Bird Demography Research Articles

Biological traits and biome features mediate responses of terrestrial bird demography to droughts.

Changing drought regimes are a rising threat to biodiversity, yet their impacts on wildlife vary greatly. Acknowledging the factors associated with these consequences brings novel insights into species vulnerability resulting from extreme climatic events and facilitates effective mitigation of climate change risks. Based on 319 observations from 29 peer-reviewed studies on birds-a well-monitored taxonomic group-we extract the responses of demographic metrics to droughts for 204 species across eight terrestrial biomes to examine the consequences of droughts. According to relevant studies, we chose the factors potentially moderating bird demography under droughts and compiled the data for these factors from published datasets. A meta-analysis is performed to determine the drought effect on bird demography at individual and population levels, accounting for the influence of species traits, timescale and severity of droughts, as well as biome features. The results show that droughts have an overall negative effect on bird demography, and the effect is mediated by different factors at each level. For individuals exposed to droughts, declines in demographic rates are found to be related to narrower extents of occurrence of species, and a significant overall reduction in demographic rates is identified for individuals residing in deserts and xeric shrublands. At the population level, declines in abundance or reproductive performance are generally identified for invertivores, frugivores, nectarivores and omnivores; short-lived species with small clutch sizes also show greater susceptibilities under the impacts of droughts. Our findings additionally suggest that the demographic vulnerability of bird individuals and populations could be affected by the duration and magnitude of drought episodes. Although our results are subject to publication bias, these conclusions advance the assessment of vulnerability to extreme climatic events that used to be based on equally weighted species traits and support bird conservation by prioritizing the declining populations of species with drought-susceptible traits.

Constant or varied survival? Effects of seasonal and unpredictable factors on bird survival in a Neotropical semi-arid region

Bird demography in the semi-arid tropics is poorly understood and potentially distinct from that in the humid tropics. In dry environments, rainfall dynamics lead to temporal concentration of food resources and to animals' responses to these resources, which together influence survival. Using six years of sampling from a dry forest in Brazil, we describe the temporal relationship among rainfall, breeding and feather moult to test how these events interact to influence the survival of the Pearly-vented Tody-Tyrant. The first four years were marked by drought. Birds seldom had brood patches in the final years of drought, whereas moulting was uncommon in the subsequent rainy years. Rainfall, breeding and moults were concentrated in the first half of the year. Apparent survival was low during the final two years of drought, increased in subsequent years and was seasonally higher during periods of rain, breeding and moulting. Our findings indicate that survival is somewhat resistant to climate unpredictability but declines during multiyear drought events. A strategy of phenotypic flexibility seems to have minimized the intense negative effects on survival. We believe that these findings provide insights that contribute to a better understanding of the demography and annual routines of birds in seasonal environments.

Diversity, abundance, and host relationships of avian malaria and related haemosporidians in New Mexico pine forests.

Avian malaria and related haemosporidian parasites (genera Haemoproteus, Plasmodium, and Leucocytozoon) affect bird demography, species range limits, and community structure, yet they remain unsurveyed in most bird communities and populations. We conducted a community-level survey of these vector-transmitted parasites in New Mexico, USA, to describe their diversity, abundance, and host associations. We focused on the breeding-bird community in the transition zone between piñon-juniper woodland and ponderosa pine forests (elevational range: 2,150–2,460 m). We screened 186 birds representing 49 species using both standard PCR and microscopy techniques to detect infections of all three avian haemosporidian genera. We detected infections in 68 out of 186 birds (36.6%), the highest proportion of which were infected with Haemoproteus (20.9%), followed by Leucocytozoon (13.4%), then Plasmodium (8.0%). We sequenced mtDNA for 77 infections representing 43 haplotypes (25 Haemoproteus, 12 Leucocytozoon, 6 Plasmodium). When compared to all previously known haplotypes in the MalAvi and GenBank databases, 63% (27) of the haplotypes we recovered were novel. We found evidence for host specificity at the avian clade and species level, but this specificity was variable among parasite genera, in that Haemoproteus and Leucocytozoon were each restricted to three avian groups (out of six), while Plasmodium occurred in all groups except non-passerines. We found striking variation in infection rate among host species, with nearly universal infection among vireos and no infection among nuthatches. Using rarefaction and extrapolation, we estimated the total avian haemosporidian diversity to be 70 haplotypes (95% CI [43–98]); thus, we may have already sampled ∼60% of the diversity of avian haemosporidians in New Mexico pine forests. It is possible that future studies will find higher diversity in microhabitats or host species that are under-sampled or unsampled in the present study. Fortunately, this study is fully extendable via voucher specimens, frozen tissues, blood smears, parasite images, and documentation provided in open-access databases (MalAvi, GenBank, and ARCTOS).

