Tropical winter habitat limits reproductive success on the temperate breeding grounds in a migratory bird

Identifying the factors that control population dynamics in migratory animals has been constrained by our inability to track individuals throughout the annual cycle. Using stable carbon isotopes, we show that the reproductive success of a long-distance migratory bird is influenced by the quality of habitat located thousands of kilometres away on tropical wintering grounds. For male American redstarts (Setophaga ruticilla), winter habitat quality influenced arrival date on the breeding grounds, which in turn affected key variables associated with reproduction, including the number of young fledged. Based on a winter-habitat model, females occupying high-quality winter habitat were predicted to produce more than two additional young and to fledge offspring up to a month earlier compared with females wintering in poor-quality habitat. Differences of this magnitude are highly important considering redstarts are single brooded, lay clutches of only three to five eggs and spend only two-and-a-half months on the breeding grounds. Results from this study indicate the importance of understanding how periods of the annual cycle interact for migratory animals. Continued loss of tropical wintering habitat could have negative effects on migratory populations in the following breeding season, minimizing density-dependent effects on the breeding grounds and leading to further population declines. If conservation efforts are to be successful, strategies must incorporate measures to protect all the habitats used during the entire annual cycle of migratory animals.     

Multiple density-dependence mechanisms regulate a migratory bird population during the breeding season

The mechanisms regulating bird populations are poorly understood and controversial. We provide evidence that a migratory songbird, the black-throated blue warbler (Dendroica caerulescens), is regulated by multiple density-dependence mechanisms in its breeding quarters. Evidence of regulation includes: stability in population density during 1969-2002, strong density dependence in time-series analyses of this period, an inverse relationship between warbler density and annual fecundity, and a positive relationship between annual fecundity and recruitment of yearlings in the subsequent breeding season. Tests of the mechanisms causing regulation were carried out within the Hubbard Brook Experimental Forest, New Hampshire, during 1997-1999. When individuals from abutting territories were experimentally removed in a homogeneous patch of high-quality habitat, the fecundity of focal pairs nearly doubled, revealing a locally operating crowding mechanism. A site-dependence mechanism was indicated by an inverse relationship between population size and mean territory quality, as well as by greater annual fecundity on the sites that were most frequently occupied and of highest quality. These site-dependence relationships were revealed by intensive monitoring of territory quality and demography at the landscape spatial scale. Crowding and site-dependence mechanisms, therefore, acted simultaneously but at different spatial scales to regulate local abundance of this migratory bird population.     

Information collected during the post-breeding season guides future breeding decisions in a migratory bird

Breeding habitat choice and investment decisions are key contributors to fitness in animals. Density of individuals is a well-known cue of habitat quality     

Maladaptive habitat selection of a migratory passerine bird in a human-modified landscape

In human-altered environments, organisms may preferentially settle in poor-quality habitats where fitness returns are lower relative to available higher-quality habitats. Such ecological trapping is due to a mismatch between the cues used during habitat selection and the habitat quality. Maladaptive settlement decisions may occur when organisms are time-constrained and have to rapidly evaluate habitat quality based on incomplete knowledge of the resources and conditions that will be available later in the season. During a three-year study, we examined settlement decision-making in the long-distance migratory, open-habitat bird, the Red-backed shrike (Lanius collurio), as a response to recent land-use changes. In Northwest Europe, the shrikes typically breed in open areas under a management regime of extensive farming. In recent decades, Spruce forests have been increasingly managed with large-size cutblocks in even-aged plantations, thereby producing early-successional vegetation areas that are also colonised by the species. Farmland and open areas in forests create mosaics of two different types of habitats that are now occupied by the shrikes. We examined redundant measures of habitat preference (order of settlement after migration and distribution of dominant individuals) and several reproductive performance parameters in both habitat types to investigate whether habitat preference is in line with habitat quality. Territorial males exhibited a clear preference for the recently created open areas in forests with higher-quality males settling in this habitat type earlier. Reproductive performance was, however, higher in farmland, with higher nest success, offspring quantity, and quality compared to open areas in forests. The results showed strong among-year consistency and we can therefore exclude a transient situation. This study demonstrates a case of maladaptive habitat selection in a farmland bird expanding its breeding range to human-created open habitats in plantations. We discuss the reasons that could explain this decision-making and the possible consequences for the population dynamics and persistence.     

