Population Of Song Sparrows Research Articles

NEST PREDATION AND COWBIRD PARASITISM IN SONG SPARROWS

Obligate brood parasitic birds may improve their reproductive success if they cause the failure of nests that they had not parasitized, because many host species renest soon after failed nesting attempts and replacement nests may be available for future parasitism. Presently there is conflicting evidence on a key correlate of this parasite-predator hypothesis, namely whether parasitized nests survive at higher rates than non-parasitized nests. Using data collected by volunteers for the Cornell Nest Record Program and by examining nest survival in a northeastern population of Song Sparrows (Melospiza melodia) where parasitism by Brown-headed Cowbirds (Molothrus ater) was common, I found that parasitized sparrow nests in the nestcard sample were more likely to survive at the early stages of the nesting cycle than non-parasitized nests. In addition, while overall reproductive success in the focal study population did not differ significantly between parasitized and non-parasitized hosts, non-parasitized nests were significantly more likely to fail due to predation prior to hatching than non-parasitized nests. Whether the correlation between the presence of parasitic eggs and decreased predation occurs due to cowbirds causing the failure of non-parasitized nests or, alternatively, preference for safe host nests and/or the protection of parasitized nests by parasitic females, will require direct observations and experimental manipulations.

Estimating the time to extinction in an island population of song sparrows.

We estimated and modelled how uncertainties in stochastic population dynamics and biases in parameter estimates affect the accuracy of the projections of a small island population of song sparrows which was enumerated every spring for 24 years. The estimate of the density regulation in a theta-logistic model (theta = 1.09 suggests that the dynamics are nearly logistic, with specific growth rate r1 = 0.99 and carrying capacity K = 41.54. The song sparrow population was strongly influenced by demographic (ŝigma2(d) = 0.66) and environmental (ŝigma2(d) = 0.41) stochasticity. Bootstrap replicates of the different parameters revealed that the uncertainties in the estimates of the specific growth rate r1 and the density regulation theta were larger than the uncertainties in the environmental variance sigma2(e) and the carrying capacity K. We introduce the concept of the population prediction interval (PPI), which is a stochastic interval which includes the unknown population size with probability (1 - alpha). The width of the PPI increased rapidly with time because of uncertainties in the estimates of density regulation as well as demographic and environmental variance in the stochastic population dynamics. Accepting a 10% probability of extinction within 100 years, neglecting uncertainties in the parameters will lead to a 33% overestimation of the time it takes for the extinction barrier (population size X = 1) to be included into the PPI. This study shows that ignoring uncertainties in population dynamics produces a substantial underestimation of the extinction risk.

Perch Proximity Correlates with Higher Rates of Cowbird Parasitism of Ground Nesting Song Sparrows

The reproductive success of avian brood parasites depends, to a great extent, on their ability to locate host nests that are at the appropriate stages of the host laying cycle. Consequently, brood parasites are expected to possess elaborate mechanisms and search modes to locate potential host nests. Through observing a population of Song Sparrows (Melospiza melodia) parasitized by the Brown-headed Cowbird (Molothrus ater) we examined two specific factors that may influence cowbird parasitism of a ground nesting host. Proximity to potential perches was a significant predictor of cowbird parasitism, but overhead nest visibility, either classified dichotomously as visible or not, or measured as the absolute area of a nest visible to an observer, was not correlated with the likelihood of parasitism. Comparisons with previous studies suggest that female cowbirds use similar nest searching mechanisms in open habitats, irrespective of the height of host nests.

Bayesian meta‐analysis of demographic parameters in three small, temperate passerines

Accurate estimates of population parameters are vital for estimating extinction risk. Such parameters, however, are typically not available for threatened populations. We used a recently developed software tool based on Markov Chain Monte Carlo methods for carrying out Bayesian inference (the BUGS package) to estimate four demographic parameters; the intrinsic growth rate, the strength of density dependence, and the demographic and environmental variance, in three species of small temperate passerines from two sets of time series data taken from a dipper and a song sparrow population, and from previously obtained frequentist estimates of the same parameters in the great tit. By simultaneously modeling variation in these demographic parameters across species and using the resulting distributions as priors in the estimation for individual species, we improve the estimates for each individual species. This framework also allows us to make probabilistic statements about plausible parameter values for small passerines temperate birds in general which is often critically needed in management of species for which little or no data are available. We also discuss how our work relates to recently developed theory on dynamic stochastic population models, and finally note some important differences between frequentist and Bayesian methods.