The future demographic niche of a declining grassland bird fails to shift poleward in response to climate change

Temperate grasslands and their dependent species are exposed to high variability in weather and climate due to the lack of natural buffers such as forests. Grassland birds are particularly vulnerable to this variability, yet have failed to shift poleward in response to recent climate change like other bird species in North America. However, there have been few studies examining the effect of weather on grassland bird demography and consequent influence of climate change on population persistence and distributional shifts. The goal of this study was to estimate the vulnerability of Henslow’s Sparrow (Ammodramus henslowii), an obligate grassland bird that has been declining throughout much of its range, to past and future climatic variability. We conducted a demographic meta-analysis from published studies and quantified the relationship between nest success rates and variability in breeding season climate. We projected the climate-demography relationships spatially, throughout the breeding range, and temporally, from 1981 to 2050. These projections were used to evaluate population dynamics by implementing a spatially explicit population model. We uncovered a climate-demography linkage for Henslow’s Sparrow with summer precipitation, and to a lesser degree, temperature positively affecting nest success. We found that future climatic conditions—primarily changes in precipitation—will likely contribute to reduced population persistence and a southwestward range contraction. Future distributional shifts in response to climate change may not always be poleward and assessing projected changes in precipitation is critical for grassland bird conservation and climate change adaptation.

A long-term study of egg dimensions in the Tree Sparrow Passer montanus in relation to climate change

Recent studies have illustrated that increasing temperatures have had considerable effects on species and entire ecosystems. In this study I examine long-term trends of egg dimensions (length, breadth and volume) in the population of Tree Sparrows (Passer montanus), investigated during 36 years (1980–2015), in relation to spring temperatures. This species is a common, sedentary, hole-nesting and socially monogamous passerine bird in research area. Research took place in Mokrice area in north-western Croatia. Average temperatures in spring (mean March–April) have significantly increased during the monitored period of time. In population in north-western Croatia there was no increase or decrease in egg dimensions. Correlations between mean spring temperatures and mean egg length, and egg breadth and egg volume were not significant. Spring precipitations (March–April) did not change over the years and did not affect egg dimensions. Data on changes in egg dimensions are relatively scarce in comparison with data on other breeding parameters in birds and, in some cases, controversial ; some authors report increase in egg dimensions while others report decreases in egg dimensions in different bird species and populations as a response to climate warming. Thus, changes in egg dimension are a very complex issue in bird demography because there are many traits apart from climate that have impact on egg size, such as female condition, genetic and habitat characteristics etc., so further investigations on this subject are necessary.

How to sustain meadow passerine populations in Europe through alternative mowing management

Two decades of agri-environmental policy did not prevent a long term decline of grassland birds in Europe. Additional measures are therefore needed to sustain the populations. This study explored alternative mowing management regimes likely to secure demographic sources in the early mown grassland systems of western Europe, and to limit habitat loss after farming abandonment in countries of the former Eastern Bloc. Postponing grass cutting until after mid-July from 2009 to 2014 in half of the area of 4 study sites (29–55ha each) in the Saône Valley (France), led to increased territory density and improved hatching success. Bird response however was species-specific: Corn Bunting Emberiza calandra territory density benefited the most from the alternative management, Yellow Wagtail Motacilla flava territory distribution tended to match the late mown areas, whereas the Whinchat Saxicola rubetra did not change its initial distribution. Temporary interruption of mowing in 8 meadow units (11.7–15.1ha) of the Moskva Valley (Central Russia) was similarly correlated with higher territory density. Whinchat territory density decreased after one single year of mowing. After two consecutive years of mowing, Whinchat hatching success was lower and the Lesser Citrine Wagtail Motacilla citreola werae virtually disappeared. The tested alternative mowing regimes may therefore locally increase population density without negative density dependent effects on hatching rates. Implementing rotational mowing could reduce habitat loss caused by farming abandonment in Russia. Postponing mowing until after mid-July in patches of hay fields may sustain meadow bird demography in the remaining strongholds of western Europe.

