Population Of Song Sparrows Research Articles

Song-type sharing in a population of Song Sparrows in the eastern United States

Male Song Sparrows (Melospiza melodia) sometimes interact with neighboring territory owners by song-type matching or repertoire matching. In some Song Sparrow populations, levels of song sharing are high and most neighbors can interact by matching, but levels of song sharing are much lower in other populations, limiting the degree to which males can match their neighbors. One explanation for variation in sharing levels is that the importance of song-type and repertoire matching, and therefore the extent of song sharing, varies geographically in North America, being greater in western populations than eastern populations. However, to date, two studies of eastern populations have provided conflicting evidence concerning levels of song sharing by Song Sparrows. Thus, we measured sharing of whole songs and introductory phrases of songs between males with adjacent territories in another population of Song Sparrows in the eastern United States (North Carolina), near the eastern and southern limits of the species’ breeding range. Males (N = 17) in our study shared an average of only 8.7% of their song types with neighbors, and more than half of neighbor pairs shared no whole songs. Sharing of introductory phrases was more common (mean = 22.8%). The level of whole song sharing in our study is the second lowest yet reported for any Song Sparrow population, supporting the hypothesis that sharing is generally lower in eastern than in western populations. Tipo de canto compartido en una población de Melospiza melodia en la parte este de los Estados Unidos Los machos del Gorrión melódico (Melospiza melodia) en ocasiones interaccionan con los dueños de territorios vecinos, pareando la canción tipo o el repertorio. En algunas poblaciones del gorrión, el nivel de canciones compartidas es alto y muchos vecinos pueden interactuar pareando el mismo, pero en otras poblaciones, el nivel de canciones compartidas es mucho menor, limitando el grado de pareo del canto de sus vecinos. Una explicación para la variabilidad en el nivel de compartir es que la importancia del tipo de canto, pareo del repertorio, y por ende la extensión de compartir las vocalizaciones, varía geográficamente en Norte América, siendo mayor en las poblaciones del oeste que en las del este del continente. Sin embargo, al presente, dos estudios de poblaciones del este han provisto evidencia conflictiva sobre los niveles del compartir del canto en el Gorrión melódico. Por ende, medimos el compartir de canciones completas y de frases de introducción entre machos de territorios adyacentes en otra población del este, en Carolina del Norte, cerca del límite este y sur del área de reproductiva geográfica de esta especie. Los machos (N = 17) en nuestra área de estudio tan solo compartieron un promedio del 8.7% de su canto con los vecinos, y más de la mitad de los vecinos, no compartieron nada de la canción. El compartir las frases introductorias resultó más común (promedio = 22.8%). El nivel de compartir la canción completa, es el segundo más bajo informado para población alguna del Gorrión melódico, lo que apoya la hipótesis, de que el compartir del canto, por lo general, es más bajo en las poblaciones del este que en las del oeste.

Distinguishing individual quality from habitat preference and quality in a territorial passerine

Theory predicts that animals breeding in heterogeneous landscapes preferentially occupy habitats likely to maximize individual fitness, but identifying those habitats has proved problematic. Many studies develop metrics of habitat quality linked to site-specific reproductive output measured in successive years, but few separate the independent effects of individual "intrinsic quality" from those due solely to the attributes of the habitats themselves. In many populations, processes such as competitive territory defense, longevity, site-fidelity, and variation in breeding density and territory size over time have the potential to limit the degree to which individual and habitat quality will be positively related in nature. However, the effects of these processes on estimates of habitat or site-specific reproductive output have not been thoroughly investigated. We show that, in an insular population of Song Sparrows (Melospiza melodia), females nested preferentially in breeding sites with high mean reproductive output assessed over 35 years, and that variation in site-specific reproductive output was positively related to female intrinsic quality, measured here as the lifetime reproductive success of individual females relative to others hatched the same year (rLRS). In contrast, vegetation traits (shrub cover, edge, and soil depth) predicted female preference for breeding sites but did not predict site-specific variation in annual reproductive output. Female quality also did not predict which females occupied more- or less-preferred breeding sites over the study period. However, mean annual reproductive output of breeding sites estimated over 35 years was strongly positively related to the quality of the females that nested in them. Overall, these results indicate that site-specific estimates of habitat quality that do not consider the quality of the individuals occupying those sites may include substantial bias due to variation in occupant quality, and thus may not reliably predict the intrinsic effects of habitat quality on individual or population fitness.