Song sharing in two populations of song sparrows ( Melospiza melodia )

Sharing song types with immediate neighbors is widespread in birds with song repertoires, and sharing songs may confer a selective advantage in some cases. Levels of song sharing vary between different geographical populations of several bird species, and ecological differences often correlate with differences in singing behavior; in particular, males in migratory subspecies often share fewer songs than males in resident subspecies. The song sparrow (Melospiza melodia) appears to fit this pattern: resident song sparrows in western North America generally share 20–40% of their repertoire (of about eight songs) with each neighbor, while migratory subspecies from eastern North America often share 10% or less. We compared song sharing in two populations within a single subspecies of song sparrow (M. m. morphna) in Washington State. These populations, separated by only 120 km, nonetheless differ in migratory tendencies and several other ecological and life history variables. We recorded complete song repertoires from 11 male song sparrows in a high-elevation, migrating population at Gold Creek in west-central Washington, and compared them to two samples (n = 15 and n = 36) from a coastal, resident population at Discovery Park, Seattle, Washington. Despite major differences in habitat, population density, and migratory tendencies, song sharing among Gold Creek males was as high as that among Discovery Park males. In both populations, sharing was highest between immediate neighbors, and declined with distance. We conclude that at the within-subspecies level, neither migration nor population density affect song sharing in song sparrows, a song repertoire species.

Ecological correlates of song learning in song sparrows

Most studies of song learning have been conducted in the laboratory, and thus little is known about how song learning is affected by ecological variables in nature. Taking an ecological perspective, we studied song learning and territory establishment in a sedentary population of song sparrows (Melospiza melodia). We examined the song repertoires of an entire cohort of males (41 subjects) hatched in 1992 and compared them to those of potential song tutors (adults who were present in the young birds’ first year). We found that a young bird learns songs from multiple tutors who were neighbors in his first year and usually establishes a territory among or near these tutors. The degree to which tutors influenced the repertoires of the young birds varied greatly. Adult males who survived into 1993 tutored more songs than those who did not survive, supporting the hypothesis that a young male’s repertoire is influenced by social interactions with adults continuing beyond the classical sensitive period of the natal summer. The final repertoire of a young bird in most cases was weighted toward one of his tutors with whom he continued to interact, as an immediate neighbor, into his first spring. We found no correlations between potential measures of male quality or vigor and degree of tutor influence.

Geographic analysis of nucleotide diversity and song sparrow (Aves: Emberizidae) population history.

Mitochondrial DNA (mtDNA) control-region (CR) sequences were analysed to address three questions regarding the evolution of geographical variation in song sparrows. (i) Are mtDNA sequences more informative about phylogenetic relationships and population history than previously published restriction fragment (RFLP) data? (ii) Are song sparrow CR sequences evolving in a selectively neutral manner? (iii) What do the haplotype cladogram and geographical pattern of nucleotide diversity (pi) suggest about the recent evolutionary history of song sparrow populations? Results from phylogenetic analyses of CR sequences corroborate RFLP results and reveal instances in which haplotypes do not group by locality. Neutrality tests (Tajima 1989a) suggest that song sparrow mtDNA is evolving in a selectively neutral manner, although exceptions are noted. A novel geographical pattern of pi suggests a model of song sparrow population history involving multiple Pleistocene refugia and colonization of some formerly glaciated regions from multiple sources. Moreover, application of coalescence theory to the haplotype cladogram suggests that two different haplotypes (48NF and 151HA) may have predominated in different parts of the song sparrow's range. This model provides insight into the current distribution of song sparrow mtDNA haplotypes and may explain the discordance between evolutionary history inferred from mtDNA and morphology in this species.