Climate change and fitness components of a migratory bird breeding in the Mediterranean region

The increase in spring temperatures in temperate regions over the last two decades has led to an advancing spring phenology, and most resident birds have responded to it by advancing their onset of breeding. The pied flycatcher (Ficedula hypoleuca) is a long‐distance migrant bird with a relatively late onset of breeding with respect to both resident birds and spring phenology in Europe. In the present correlational study, we show that some fitness components of pied flycatchers are suffering from climate change in two of the southernmost European breeding populations. In both montane study areas, temperature during May increased between 1980 and 2000 and an advancement of oak leafing was detected by using the normalized difference vegetation index (NDVI) to assess tree phenology. This might result in an advancement of the peak in availability of caterpillars, the main prey during the nestling stage. Over the past 18 yr, the time of egg laying and clutch size of pied flycatchers were not affected by the increase in spring temperatures in these Mediterranean populations. However, this increase seems to have an adverse effect on the reproductive output of pied flycatchers over the same period. Our data suggest that the mismatch between the timing of peak food supply and nestling demand caused by recent climate change might result in a reduction of parental energy expenditure that is reflected in a reduction of nestling growth and survival of fledged young in our study populations. The data seem to indicate that the breeding season has not shifted and it is the environment that has shifted away from the timing of the pied flycatcher breeding season. Mediterranean pied flycatchers were not able to advance their onset of breeding, probably, because they are constrained by their late arrival date and their restricted high altitude breeding habitat selection near the southern border of their range.     

Seasonal survival estimation for a long-distance migratory bird and the influence of winter precipitation

Conservation of migratory animals requires information about seasonal survival rates. Identifying factors that limit populations, and the portions of the annual cycle in which they occur, are critical for recognizing and reducing potential threats. However, such data are lacking for virtually all migratory taxa. We investigated patterns and environmental correlates of annual, oversummer, overwinter, and migratory survival for adult male Kirtland’s warblers (Setophaga kirtlandii), an endangered, long-distance migratory songbird. We used Cormack–Jolly–Seber models to analyze two mark–recapture datasets: 2006–2011 on Michigan breeding grounds, and 2003–2010 on Bahamian wintering grounds. The mean annual survival probability was 0.58 ± 0.12 SE. Monthly survival probabilities during the summer and winter stationary periods were relatively high (0.963 ± 0.005 SE and 0.977 ± 0.002 SE, respectively). Monthly survival probability during migratory periods was substantially lower (0.879 ± 0.05 SE), accounting for ~44% of all annual mortality. March rainfall in the Bahamas was the best-supported predictor of annual survival probability and was positively correlated with apparent annual survival in the subsequent year, suggesting that the effects of winter precipitation carried over to influence survival probability of individuals in later seasons. Projection modeling revealed that a decrease in Bahamas March rainfall >12.4% from its current mean could result in negative population growth in this species. Collectively, our results suggest that increased drought during the non-breeding season, which is predicted to occur under multiple climate change scenarios, could have important consequences on the annual survival and population growth rate of Kirtland’s warbler and other Neotropical–Nearctic migratory bird species.     

Breeding phenology and winter activity predict subsequent breeding success in a trans-global migratory seabird

Inter-seasonal events are believed to connect and affect reproductive performance (RP) in animals. However, much remains unknown about such carry-over effects (COEs), in particular how behaviour patterns during highly mobile life-history stages, such as migration, affect RP. To address this question, we measured at-sea behaviour in a long-lived migratory seabird, the Manx shearwater (Puffinus puffinus) and obtained data for individual migration cycles over 5 years, by tracking with geolocator/immersion loggers, along with 6 years of RP data. We found that individual breeding and non-breeding phenology correlated with subsequent RP, with birds hyperactive during winter more likely to fail to reproduce. Furthermore, parental investment during one year influenced breeding success during the next, a COE reflecting the trade-off between current and future RP. Our results suggest that different life-history stages interact to influence RP in the next breeding season, so that behaviour patterns during winter may be important determinants of variation in subsequent fitness among individuals.     