From eggs to fledging: negative impact of urban habitat on reproduction in two tit species

The exploration of the effects of urbanization on bird demography has attracted much attention, and several studies found lower reproductive success in towns, which suggested strong environmental constraints. Here, we conducted a 3-year study to explore the consequences of urbanization on the breeding success of two species that originated in forests, the Blue Tit Cyanistes caeruleus and the Great Tit Parus major. In two replicates of urban and forest habitats, we studied the components of reproductive success. In one replicate of each habitat, we quantified nestling growth over the three breeding seasons, and we collected data on egg quality during one breeding season. The general picture that emerges from our finding is that in urban sites breeding success was lower with smaller clutch sizes, higher clutch, higher brood failure rates and lower survival rates. Our results also showed reduced growth in urban habitats, at the embryonic and nestling stages, with potential adverse consequences on fitness. Crucial ecological factors could explain the observed contrasts between the habitats, and food limitation is among the most likely. Overall, we demonstrated the negative effects of urbanization on the reproductive success of forest birds, and our results were consistent between species and geographic areas for these negative effects. Our results suggest a mismatch between urban environments and the habitat exploitation abilities that birds have evolved in their native forest ecosystems.

Mowing management and density dependence in meadow passerine hatching success

Capsule Improving habitat quality through late or intermittent mowing may increase passerine population density without reducing reproductive success.Aim To describe the relationship between passerine territory density (TD) and Whinchat Saxicola rubetra and Yellow Wagtail Motacilla flava hatching success, under different management regimes.Methods TD was defined by the Territory Mapping method in nine sites selected in five French flood plains, in four intermittently mown meadows in Russia and in four additional French sites where mowing was partially postponed. Hatching success was assessed by the observation of prey carrying. Generalized Linear Mixed Models were used to analyse the influence on hatching rates of mowing management, early mowing and TD. Linear Mixed Models were used to study the effect of mowing management on TD.Results In French early mown meadows, TD affected reproductive success. Higher hatching rates and TD were observed in Russia. Similar results in French study sites with postponed mowing confirmed the influence of mowing management on carrying capacity.Conclusion In west-European grassland systems, late or intermittently mown patches within or at the edge of meadow areas, with potentially improved invertebrate-prey availability, should help to sustain bird demography and improve the efficiency of current conservation programmes.

The persistent problem of lead poisoning in birds from ammunition and fishing tackle

Lead (Pb) is a metabolic poison that can negatively influence biological processes, leading to illness and mortality across a large spectrum of North American avifauna (.120 species) and other organisms. Pb poisoning can result from numerous sources, including ingestion of bullet fragments and shot pellets left in animal carcasses, spent ammunition left in the field, lost fishing tackle, Pb-based paints, large-scale mining, and Pb smelting activities. Although Pb shot has been banned for waterfowl hunting in the United States (since 1991) and Canada (since 1999), Pb exposure remains a problem for many avian species. Despite a large body of scientific literature on exposure to Pb and its toxicological effects on birds, controversy still exists regarding its impacts at a population level. We explore these issues and highlight areas in need of investigation: (1) variation in sensitivity to Pb exposure among bird species; (2) spatial extent and sources of Pb contamination in habitats in relation to bird exposure in those same locations; and (3) interactions between avian Pb exposure and other landscape-level stressors that synergistically affect bird demography. We explore multiple paths taken to reduce Pb exposure in birds that (1) recognize common ground among a range of affected interests; (2) have been applied at local to national scales; and (3) engage governmental agencies, interest groups, and professional societies to communicate the impacts of Pb ammunition and fishing tackle, and to describe approaches for reducing their availability to birds. As they have in previous times, users of fish and wildlife will play a key role in resolving the Pb poisoning issue.