Projecting demographic responses to climate change: adult and juvenile survival respond differently to direct and indirect effects of weather in a passerine population

Few studies have quantitatively projected changes in demography in response to climate change, yet doing so can provide important insights into the processes that may lead to population declines and changes in species distributions. Using a long-term mark-recapture data set, we examined the influence of multiple direct and indirect effects of weather on adult and juvenile survival for a population of Song Sparrows (Melospiza melodia) in California. We found evidence for a positive, direct effect of winter temperature on adult survival, and a positive, indirect effect of prior rainy season precipitation on juvenile survival, which was consistent with an effect of precipitation on food availability during the breeding season. We used these relationships, and climate projections of significantly warmer and slightly drier winter weather by the year 2100, to project a significant increase in mean adult survival (12-17%) and a slight decrease in mean juvenile survival (4-6%) under the B1 and A2 climate change scenarios. Together with results from previous studies on seasonal fecundity and postfledging survival in this population, we integrated these results in a population model and projected increases in the population growth rate under both climate change scenarios. Our results underscore the importance of considering multiple, direct, and indirect effects of weather throughout the annual cycle, as well as differences in the responses of each life stage to climate change. Projecting demographic responses to climate change can identify not only how populations will be affected by climate change but also indicate the demographic process(es) and specific mechanisms that may be responsible. This information can, in turn, inform climate change adaptation plans, help prioritize future research, and identify where limited conservation resources will be most effectively and efficiently spent.

Climate, ecological release and bill dimorphism in an island songbird

Sexual size dimorphism is expected to be more pronounced in vertebrates on islands, particularly in trophic characters, as a response to decreased interspecific competition for food. We found (based on measurements of 1423 museum specimens) that bill size dimorphism was greater in island than mainland populations of song sparrows. However, dimorphism varied among islands and was positively correlated with high summer temperature and island size. Island song sparrow bills follow the overall positive temperature bill size relationship for California song sparrows, which includes larger bills on large, warmer islands. Large bills dissipate more heat and may be an adaptation to summer heat stress. Dimorphism increases because the slope for males is greater than females. Thus, the greater magnitude of bill dimorphism on islands with warmer summers may result from males experiencing greater thermal stress during territorial activity, creating different thermal optima. In contrast, bill dimorphism was unrelated to climate on the mainland. We hypothesize that reduced interspecific competition releases island populations from a constraint so that sex-specific physiological optima can be achieved, whereas mainland birds are constrained.

HPA axis regulation, survival, and reproduction in free-living sparrows: Functional relationships or developmental correlations?

A growing body of theoretical and empirical work has addressed the relationship between hypothalamus–pituitary–adrenal (HPA) function and fitness. For example, the corticosterone (CORT)-fitness and CORT-condition hypotheses predict that baseline and/or stress-induced levels of glucocorticoids should relate to fitness, and recent empirical studies have reported relationships between HPA function and fitness-related sexually selected traits. Here we introduce a framework for evaluating whether such relationships reflect functional relationships or developmental correlations. We then address this framework using data from a free-living population of song sparrows (Melospiza melodia). In two independent studies we have found that song complexity (a sexually selected trait) is correlated with stress reactivity: males with more complex vocal repertoires show reduced CORT response to standardized restraint stress. This pattern likely results from the early life environment concurrently affecting development of both song and the HPA axis. Suppression of CORT by dexamethasone was also correlated to measures of body condition and immune function, and females paired to males with higher stress-induced levels of CORT initiated egg-laying later. Finally, stress reactivity predicted overwinter survival in one year, although not in another. Thus, the relationship between HPA axis function and fitness likely varies temporally and by context. Some fitness-related traits may be functionally related to HPA regulation, but many others may be related through developmental correlation.