No Evidence for Inbreeding Avoidance in a Natural Population of Song Sparrows (Melospiza melodia)

We studied mate choice and inbreeding avoidance a natural population of song sparrows (Melospiza melodia) on Mandarte Island, Canada. Inbreeding occurred regularly: 59% all matings were between known relatives. We tested for inbreeding avoidance by comparing the observed levels of inbreeding to those expected if mate choice had been random with respect to relatedness. Independent of our assumptions about the availability of mates in the random mating model, we found that the expected and observed distributions of inbreeding coefficients were similar, as was the expected and observed frequency of close (f >/= 0.125) inbreeding. Furthermore, there was no difference in relatedness observed pairs and those that would have resulted had birds mated instead with their nearest neighbors. The only evidence to suggest any inbreeding avoidance was a reduced rate of parent-offspring matings as compared to one random mating model but not the other. Hence, despite substantial inbreeding depression in this population, we found little evidence for inbreeding avoidance through mate choice. We present a simple model to suggest that variation in inbreeding avoidance behaviors in birds may arise from differences in survival rates: in species with low survival rates, the costs of forfeiting matings to avoid inbreeding may exceed the costs of inbreeding.

Song-type sharing in song sparrows: implications for repertoire function and song learning

One hypothesis for the function of song repertoires is that males learn multiple song types so that they may share songs with neighbors, allowing them to match during territorial interactions. In at least one song sparrow population, in Washington, territorial males share a high proportion of song types with their neighbors and use these shared songs in matching. We recorded song sparrows in Pennsylvania and quantified sharing of whole songs and song segments. We found that song sharing is an order of magnitude less common in the Pennsylvania population. We found sharing of song segments to be significantly more common than the sharing of whole songs in three of the five fields we examined, while we found no significant differences between whole and partial song sharing in the remaining two fields. Finally, we found no evidence that sharing is greater between birds in the same field compared to birds in different fields. Taken with the data from Washington song sparrows, these results provide evidence for intraspecific geographic variation in the organization of song repertoires, and suggest that song sharing has not been a strong selective force in the evolution of song repertoires in song sparrows as a species. Furthermore, Washington and Pennsylvania song sparrows differ in how they learn song, in that Washington birds copy whole songs, while Pennsylvania birds appear to copy and recombine song segments, as has been found in laboratory studies of song learning. Thus both song learning and the function of song repertoires differ between populations of song sparrows. Such intraspecific geographic variation offers a unique opportunity to explore the ecological and historical factors which have influenced the evolution of song.

Allometric influence on phenotypic variation in the Song Sparrow (Melospiza melodia)

Song Sparrow (Melospiza melodia) populations found along the Pacific Coast of North America, from Baja California to the islands off the coast of Alaska, exhibit extensive morphological variation. With a multivariate analysis of size and shape, I describe a portion of this pattern and examine how it could be maintained despite gene flow among the populations. Because shape differences fall along geographic barriers, I suggest that similarities among Song Sparrow populations in multivariate shape reflect their pattern of genetic relatedness. A general pattern of Song Sparrow post-nestling growth allometry has been discovered: bill characteristics are positively allometric and all other characteristics are negatively allometric. In contrast to shape, multivariate patterns of body size variation do not correspond to geographic relationships. In combination with evidence of Song Sparrow phenotypic plasticity, it is proposed that multivariate body size is an environmentally plastic trait and that specific traits exhibit levels of phenotypic plasticity in proportion to their rate of growth with respect to body size. In this way local environmental factors which alter body size may change an entire suite of allometrically related traits and thus create striking patterns of morphological variation.