Avian functional group responses to rainfall across four vegetation types in theSimpsonDesert, centralAustralia

AbstractTemporal and spatial variability in rainfall and subsequent resource flows often drive large changes in bird demography and the structure of avian assemblages in arid regions. Here, we describe broad avian functional group responses to rainfall across four vegetation classes in theSimpsonDesert, centralAustralia, over a period when both dry and wet conditions prevailed. Birds were surveyed in eucalypt woodlands, gidyea woodlands, mixedAcacia(mulga) shrublands and spinifex‐dominated hummock grasslands in this region between 2006 and 2008 using a spatially nested hierarchical sampling design with fixed radius point counts as replicates. Predictably, the composition of bird communities varied across vegetation classes as well as over time. Generalist feeders and insectivorous birds dominated assemblages during dry periods, whereas granivorous species were most abundant and widespread following heavy rains. Carnivores and specialist nectarivores responded variably throughout the sampling period regardless of habitat. The dynamics of granivores were especially interesting; large numbers of nomadic species moved into the study area in response to rainfall, highlighting a key role for this group in shaping community structure. This study represents the first attempt to investigate avian functional group dynamics across differing vegetation types in a variable but low productivity landscape. The findings confirm that community composition is spatially and temporally dynamic, and prompt further investigation into the roles that habitat and resource limitation play in defining bird communities in aridAustralia.

Demographic consequences of migratory stopover: linking red knot survival to horseshoe crab spawning abundance

Understanding how events during one period of the annual cycle carry over to affect survival and other fitness components in other periods is essential to understanding migratory bird demography and conservation needs. Previous research has suggested that western Atlantic red knot (Calidris canutus rufa) populations are greatly affected by horseshoe crab (Limulus polyphemus) egg availability at Delaware Bay stopover sites during their spring northward migration. We present a mass-based multistate, capture-recapture/resighting model linking (1) red knot stopover mass gain to horseshoe crab spawning abundance and (2) subsequent apparent annual survival to mass state at the time of departure from the Delaware Bay stopover area. The model and analysis use capture-recapture/resighting data with over 16,000 individual captures and 13,000 resightings collected in Delaware Bay over a 12 year period from 1997–2008, and the results are used to evaluate the central management hypothesis that red knot populations can be influenced by horseshoe crab harvest regulations as part of a larger adaptive management effort. Model selection statistics showed support for a positive relationship between horseshoe crab spawning abundance during the stopover and the probability of red knots gaining mass (parameter coefficient from the top model b̂ = 1.71, = 0.46). Our analyses also supported the link between red knot mass and apparent annual survival, although average estimates for the two mass classes differed only slightly. The addition of arctic snow depth as a covariate influencing apparent survival improved the fit of the data to the models (parameter coefficient from the top model b̂ = 0.50, = 0.08). Our results indicate that managing horseshoe crab resources in the Delaware Bay has the potential to improve red knot population status.

Towards a vertebrate demographic data bank

The development of computers, appropriate statistical methodology and specialized software has induced an explosion in empirical research on vertebrate population dynamics. Many long-term programs have led to impressive datasets and to the publication of hundreds of estimates of vital rates critical to many areas of ecology: evolution of life history strategies, conservation biology, behavioral ecology, population management, etc. Such estimates are still usually available through regular scientific articles, and their use for comparative purposes suffers from several shortcomings: duplication of technical work, lack of evaluation of methodological bias, and difficulties in linking vital rates estimates with other basic traits such as body size. It thus seems it is time to propose a demographic databank to collect the information on vertebrate demography published and being published and make it widely available. The resulting database should become the equivalent for vertebrate demography to what “Genbank” is for DNA sequences. Bird demography has a critical mass of knowledge adequate for a first step. This paper reviews, based on a prototype database, the outline of such a project of demographic database: type of data and estimates stored, assessment of methodology and data quality, data documentation, taxonomical and phylogenetical information, link with other existing biodiversity databases, procedures for depositing information, links with scientific journals, etc. The contours of a collaborative group to launch such a project are also discussed.

Changes in ring recovery rates over the last 50 years: shall we continue to ring ducks?

Although knowledge of ring recovery rate is of crucial importance to establish demographic parameters, such as survival probability, this information is generally unknown for the dabbling ducks. The almost single existing value from the North American Mallard (Anas platyrhynchos) is thus generally applied to other species or geographic areas, even though this assumption may be strongly misleading. In the study reported here, we have relied on a proxy for actual ring recovery rate, namely, the proportion of rings fitted each year that were eventually reported, to test for differences between duck species (Mallard and Teal Anas crecca) and between countries [England (UK) and France] for Teal. Potential trends over time (1952–2005) were also assessed. Ring recovery rate was found to have sharply decreased and at a similar rate over time in both species in France, and in both countries for Teal (e.g. from 22.7% in 1952 to 7.2% in 1992 for the British Teal). Teal rings were, however, reported more frequently than Mallard ones, and Teal rings were more frequently reported in the UK than in France. In recent years, a phone number for reporting the information has been included on the bird mark, with the immediate result of a doubling in the recovery rate in France. Adaptations to current ringing procedures that would improve ring reporting would appear to be necessary in the future if researchers are to keep ringing as a valuable source of information for bird demography studies.