The ecological-evolutionary interplay: density-dependent sexual selection in a migratory songbird

Little is understood about how environmental heterogeneity influences the spatial dynamics of sexual selection. Within human-dominated systems, habitat modification creates environmental heterogeneity that could influence the adaptive value of individual phenotypes. Here, we used the gray catbird to examine if the ecological conditions experienced in the suburban matrix (SM) and embedded suburban parks (SP) influence reproductive strategies and the strength of sexual selection. Our results show that these habitats varied in a key ecological factor, breeding density. Moreover, this ecological factor was closely tied to reproductive strategies such that local breeding density predicted the probability that a nest would contain extra-pair offspring. Partitioning reproductive variance showed that while within-pair success was more important in both habitats, extra-pair success increased the opportunity for sexual selection by 39% at higher breeding densities. Body size was a strong predictor of relative reproductive success and was under directional selection in both habitats. Importantly, our results show that the strength of sexual selection did not differ among habitats at the landscape scale but rather that fine-scale variation in an ecological factor, breeding density, influenced sexual selection on male phenotypes. Here, we document density-dependent sexual selection in a migratory bird and hypothesize that coarse-scale environmental heterogeneity, in this case generated by anthropogenic habitat modification, changed the fine-scale ecological conditions that drove the spatial dynamics of sexual selection.     

Waders in winter: long-term changes of migratory bird assemblages facing climate change

Effects of climate change on species occupying distinct areas during their life cycle are still unclear. Moreover, although effects of climate change have widely been studied at the species level, less is known about community responses. Here, we test whether and how the composition of wader (Charadrii) assemblages, breeding in high latitude and wintering from Europe to Africa, is affected by climate change over 33 years. We calculated the temporal trend in the community temperature index (CTI), which measures the balance between cold and hot dwellers present in species assemblages. We found a steep increase in the CTI, which reflects a profound change in assemblage composition in response to recent climate change. This study provides, to our knowledge, the first evidence of a strong community response of migratory species to climate change in their wintering areas.     

Ecological factors associated with the breeding and migratory phenology of high-latitude breeding western sandpipers

Environmental conditions influence the breeding and migratory patterns of many avian species and may have particularly dramatic effects on long-distance migrants that breed at northern latitudes. Environment, however, is only one of the ecological variables affecting avian phenology, and recent work shows that migration tactics may be strongly affected by changes in predator populations. We used long-term data from 1978 to 2000 to examine the interactions between snowmelt in western Alaska in relation to the breeding or migration phenologies of small shorebirds and their raptor predators. Although the sandpipers’ time of arrival at Alaskan breeding sites corresponded with mean snowmelt, late snowmelts did delay breeding. These delays, however, did not persist to southward migration through British Columbia, likely due to the birds’ ability to compensate for variance in the length of the breeding season. Raptor phenology at an early stopover site in British Columbia was strongly related to snowmelt, so that in years of early snowmelt falcons appeared earlier during the sandpipers’ southbound migration. These differential effects indicate that earlier snowmelt due to climate change may alter the ecological dynamics of the predator–prey system.     

Rapid changes in phenotype distribution during range expansion in a migratory bird

The capacity of species to track changing environmental conditions is a key component of population and range changes in response to environmental change. High levels of local adaptation may constrain expansion into new locations, while the relative fitness of dispersing individuals will influence subsequent population growth. However, opportunities to explore such processes are rare, particularly at scales relevant to species-based conservation strategies. Icelandic black-tailed godwits, Limosa limosa islandica, have expanded their range throughout Iceland over the last century. We show that current male morphology varies strongly in relation to the timing of colonization across Iceland, with small males being absent from recently occupied areas. Smaller males are also proportionately more abundant on habitats and sites with higher breeding success and relative abundance of females. This population-wide spatial structuring of male morphology is most likely to result from female preferences for small males and better-quality habitats increasing both small-male fitness and the dispersal probability of larger males into poorer-quality habitats. Such eco-evolutionary feedbacks may be a key driver of rates of population growth and range expansion and contraction.     