Allometric influence on phenotypic variation in the Song Sparrow (Melospiza melodia)

Song Sparrow (Melospiza melodia) populations found along the Pacific Coast of North America, from Baja California to the islands off the coast of Alaska, exhibit extensive morphological variation. With a multivariate analysis of size and shape, I describe a portion of this pattern and examine how it could be maintained despite gene flow among the populations. Because shape differences fall along geographic barriers, I suggest that similarities among Song Sparrow populations in multivariate shape reflect their pattern of genetic relatedness. A general pattern of Song Sparrow post-nestling growth allometry has been discovered: bill characteristics are positively allometric and all other characteristics are negatively allometric. In contrast to shape, multivariate patterns of body size variation do not correspond to geographic relationships. In combination with evidence of Song Sparrow phenotypic plasticity, it is proposed that multivariate body size is an environmentally plastic trait and that specific traits exhibit levels of phenotypic plasticity in proportion to their rate of growth with respect to body size. In this way local environmental factors which alter body size may change an entire suite of allometrically related traits and thus create striking patterns of morphological variation.

Inbreeding and Its Fitness Effects in an Insular Population of Song Sparrows (Melospiza melodia)

Inbreeding and Its Fitness Effects in an Insular Population of Song Sparrows (Melospiza melodia)

INBREEDING AND ITS FITNESS EFFECTS IN AN INSULAR POPULATION OF SONG SPARROWS (MELOSPIZA MELODIA).

Inbreeding depression is thought to be a major factor affecting the evolution of mating systems and dispersal. While there is ample evidence for inbreeding depression in captivity, it has rarely been documented in natural populations. In this study, I examine data from a long-term demographic study of an insular population of song sparrows (Melospiza melodia) and present evidence for inbreeding depression. Forty-four percent of all matings on Mandarte Island, British Columbia, were among known relatives. Offspring of a full-sib mating (f = 0.25) experienced a reduction in annual survival rate of 17.5% on average. Over their lifetime, females with f = 0.25 produced 48% fewer young that reached independence from parental care. In contrast, male lifetime reproductive success was not affected by inbreeding. Reduced female lifetime reproductive success was mostly due to reduced hatching rates of the eggs of inbred females. Relatedness among the parents did not affect their reproductive success. Using data on survival from egg stage to breeding age, I estimated the average song sparrow egg on Mandarte Island to carry a minimum of 5.38 lethal equivalents (the number of deleterious genes whose cumulative effect is equivalent to one lethal); 2.88 of these lethal equivalents were expressed from egg stage to independence of parental care. This estimate is higher than most estimates reported for laboratory populations and lower than those reported for zoo populations. Hence, the costs of inbreeding in this population were substantial and slightly above those expected from laboratory studies. Variability in estimates of lethal equivalents among years showed that costs of inbreeding were not constant across years.

No Evidence for Inbreeding Avoidance in a Natural Population of Song Sparrows (Melospiza melodia)

We studied mate choice and inbreeding avoidance a natural population of song sparrows (Melospiza melodia) on Mandarte Island, Canada. Inbreeding occurred regularly: 59% all matings were between known relatives. We tested for inbreeding avoidance by comparing the observed levels of inbreeding to those expected if mate choice had been random with respect to relatedness. Independent of our assumptions about the availability of mates in the random mating model, we found that the expected and observed distributions of inbreeding coefficients were similar, as was the expected and observed frequency of close (f ≥ 0.125) inbreeding. Furthermore, there was no difference in relatedness observed pairs and those that would have resulted had birds mated instead with their nearest neighbors. The only evidence to suggest any inbreeding avoidance was a reduced rate of parent‐offspring matings as compared to one random mating model but not the other. Hence, despite substantial inbreeding depression in this population, we found little evidence for inbreeding avoidance through mate choice. We present a simple model to suggest that variation in inbreeding avoidance behaviors in birds may arise from differences in survival rates: in species with low survival rates, the costs of forfeiting matings to avoid inbreeding may exceed the costs of inbreeding.