Fortune favours the aggressive: territory quality and behavioural syndromes in song sparrows, Melospiza melodia

While the relationship between aggression and resource value in competitions for discrete resources is well established, the relationship between aggression and territory quality is less well understood. Territoriality imposes spatial structure on social interactions: if spatial heterogeneity in habitat quality leads to clustering of high- and low-quality territories, and if consistently more aggressive individuals are more likely to obtain higher-quality territories, the resulting clusters of territory holders of similar aggression are likely to influence a wide array of social dynamics in territorial species, particularly when aggressive behaviour is consistently associated with variation in other behaviours, such as boldness. Using 8 years of historical nesting data in a northeastern population of song sparrows, we here determine (1) whether historical nesting success (defined both as clutch size and survival) is spatially clustered; and (2) whether aggression or boldness in first-year territorial males is correlated with the historical clutch size or nest survival of each male's territory. We found significant clustering of both predation risk and large clutch sizes, even when controlling for the identities of historical territory owners. The aggressive behaviour of new territorial males was correlated with historical clutch size, but not with historical nest survival; in contrast, boldness was not correlated with either historical clutch size or nest survival. Thus, spatial variation in habitat quality is correlated with territorial aggression, but territory holders may either prioritize or be unable to assess all variables contributing to territory quality. In addition, a boldness-aggression syndrome does not necessarily result in both behaviours covarying with territory quality.

Effects of urbanization on Song Sparrow (Melospiza melodia) population connectivity

Urbanization may affect genetic differentiation among animal populations because it converts native vegetation to novel land cover types that can affect population connectivity. The effect of land cover change on genetic differentiation may vary among taxa; mobile birds may be least affected. Regardless, genetic differentiation between populations should be best predicted by measures of distance that incorporate the effect of land cover on movement. We studied the relationship between land cover and genetic differentiation in Song Sparrows (Melospiza melodia) at eighteen sites in the Seattle metropolitan region. We generated a series of hypothetical “resistance surfaces” based on land cover and development age, calculated “resistance distances” between pairs of sampling sites, and related them to pairwise genetic differentiation. Genetic differentiation was best described by a multiple regression model where resistance to gene flow (1) linearly increased with age of development and (2) was greater in high- and medium-density urbanization than in native forest land cover types (R 2 = 0.15; p = 0.003). The single variable with the highest correlation with genetic differentiation was derived from a linear relationship between development age and resistance (R 2 = 0.08; p = 0.007). Our results thus suggested that urban development reduced population connectivity for Song Sparrows. However, the relation of development age to genetic differentiation suggested that equilibrium was not yet reached. Hence, the effects of lost connectivity will increase. Our understanding of the landscape genetics of this recently anthropogenically modified landscape benefited from considering population history.

EXTRA-PAIR PATERNITY AND THE VARIANCE IN MALE FITNESS IN SONG SPARROWS (MELOSPIZA MELODIA)

The variance in fitness across population members can influence major evolutionary processes. In socially monogamous but genetically polygynandrous species, extra-pair paternity (EPP) is widely hypothesized to increase the variance in male fitness compared to that arising given the socially monogamous mating system. This hypothesis has not been definitively tested because comprehensive data describing males' apparent (social) and realized (genetic) fitness have been lacking. We used 16 years of comprehensive social and genetic paternity data for an entire free-living song sparrow (Melospiza melodia) population to quantify and compare variances in male apparent and realized fitness, and to quantify the contribution of the variances in within-pair reproductive success (WPRS) and extra-pair reproductive success (EPRS) and their covariance to the variance in realized fitness. Overall, EPP increased the variance in male fitness by only 0-27% across different fitness and variance measures. This relatively small effect reflected the presence of socially unpaired males with zero apparent and low realized fitness, small covariance between WPRS and EPRS, and large variance in WPRS that was relatively unaffected by EPP. Therefore, although EPP altered individual males' contributions to future generations, its impact on population-level parameters such as the opportunity for selection and effective population size was limited.

Behavioral Syndromes Break Down in Urban Song Sparrow Populations

Animals in urban habitats face a number of unique stresses, including the necessity of dealing with high levels of human activity. Growing research suggests that: (1) inherent traits, as opposed to learned behavior, influence which species invade urban habitats, and (2) individuals exhibit behavioral syndromes that limit behavioral flexibility. As a result, perhaps only animals with inherently bold personalities successfully settle in areas of high human activity, and such animals may also exhibit correlated variation in other behavioral traits, such as territorial aggression. In this study, we examine boldness and aggression in several urban and rural populations of song sparrows (Melospiza melodia). We found that urban birds were both bolder toward humans and also showed higher levels of aggression. We found a correlation between boldness and aggression in all populations combined, but no correlation within urban populations. Our results agree with other recent studies of song sparrow behavior, suggesting that greater boldness and aggression are general features of urban song sparrow populations, and a lack of a correlation between boldness and aggression in urban habitats is a general phenomenon as well. Urban habitats may select for bold and aggressive birds, and yet the traits can vary independently. These results add to a small number of studies which find that behavioral syndromes break down in potentially high quality habitats.

