tree Swallow chicks in a full factorial design. Diets were either high-LCPUFA or low in LCPUFA but high in ALA, allowing us to separate the effects of direct LCPUFA in diet from the ability of Tree Swallows to convert their precursor, ALA, into LCPUFA. We found that fatty acid composition was more important for Tree Swallow chick performance than food quantity. On high-LCPUFA diets, chicks grew faster, were in better condition, and had greater immunocompetence and lower basal metabolic rates compared with chicks on both low LCPUFA diets. Increasing the quantity of high-LCPUFA diets resulted in improvements to all metrics of performance while increasing the quantity of low-LCPUFA diets only resulted in greater immunocompetence and lower metabolic rates. Chicks preferentially retained LCPUFA in brain and muscle when both food quantity and LCPUFA were limited. Our work suggests that fatty acid composition is an important dimension of aerial insectivore nutritional ecology and reinforces the importance of high-quality aquatic habitat for these declining birds.

Across North America, tree swallows have advanced their mean date of clutch initiation (lay date) by Ϸ9 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-size͞ 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. O ver the past century, global surface temperature has increased by an average of 0.6°C (1). It is now widely accepted that air temperatures have been increasing as a result of increased anthropogenic carbon dioxide in the atmosphere. Air temperature could be one of the key environmental cues affecting the seasonal timing of egg laying in birds, and there is evidence that increases in mean temperatures have led to earlier lay dates in birds (2-6) and accelerated phenologies in many other taxa (7-9).One of the most general patterns in the life histories of birds is that females that lay later in a breeding season tend to lay smaller clutches (refs. 10 and 11, but see refs. 12 and 13 for exceptions in multiple-brooded species), raising the possibility that earlier breeding may be leading to larger clutches being laid. Clutch size is an important life-history trait, because it sets a hard upper limit on offspring production. It is also the easiest life-history trait to measure, and data on clutch size variation often are available when data on other life-history traits are not.There is considerable evidence that the correlation between lay date and clutch size may be the result of a strong biological coupling (7,14), yet the nature of this biological link is very poorly understood. In an early attempt to study this connection between lay date and clutch size, von Haartman (15) suggested a dichotomy in the way that birds adjust their clutch sizes to the timing of breeding. At a study site in Lemsjöholm, Finland, both pied flycatchers (Ficedula hypoleuca) and starlings (Sturnus vulgaris) laid smaller clutches as each breeding season progressed. However, wh...