Blood lead concentrations in wild birds from a polluted mining region at Villa de La Paz, San Luis Potosí, Mexico

This investigation was undertaken to determine the concentrations of lead in bird blood samples from a mining region in central Mexico and to compare concentrations among several different feeding guilds. The study took place in the Mexican state of San Luis Potosi in a region known as “Villa de la Paz.” This is one of the most intensely exploited mining regions in central Mexico and has been actively mined for over four centuries. Lead concentrations from bird blood samples taken from four polluted sites were significantly higher than those from a control, unpolluted site (F = 6.3, P < 0.0002). Similarly, mean blood lead concentrations in birds from a highly polluted site were higher than those from a site that has intermediate pollution levels (P < 0.05). In addition, samples from insectivorous birds had significantly lower lead concentrations compared to granivores, frugivores–insectivores, and omnivores (F = 4.86, P = 0.004), and a large proportion of all individuals had blood lead concentrations indicative of low, sub-lethal toxic effects. Finally, in two polluted sites, remarkably small numbers of insectivore–frugivores, and granivores were trapped, and in one polluted site a large number of insectivores was trapped (X2 = 29.9, P = 0.03), and no differences in proportions of migrants and non-migrants were found among sampling sites (X2 = 0.6, P = 0.96). To date, it has not been determined to what extent constant exposure to these levels of pollution can influence health at the individual level, lifespan, and, therefore, population demography of birds from this region.

Regional and National Issues for Forest Wildlife Research and Management

Abstract Biologists and managers face a broad range of issues related to research and management of forest wildlife. To assess current trends in research, I reviewed 12 scientific journals (1992–2001) that published articles on wildlife, general ecology, and forestry and summarized articles by location, topic, issue, and species. To assess management concerns and issues, I interviewed professional biologists from across the United States, asking their opinions on important forest wildlife topics, approaches to improve research design, how management differs on public versus private lands, and which species they thought were underrepresented, overrepresented, or of special concern in their region. Most scientific articles focused on habitat selection, effects of forest alteration, and demography of birds and mammals. Herpetofauna (i.e., amphibians and reptiles) and invertebrates, although the subjects of some studies, made up only 10% each of all research papers. Biologists were most concerned with the alteration of forest vegetative structure due to timber harvest and its effects on wildlife populations and communities, but also recognized other issues such as changes in natural disturbance patterns such as fire, effects of road building, and loss of early successional vegetative communities as well as fragmentation of older forests. Most biologists agreed that long-term, large-scale, experimental studies, which document the demographic response of forest wildlife to alterations in forest cover, are needed. FOR. SCI. 48(2):181–189.

AVIAN LIFE HISTORY VARIATION AND CONTRIBUTION OF DEMOGRAPHIC TRAITS TO THE POPULATION GROWTH RATE

A central goal in ecology is to predict population dynamics from demographic information. Based on the asymptotic population growth rate λ, calculated from a projection matrix model as a descriptor of the population dynamics, we analyze published data of 49 species of birds to determine how λ is influenced by variation in different demographic traits. Across species, the mean elasticity of the adult survival rate was significantly larger than the mean elasticity of the fecundity rate. The contribution of the fecundity rate to the population growth rate increased with increasing clutch size and decreasing adult survival rate, while the greatest contribution of adult survival rate occurred among long-lived species that matured late and laid few eggs. This represents a continuum from “highly reproductive species” at one end to “survivor species” at the other end. In addition, a high contribution of adult survival rate was found in some relatively long-lived species with early age at maturity (and a large clutch size) which was assumed to represent a bet-hedging strategy, i.e., producing a large number of offspring in some occasional good years. In a retrospective analysis, interspecific differences in the effects of actual temporal variation in adult survival rate and fecundity rate on the variability of λ were analyzed. These effects are expected to be large when the variance or the sensitivity of the trait is large. Because there was a negative relationship among species, both for the adult survival rate and the fecundity rate between the variability and the sensitivity of the trait, contribution of a trait to the variance in λ decreased with sensitivity. Similarly, within species, less temporal variation was found in traits with high elasticities than in traits with less contribution to λ. In some species, covariance among matrix elements also influenced the contribution of a demographic trait to λ. Monitoring schemes of bird demography should be designed in such a way that temporal variances and covariances among demographic traits can be estimated. Furthermore, it is important in such schemes to include data from a combination of traits that either have large sensitivities or high temporal variation.