Disentangling the effect of genes, the environment and chance on sex ratio variation in a wild bird population

Sex ratio theory proposes that the equal sex ratio typically observed in birds and mammals is the result of natural selection. However, in species with chromosomal sex determination, the same 1 : 1 sex ratio is expected under random Mendelian segregation. Here, we present an analysis of 14 years of sex ratio data for a population of song sparrows (Melospiza melodia) on Mandarte Island, at the nestling stage and at independence from parental care. We test for the presence of variance in sex ratio over and above the binomial variance expected under Mendelian segregation, and thereby quantify the potential for selection to shape sex ratio. Furthermore, if sex ratio variation is to be shaped by selection, we expect some of this extra-binomial variation to have a genetic basis. Despite ample statistical power, we find no evidence for the existence of either genetic or environmentally induced variation in sex ratio, in the nest or at independence. Instead, the sex ratio variation observed matches that expected under random Mendelian segregation. Using one of the best datasets of its kind, we conclude that female song sparrows do not, and perhaps cannot, adjust the sex of their offspring. We discuss the implications of this finding and make suggestions for future research.

Micro-spatial genetic structure in song sparrows (Melospiza melodia)

The spatial genetic structure of populations is strongly influenced by current and historical patterns of gene flow and drift, which in the simplest case, is limited by geographic distance. We examined the microspatial genetic structure within 33 populations of song sparrows (Melospiza melodia) which included eight subspecies located across coastal areas in southern British Columbia (BC) and California. We also examined the effect of water barriers and local density estimates on genetic structuring. Across both regions, positive genetic structure was detectable at distances of less than 10 km. Genetic divergence was highest in Californian subspecies, perhaps due to reduced gene flow across sub-specific contact zones. In BC, populations distributed across islands displayed greater genetic structuring over similar spatial scales than those across mainland sites, supporting the prediction that water barriers reduce gene flow in this species. Our results confirm both the expectation for fine-scale genetic structure in these generally sedentary subspecies, and the role of landscape features in generating geographic variation in genetic structure.

Comprehensive paternity assignment: genotype, spatial location and social status in song sparrows, Melospiza Melodia

Comprehensive, accurate paternity assignment is critical to answering numerous questions in evolutionary ecology. Yet, most studies of species with extra-pair paternity (EPP) fail to assign sires to all offspring. Common limitations include incomplete and biased sampling of offspring and males, particularly with respect to male location and social status, potentially biasing estimated patterns of paternity. Studies that achieve comprehensive sampling and paternity assignment are therefore required. Accordingly, we genotyped virtually all males and >99% of 6-day-old offspring over 16 years in a song sparrow (Melospiza melodia) population and used three complementary statistical methodologies to attempt complete paternity assignment for all 2207 offspring. Assignments were highly consistent across maximum likelihood methods that used solely genotype data, and heuristic and integrated Bayesian analyses that included data describing individual locations. Sires were assigned to >99% of all genotyped offspring with ≥95% confidence, revealing an EPP rate of c. 28%. Extra-pair sires primarily occupied territories neighbouring their extra-pair offspring; spatial location was therefore highly informative for paternity assignment. EPP was biased towards paired territorial males, although unpaired territorial and floater males sired c. 13% of extra-pair offspring. Failing to sample and include unpaired males as candidate sires would therefore substantially reduce assignment rates. These analyses demonstrate the integration of genetic and ecological information to achieve comprehensive paternity assignment and direct biological insight, illustrate the potential biases that common forms of incomplete sampling could have on estimated patterns of EPP, and provide an essential basis for understanding the evolutionary causes and consequences of EPP.