Nest Predation and Cowbird Parasitism Create a Demographic Sink in Wetland-Breeding Song Sparrows

We studied mechanisms underlying poor breeding performance in a color-banded population of Song Sparrows (Melospiza melodia) in coastal British Columbia for four years. Only 21% of sparrow nests produced fledglings. The main causes of nest failure were complete predation (42% of nests), and abandonment after parasitism by Brown-headed Cowbirds (Molothrus ater) coincided with partial predation (34%). Increasingly fewer sparrow young fledged as the number of cowbird eggs per nest increased, but nests with up to five cowbird eggs fledged cowbirds. Many nests produced only cowbird fledglings. Annual production of recruits was seven times below the annual rate of adult disappearance. Nevertheless, Song Sparrow numbers at the study site remained stable due to immigration. When reproductive success was estimated in the absence of cowbird parasitism, production of young remained poor. Thus, high levels of predation on nest contents explained the poor breeding performance. Experiments with dummy Song Sparrow eggs in Song Sparrow nests suggested that mice, shrews, and medium-sized birds were the principal egg predators, but these small mammals probably were not frequent predators on Song Sparrow eggs. Damage by bird predators coincided with the laying season of the cowbird, suggesting that cowbirds contributed to predation on sparrow nests. The study site is a population sink for Song Sparrows because of high rates of nest predation and brood parasitism. These ecological factors may be contributing to the significant long-term decline (1966-1994) of the Song Sparrow in British Columbia and throughout Canada.

Nest predation by cowbirds and its consequences for passerine demography.

Brood parasitism by brown-headed cowbirds (Molothrus ater) reduces reproductive success in many passerines that nest in fragmented habitats and ecological edges, where nest predation is also common. We tested the hypothesis that parasitism and predation are often linked because cowbirds depredate nests discovered late in the host's nesting cycle to enhance future opportunities for parasitism. Over a 20-year study period, brood parasitism by cowbirds was a prerequisite to observing marked inter- and intraannual variation in the rate of nest failure in an insular song sparrow (Melospiza melodia) population. Nest failure increased with the arrival and laying rate of cowbirds and declined when cowbirds ceased laying. The absence or removal of cowbirds yielded the lowest nest failure rates recorded in the study. The absence of cowbirds also coincided with the absence of an otherwise strong positive correlation between host numbers and the annual rate of nest failure. Host numbers, cowbird parasitism, and nest failure may be correlated because cowbirds facilitate nest failure rather than cause it directly. However, an experiment mimicking egg ejection by cowbirds did not affect nest failure, and, contrary to the main prediction of the predation facilitation hypothesis, naturally parasitized nests failed less often than unparasitized nests. Higher survival of parasitized nests is expected under the cowbird predation hypothesis when female cowbirds defend access to hosts because cowbirds should often depredate unparasitized nests but should not depredate nests they have laid in. Where female cowbirds have overlapping laying areas, we expect parasitized nests to fail more often than others if different cowbirds often discover the same nests. We suggest that nest predation by cowbirds represents an adaptation for successful parasitism and that cowbirds influence host demography via nest predation.

Effects of granular formulations of terbufos and fonofos applied to cornfields on mortality and reproductive success of songbirds

: This field study used a precise method of assessing effects of insecticides on bird populations by establishing a marked population before application and determining survival rates of known individuals after application of the insecticides. We investigated the effects of two organophosphate granular insecticides applied to cornfields in southwestern Ontario to combat the corn rootworm (Diabrotica spp.). Nine control fields and 11 treated fields, six of which were treated with Fonofos (Dyfonate 20G) and five treated with Terbufos (Counter 15G), were chosen. A colour-banded population of 228 Song Sparrows (Melospiza melodia) was established and numbers monitored before and after application of the insecticides. Activity watches showed that territorial individuals spent at least some of their time foraging on the cornfields. There was no evidence that the insecticides affected the survivorship of either male or female Song Sparrows in the test population. Results from analyses of counts suggest no dramatic impact in three other species of passerines (Horned Lark Eremophila alpestris, Savannah Sparrow Passerculus sandwichensis and Vesper Sparrow Pooecetes gramineus). Sublethal effects of the insecticides on reproductive success of the Song Sparrow population were investigated. Ninety-one nests were found and their outcome (successful or failed) determined. Nest watches showed that parents collected some food for their young from cornfields in at least half of the nests. None of the reproductive parameters measured - reproductive success, clutch size, number of unhatched eggs, number of young fledged per nest - differed among the two types of treated fields and the control fields. In conclusion, the Song Sparrows studied were not ingesting the insecticide granules in lethal doses or in amounts sufficient to affect reproductive success and, hence, no differences in survivorship of adults nor in those reproductive parameters measured among the three field types were detected.