The use of ringing to monitor changes in the numbers and demography of birds

Knowledge of changes in the abundance and demography (productivity and survival) of birds is essential if conservation problems are to be identified and remedied. Bird ringing is the only way to monitor avian survival rates and can provide an effective means of monitoring changes in numbers and productivity. In this paper, we briefly review past and present applications of bird ringing for the purpose of monitoring numbers and demography, and suggest ways in which current monitoring may be improved. In order to monitor survival from recoveries of dead birds it is important that more than one age class of birds are ringed, and serious statistical problems can arise when most birds are ringed as chicks. While the breeding season is generally the most useful time of year to ring birds for the purpose of monitoring, organised winter ringing may be useful for species which show strong fidelity to wintering sites and are therefore likely to be re trapped in subsequent winters. The Constant Effort Sites philosophy of maximising retrap rates while standardising (or at least measuring) catching effort should be extended to other groups of birds.

Demography of Birds in a Neotropical Forest: Effects of Allometry, Taxonomy, and Ecology

Comparative demographic studies of terrestrial vertebrates have included few samples of species from tropical forests. We analyzed 9 yr of mark—recapture data and estimated demographic parameters for 25 species of birds inhabiting lowland forests in central Panama. These species were all songbirds (Order Passeriformes) ranging in mass from 7 to 57 g. Using Jolly—Seber stochastic models for open populations, we estimated annual survival rate, population size, and recruitment between sampling periods for each species. We then explored relationships between these parameters and attributes such as body size, phylogenetic affiliation, foraging guild, and social behavior. Larger birds had comparatively long life—spans and low recruitment, but body size was not associated with population size. After adjusting for effects of body size, we found no association between phylogenetic affiliation and any demographic trait. Ecological attributes, especially foraging guild, were more clearly associated with interspecific variation in all demographic traits. Ant—followers had comparatively long life—spans, but species that participate in flocks did not live longer than solitary species. The allometric associations we observed were consistent with those demonstrated in other studies of vertebrates; thus, these relationships appear to be robust. Our findings that ecological factors were more influential than phylogenetic affiliation contrasts with comparative studies of temperate—zone birds and suggests that the relative importance of environmental vs. historical factors varies geographically.

Effects of a Severe Drought on the Population Biology of a Tropical Hummingbird

At Finca La Selva, in the Caribbean lowlands of Costa Rica, the Long—tailed Hermit (Phaethornis superciliosus) has a long breeding season (December or January through August or September) during which flowers are abundant, followed by a severe but predictable season of flower scarcity in late October—November, during which most of the annual mortality normally occurs. In 1973, the third of a 4—yr population study of marked lek males, an unusual drought caused a severe flower shortage at the height of the breeding season. During the latter part of the breeding season lek activity of males was reduced, successful breeding by females was drastically curtailed, and masses of both sexes dropped to levels lower than those of the lean season. Interrupted molt, an indicator of energy stress normally seen only in a minority of breeding females, was observed in 25% of adult males, and 50% of females, 1973. Survivorship of lek males through the breeding season dropped from 90% in normal years to °60%, and the total lek population at the start of the 1974 breeding season was one—third lower than before the drought in 1973; the cohort of yearling males was most strongly affected. Recovery of numbers and age structure of the lek male populations to pre—1973 levels required 3—4 yr. Jolly—Seber analyses showed close agreement with survivorship as determined by direct observation (possible because of the lek social system), and permitted estimation of other parameters of interest: populations of marked and unmarked males, probability of recapture of marked males, and recruitment. The average annual survivorship of male P. superciliosus (in normal years) is near the mean value reported for a series of tropical species. A simple model of the reproductive output of female P. superciliosus, assuming no sexual differences in survivorship, suggests that a female breeding only in the prime breeding season (roughly February—July, when over 80% of the fledglings are normally produced) will no more than replace herself in the population; breeding throughout 8—9 mo will permit her to increase her reproductive output by up to 10%, which agrees well with the 3+ yr required by the (male) population to recover from the drought. Possible implications of these findings for tropical bird demography and conservation are discussed.