Behavioural Syndromes in Urban and Rural Populations of Song Sparrows

AbstractAnimals in urban habitats are often noticeably bold in the presence of humans. Such boldness may arise due to habituation, as urban animals learn, through repeated exposure, that passing humans do not represent a threat. However, there is growing research suggesting that: (1) inherent traits, as opposed to learned behaviour, influence which species invade urban habitats, and (2) individuals exhibit individual personality traits that limit behavioural flexibility, with the possible result that not all individuals would be able to demonstrate an appropriate level of boldness in urban environments. As a result, perhaps only birds with inherently bold personalities could successfully settle in an area of high human disturbance, and further, we might also expect to see the existence of behavioural syndromes, where boldness is correlated with variation in other behavioural traits such as aggression. In this study, we examined boldness and territorial aggression in urban and rural populations of song sparrows. We found that urban birds were bolder towards humans and that urban birds also showed higher levels of territorial aggression. We also found an overall correlation between boldness and territorial aggression, suggesting that urban boldness may be part of a behavioural syndrome. However, we see no correlation between boldness and aggression in the urban population, and thus, more work is needed to determine the mechanisms accounting for high levels of boldness and aggression urban song sparrows.

Mate choice for nonadditive genetic benefits and the maintenance of genetic diversity in song sparrows

The lek paradox asserts that strong directional selection via female choice should deplete additive genetic variation in fitness and consequently any benefit to females expressing the preference. Recently, we have provided a novel resolution to the paradox by showing that nonadditive genetic effects such as overdominance can be inherited from parent to offspring, and populations with females that express a mating preference for outbred males maintain higher genetic variation than populations with females that mate randomly. Here, we test our dynamic model using empirical data previously published from a small island population of song sparrows (Melospiza melodia). The model assumes that fitness and male trait expression display overdominance effects. The results demonstrate that female choice for outbred males mediated by directional selection on song repertoire size provides a heritable benefit to offspring through reduced inbreeding depression. Within the population, we estimate the heritability of the inbreeding coefficient to be 0.18 +/- 0.08 (SD). Furthermore, we show that mate choice for outbred males increases fitness-related genetic variation in the population by 12% and thereby reduces inbreeding depression by 1% per generation in typical years and upwards of 15% in severe years. Thus, mate choice may help to stave off population extinction in this and other small populations.

Consequences of parasite invasion and land use on the spatial dynamics of host populations

Summary 1. Conversion of natural habitats to human use can affect the abundance and distribution of predators and parasites, create population sinks, and reduce the viability of valued prey and host species. We asked how the distribution of brown-headed cowbirds Molothrus ater , a generalist brood parasite, has influenced the source‐sink dynamics of song sparrow Melospiza melodia populations and their regional population trends. 2. We intensively studied 17 host populations subject to varying levels of parasitism for 1‐36 years. We linked these data to spatial and demographic models to predict growth rate in song sparrow populations in the Southern Gulf Islands, BC, Canada. 3. Patterns of growth in song sparrow populations were closely related to cowbird distribution, which in turn depended on land use patterns at landscape scales. Locally, sparrow populations were expected to increase in areas far from cowbird feeding areas, where parasitism was low, but to decline where parasitism exceeded 20%. The predicted population trends were similar to those recorded locally and via the North American Breeding Bird Survey. 4. Synthesis and applications . We show that the distribution of habitats favourable to brood parasites can affect whether host populations grow or decline regionally. In highly sedentary hosts like the sparrows in this study, density-dependent juvenile dispersal and marked spatial variation in the probability of parasitism can give rise to source-and-sink dynamics. Our results illustrate how the application of spatial models and empirical data can predict how land use decisions may influence host dynamics. We identify ways in which applied ecologists might influence land use to enhance the persistence of valued hosts, and suggest that our approach provides a promising framework for exploring regional-scale spatial dynamics of species in order to identify critical habitat and prioritize investments in conservation.

The contribution of island populations to in situ genetic conservation

Genetic variation is often lower within island populations, however islands may also harbor divergent genetic variation. The likelihood that insular populations are genetically diverse or divergent should be influenced by island size and isolation. We tested this assumption by comparing patterns of genetic variation across all major island song sparrow populations along the Pacific North American coast. Allelic richness was moderately lowered even on islands which are close to large, potential sources. The most significant differences in allelic richness occurred on very small or highly remote islands. Gene diversity was significantly lower only on remote or very small islands. We found that island populations contribute to regional genetic variation through both the amount of genetic variation and the uniqueness of that variation. The partitioning of this contribution was associated with the size and isolation of the island populations.

