Demography of Songbird Populations in a Rapidly Changing World: The Importance of Long-term Studies

A major goal of ecological research is to understand how ecological factors, such as food and predator abundance, interact to shape birth and death rates. Case studies describing this research can provide students with an understanding of how ecological conditions affect demographic rates, as well as an opportunity to explore and interpret real data. We have developed a Web-based teaching module based on a long-term study of a migratory songbird, the black-throated blue warbler (Dendroica caerulescens). The website describes this species and the ecological factors that affect its population growth and provides three exercises developed to span a range of student levels.

Roosting behaviour of a migratory songbird on Jamaican coffee farms: landscape composition may affect delivery of an ecosystem service

SummaryRecent evidence indicates that insectivorous birds in shade coffee farms provide economically significant ecosystem services by reducing insect pests, which should encourage cultivation practices favouring birds and other wildlife. However, the provisioning of this service may be dependent on landscape composition and movement patterns of these mobile consumers. Very little information is currently available on bird movements in coffee-dominated landscapes. We examined roosting behaviour in Black-throated Blue Warblers Dendroica caerulescens on two Jamaican coffee farms using radio telemetry in order to determine whether birds commute between nocturnal roosts and diurnal foraging ranges in coffee habitat. Nocturnal tracking revealed most birds moved outside diurnal foraging ranges on the farms to roost in forested habitat patches, sometimes up to 1 km away. Of 42 roost locations, only eight roosts were within the coffee farm perimeter: one individual in a coffee bush, and seven others in shade trees or vegetated riparian strips. Logistic regression confirmed that birds strongly selected continuous forest and individual trees over coffee for roosting. These findings reflect the ecological connectivity between coffee habitats and the adjacent landscape and raise the possibility that the provisioning of pest reduction services could be at least partly dependent on a farm’s proximity to forest patches.

Mass Loss by Breeding Female Songbirds: Food Supplementation Supports Energetic Stress Hypothesis in Black-Throated Blue Warblers

Reproduction is physiologically stressful for many animals. Female birds often lose body mass after their young hatch, which has been commonly attributed either to an energetic deficit incurred while breeding (the energetic stress hypothesis) or to an adaptive reduction in wing-loading to save energy during flight (the wing-loading hypothesis). We tested these two hypotheses for an open-cup nesting passerine, the Black-throated Blue Warbler (Dendroica caerulescens), using a food-supplementation experiment. We found that females provided with extra food lost less body mass between the incubation and nestling periods than did control females and that mass loss varied seasonally, with females in both treatments that nested earlier in the season losing more mass. We conclude that greater mass loss in control females supports the energetic stress hypothesis in Black-throated Blue Warblers.

Malarial parasites as geographical markers in migratory birds?

We tested the hypothesis that malarial parasites ( Plasmodium and Haemoproteus ) of black-throated blue warblers ( Dendroica caerulescens ) provide sufficient geographical signal to track population movements between the warbler's breeding and wintering habitats in North America. Our results from 1083 warblers sampled across the species' breeding range indicate that parasite lineages are geographically widespread and do not provide site-specific information. The wide distribution of malarial parasites probably reflects postnatal dispersal of their hosts as well as mixing of breeding populations on the wintering range. When compared to geographically structured parasites of sedentary Caribbean songbirds, patterns of malarial infections in black-throated blue warblers suggest that host–malaria dynamics of migratory and sedentary bird populations may be subject to contrasting selection pressures.

Habitat occupancy patterns of a forest dwelling songbird: causes and consequences

We examined patterns of habitat use and reproductive performance of a migratory songbird, the black-throated blue warbler (Dendroica caerulescens (Gmelin, 1789)), within a 3160-ha forested landscape. We surveyed 371 sites over a 3-year period. Some sites were never occupied, while others were occupied for 1, 2, or 3 years. For these 3 years we found that warbler abundance increased with frequency of occupancy. Additionally, we found that (i) deciduousness and understory shrub density increased with frequency of occupancy; (ii) in 1 of 3 years, food abundance was higher at the most frequently occupied sites; and (iii) nest predators exhibited predator-specific abundance patterns across occupancy categories. We next used occupancy patterns documented in the first 3 years of the study to predict settlement, age structure, and reproductive performance at a subset of sites in the final year of the study. We found that males settled earlier in the breeding season at sites with a high frequency of occupancy. There were no differences in arrival times of females. Additionally, age structure did not vary for either males or females across sites with different occupancy levels. Although we found no difference in mean reproductive output across sites with different occupancy levels, over 50% of the young produced fledged from territories overlapping the high occupancy sites.

To Double-Brood or not? Individual Variation in the Reproductive Effort in Black-Throated Blue Warblers (Dendroica Caerulescens)

Individuals within a population vary in important fitness components, such as reproductive success. In general, females can maximize the number of young they produce by altering either the number of young per breeding attempt or the number of breeding attempts per season. In short-lived species, and especially in small passerine birds, number of breeding attempts per season varies markedly among individuals. Here, we evaluated factors influencing whether female Blackthroated Blue Warblers (Dendroica caerulescens) initiated additional nests after a successful breeding attempt (i.e. double-brooded). The percentage of females that laid a second clutch after successfully fledging a first brood ranged from 0 to 87% and averaged 53% (n = 7 years). Multiple logistic regression and AICc model selection indicated that double-brooded females bred in territories with greater food availability and produced heavier nestlings than single-brooded females. Female age, male age, date of first breeding attempt, and number of young in the first clutch were not included in the best-fit model. Older females, however, produced heavier fledglings, and females mated to older males occurred on territories with greater food availability, indicating that age contributed to individual variation in reproductive output. Because the proportion of females that produce multiple broods within a season can have a substantial effect on the annual fecundity of a population, variation among females and among the territories they occupy (i.e. habitat quality) are key factors influencing population dynamics in this and other multibrooded, shortlived species.Poner Nidadas Dobles o No? Variación Individual en el Esfuerzo Reproductivo en Dendroica caerulescens