Ecological association of habitats and bird species during the breeding season in southeastern New South Wales

Bird communities in southeastern New South Wales, as determined by classification and ordination techniques, were seldom discrete. The most cohesive assemblage was associated with wet sclerophyll and rainforest habitats. These moist forests also supported the richest avifaunas and had high population densities. Although most birds occurred throughout the region, most species were uncommon. Of the 116 species recorded, 51 (44%) were found on fewer than 10% of the 143 plots censused. Only 19 species occurred on more than half the plots, with six of these recorded on more than 90% of the plots. Other than a small group of birds primarily restricted to moist forest, only the most common and widely distributed species regularly occurred together. Species were grouped with habitats on an elevational gradient from coastal forests to the tablelands. Within this elevational range, species were associated with major structural features of the vegetation. In terms of bird assemblages, open habitats were distinguished from those with dense vegetation. Plots which had been recently logged were separated from mature forest. The results indicate that plans for wildlife management should consider both the assemblages of species which are associated with major forest habitats and the requirements of individual species.     

Migration strategy and divergent sexual selection on bird song

Migratory birds are assumed to be under stronger sexual selection pressure than sedentary populations, and the fact that their song is more complex has been taken as confirmation of this fact. However, this assumes that sexual selection pressure due to both male competition and female choice increase together. A further issue is that, in many species, songs become less complex during competitive encounters; in contrast, female choice selects for more complex song, so the two selection pressures may drive song evolution in different directions. We analysed song in two sedentary and two migratory populations of blackcaps (Sylvia atricapilla), a species in which different song parts are directed to males and females. We found that migratory populations produce longer, female-directed warbles, indicating sexual selection through female choice is the strongest in these populations. However, the part of the song directed towards males is shorter and more repetitive (as observed in individual competitive encounters between males) in non-migratory populations, indicating sedentary populations, are under stronger selection due to male competition. We show for the first time that the intensity of selection pressure from male competition and female choice varies independently between populations with different migratory behaviours. Rapid alterations in the migration patterns of species are thus likely to lead to unexpected consequences for the costs and benefits of sexual signals.     

Elevated testosterone levels affect female breeding success and yolk androgen deposition in a passerine bird

Although it is well documented that testosterone (T) is an important mediator in the regulation of behaviour in male vertebrates, its functional significance in females is less understood. Experimentally increased T in adult female birds has been found to have both advantageous and detrimental effects on behaviour and fitness. In addition, T may also mediate maternal effects when it is deposited into the egg yolk, and variations in androgen concentration between eggs contribute to differences in offspring phenotype and fitness. In this study we examined the effects of experimentally elevated female T on reproductive success and yolk androgen deposition in the spotless starling. The administration of exogenous T in female spotless starlings before egg laying caused negative effects on reproductive performance: when compared to control females T-females laid fewer eggs and raised fewer chicks. We also found an effect of elevated female T on yolk androgen deposition: T-females laid eggs with greater amounts of yolk T than control females, whereas yolk androstenedione levels were not affected. Although some of these effects likely involved a direct interference of female T with female reproductive function, some of them could be due to effects operating in eggs through maladaptive high T levels.     

Extrapair paternity in relation to sexual ornamentation, arrival date, and condition in a migratory bird

We tested the novel hypothesis that arrival date in migratorybirds represents a reliable indicator of male quality that canbe used by females as a cue in extrapair mating decisions. Secondarysexual characters are often condition-dependent, and competitionfor early arrival leads to condition-dependent migration. Hence,both secondary sexual characters and arrival date are predictedto be condition-dependent indicators of male phenotypic quality.We studied the relationship between expression of a secondarysexual character, arrival date, and condition, respectively,and extrapair paternity in a Spanish population of barn swallows,Hirundo rustica. By using microsatellite markers to determinepaternity, we showed that 17.8% of all offspring (N = 674) and32.4% of all broods (N = 170) were due to extrapair paternity.Quasi-parasitism (in which the male nest owner fathered theoffspring, but the eggs were laid by another female) occurredin 2.6% of all nestlings and 2.9% of all broods. Individualswere consistent in the frequency of extrapair paternity amongfirst, second, and third broods. Males with long outermost tailfeathers, arriving early and in prime body condition, had littleextrapair paternity in their nests. This was also the case whencontrolling for the confounding effects of male age. Partialcorrelation analysis was used to investigate the direct andindirect effects of tail length, arrival date, and body conditionon extrapair paternity. Body condition accounted for most ofthe variance in extrapair paternity, whereas tail length andarrival date accounted for a smaller proportion of the variance.Body condition was strongly correlated with tail length andarrival date. However, because females cannot directly assesscondition or arrival date (males arrive before females), femalesmay obtain an indirect measure of condition and migration abilityfrom tail length and other phenotypic traits of males. Thissuggests that extrapair paternity depends on the effects ofcondition, through its indirect effects on arrival date, taillength, and other variables.     