Evolutionary genetics of ageing in the wild: empirical patterns and future perspectives

Summary 1 Classical evolutionary theory states that senescence should arise as a consequence of the declining force of selection late in life. Although the quantitative genetic predictions of hypotheses derived from this theory have been extensively tested in laboratory studies of invertebrate systems, relatively little is known about the genetics of ageing in the wild. 2 Data from long-term ecological studies is increasingly allowing quantitative genetic approaches to be used in studies of senescence in free-living populations of vertebrates. We review work to date and argue that the patterns are broadly consistent with theoretical predictions, although there is also a clear need for more empirical work. 3 We argue that further advances in this field of research might be facilitated by increased use of reaction norm models, and a decreased emphasis on attempting to discriminate between mutation accumulation and antagonistic pleiotropy models of senescence. We also suggest a framework for the better integration of environmental and genetic effects on ageing. 4 Finally, we discuss some of the difficulties in applying quantitative genetic models to studies of senescence outside the laboratory. In particular we highlight the problems that viability selection can cause for an accurate estimation of parameters used in the prediction of age-trajectory evolution.

The effects of contemporary processes in maintaining the genetic structure of western song sparrows (Melospiza melodia)

Historic events and contemporary processes work in concert to create and maintain geographically partitioned variation and are instrumental in the generation of biodiversity. We sought to gain a better understanding of how contemporary processes such as movement and isolation influence the genetic structure of widely distributed vagile species such as birds. Song sparrows (Melospiza melodia) in western North America provide a natural system for examining the genetics of populations that have different patterns of geographic isolation and migratory behavior. We examined the population genetics of 576 song sparrows from 23 populations using seven microsatellite loci to assess genetic differentiation among populations and to estimate the effects of drift and immigration (gene flow) on each population. Sedentary, isolated populations were characterized by low levels of immigration and high levels of genetic drift, whereas those populations less isolated displayed signals of high gene flow and little differentiation from other populations. Contemporary dispersal rates from migratory populations, estimated by assignment test, were higher and occurred over larger distances than dispersal from sedentary populations but were also probably too low to counter the effects of drift in most populations. We suggest that geographic isolation and limited gene flow facilitated by migratory behavior are responsible for maintaining observed levels of differentiation among Pacific coastal song sparrow populations.

Influential factors for natal dispersal in an avian island metapopulation

Dispersal is a key parameter in evolutionary, demographic and conservation theory, but the factors influencing dispersal between populations are rarely known, and the contribution of immigrants to population stability remains uncertain. Using dispersal data from nine island populations of song sparrows, we show that female and male immigrants responded differently to population structure: in females, immigration varied with adult sex ratio; whereas immigration by males was more influenced by population density. These patterns are consistent with the hypothesis that intra‐sexual competition for breeding resources influenced recruitment patterns. Immigrants often constituted a substantial fraction of local population size, and in six cases immigration by females prevented the extirpation of that sex from the island. Breeding vacancies and extirpations may have been more likely in females because their apparent survival was lower than in males. Local recruitment and immigration varied markedly among islands, perhaps as consequence of island size and isolation. Overall, our results suggest that immigration varied with local demography in a sex‐specific way, stabilized population numbers and reduced extinction rates in the smallest populations.

Testing the function of song-matching in birds: responses of eastern male song sparrows Melospiza melodia to partial song-matching

AbstractSong-matching has been hypothesized to be a signal of aggressive intentions whereby matching an opponent signals that the singer is likely to attack. Theory predicts that an aggressive signal should impose a cost that enforces the signal's reliability. A receiver-dependent cost imposed by the matched bird's aggressive retaliation has been proposed for song-matching. We tested for such a cost for partial song-matching in an eastern population of song sparrows where males lack the shared song types necessary for song type matching, but can perform partial song-matching using shared song segments. We tested aggressive response, as measured by average distance to a playback speaker, to partial-matching songs and non-matching songs. We predicted a stronger aggressive response to partial-matching songs, as has been shown for whole song-matching in western song sparrow populations. The birds in our study responded no differently to partial-matching and non-matching songs. Neither the distance to the playback speaker nor singing responses differed between playback treatments. Our results do not support a receiver-dependent cost to partial song-matching, as would be expected if partial-matching is a direct threat. Instead, we suggest that partial song-matching functions as a signal of attention.