Practical application of genomic selection in a doubled-haploid winter wheat breeding program

Crop improvement is a long-term, expensive institutional endeavor. Genomic selection (GS), which uses single nucleotide polymorphism (SNP) information to estimate genomic breeding values, has proven efficient to increasing genetic gain by accelerating the breeding process in animal breeding programs. As for crop improvement, with few exceptions, GS applicability remains in the evaluation of algorithm performance. In this study, we examined factors related to GS applicability in line development stage for grain yield using a hard red winter wheat (Triticum aestivum L.) doubled-haploid population. The performance of GS was evaluated in two consecutive years to predict grain yield. In general, the semi-parametric reproducing kernel Hilbert space prediction algorithm outperformed parametric genomic best linear unbiased prediction. For both parametric and semi-parametric algorithms, an upward bias in predictability was apparent in within-year cross-validation, suggesting the prerequisite of cross-year validation for a more reliable prediction. Adjusting the training population’s phenotype for genotype by environment effect had a positive impact on GS model’s predictive ability. Possibly due to marker redundancy, a selected subset of SNPs at an absolute pairwise correlation coefficient threshold value of 0.4 produced comparable results and reduced the computational burden of considering the full SNP set. Finally, in the context of an ongoing breeding and selection effort, the present study has provided a measure of confidence based on the deviation of line selection from GS results, supporting the implementation of GS in wheat variety development.     

Heritability of arrival date in a migratory bird

The behaviour of long-distance migratory birds is assumed to partly be under the influence of genes, as demonstrated by selection experiments. Furthermore, competition for early arrival among males may lead to condition-dependent migration associated with fitness benefits of early arrival achieved by individuals in prime condition. Here I present field data on the repeatability and the heritability of arrival date in a trans-equatorial migratory bird, the barn swallow Hirundo rustica, and I test for a genetic correlation between arrival date and the expression of a condition-dependent secondary sexual character. The repeatability was statistically significant and the heritability of arrival date was estimated to be 0.54 (s.e. = 0.15). There was no significant evidence of this estimate being inflated by environmental or maternal condition during rearing. Arrival date and migration are condition dependent in the barn swallow, with males with the most exaggerated secondary sexual characters also arriving the earliest. There was a significant genetic correlation between arrival date and tail length in male barn swallows, providing indirect evidence for a genetic basis of this condition dependence. Given the high level of heritability, arrival date could readily respond to selection caused by environmental change.     

Feather mass and winter moult extent are heritable but not associated with fitness-related traits in a long-distance migratory bird

In birds, the allocation of resources to plumage production may have important fitness consequences. However, we have only a limited understanding of how plumage traits respond to natural selection, making it difficult to predict how variation in plumage traits may contribute to the adaptation of birds to environmental change. In this study, we collected plumage-related data in a pedigreed population of a long-distance migratory bird (the Pied Flycatcher Ficedula hypoleuca) to estimate the heritability of two plumage traits: feather mass (as a proxy of feather quality) and the extent of winter moult. We further explored whether these plumage features were associated with some fitness-related traits. Variation in plumage characteristics could be explained by differences in sex, age and year, which indicates a high degree of plasticity in these traits. After controlling for these effects, however, feather mass and winter moult extent were highly repeatable (r = 0.58–0.82) and heritable (h² = 0.59–0.65), suggesting that additive genetic variation accounts for a significant proportion of the residual phenotypic variation of plumage traits in this population. Although the studied characteristics showed evolutionary potential, we did not find any relationship between plumage features and fitness-related traits like spring arrival date, egg-laying date, mating success or mating-time. We conclude that current selection on feather mass and moult extent, if existing, is weak, and that these moult-related traits are currently of minor importance for the adaptation of our study population to global warming.