Brown-Headed Cowbirds and an Island Population of Song Sparrows: A 16-Year Study

We studied brood parasitism by Brown-headed Cowbirds (Molothrus ater) in a strongly fluctuating island population of Song Sparrows (Melospiza melodia). One to three cowbird females visited the island daily to search for host nests in 13 of 16 study years. Individual cowbirds visited on more days and laid more eggs at high sparrow densities, but some factor, perhaps aggression by territorial female cowbirds, limited cowbird numbers at high sparrow densities. Female cowbirds exhibited natal and breeding philopatry. Individual cowbirds laid about every other day on average, began to lay well after sparrows in most years, and avoided laying two eggs in the same nest. Cowbirds rarely laid after hosts had completed laying, and their eggs hatched as often as host eggs. Parasitism rates increased with host age, probably because of age-related differences in host behavior. Cowbirds removed a host egg from two-thirds of parasitized nests, and damaged two per cent ofremaining host eggs. Parasitism reduced production of fledgling sparrows per nest by 0.79, but only by 0.27 fledglings in years when food was added experimentally to some territories. Nests failed more often during incubation in years with than without parasitism, supporting our earlier suggestion that cowbirds destroy eggs in host nests to enhance future laying opportunities. Colonization of Mandarte Island by cowbirds had little effect on sparrow numbers because of: (1) incomplete overlap in laying seasons; (2) multiple broods in sparrows; (3) functional and numerical responses of cowbirds to host density; (4) the sparrows' ability to rear their young with young cowbirds; and (5) density-dependent host reproductive success.

Control of territorial aggression in a changing environment

A sedentary population of song sparrows, Melospiza melodia morphna, in western Washington State shows year-round territoriality but in different contexts. During the breeding season (March–August), all defend multiple purpose territories as monogamous pairs, or alone, but only about 30% of individuals remain on the same territory during the nonbreeding season (September–February). During the breeding season, territorial behavior is tightly correlated with circulating levels of luteinizing hormone (LH) and testosterone, but identical behaviors during the nonbreeding season appear to be expressed independently of gonadal hormones. First summer males establish territories with basal plasma levels of LH and testosterone. Castrated male song sparrows defend territories equally well as intact males in autumn and continue to do so even through onset of the breeding season. It can therefore be questioned whether testosterone has any role in activation of territorial behavior in this species. Experiments in which testosterone levels were manipulated indicate that testosterone does not simply activate territorial behavior but enhances “persistence” of aggression during the breeding season. It is suggested that although territorial behavior may be expressed year-round, the context, neural, and hormonal bases of territoriality may change dramatically.

Repeatable Reproduction in Song Sparrows

I looked for evidence of consistent variation in individual quality of breeding females in a population of Song Sparrows (Melospiza melodia). Consistent differences in in3;2;­ dividual quality were detected by looking for correlations in reproductive parameters between nesting attempts. Seven reproductive traits were examined for evidence of variation in in3;2;­ dividual quality. In addition, I determined whether the degree of repeatability was affected by differences in conditions among years, by female age, or by success of previous nesting attempts. There were consistent differences among females in the date of initiation of nesting, clutch size, egg mass, and nutritional condition of offspring. The differences in egg mass between females may be heritable. There was evidence that repeatability of nestling condition is influenced by the success of previous nesting attempts. Although consistent differences were found in quality of female sparrows, there were no "superparents." Females that were good at one given aspect of reproduction were not proficient at all facets of reproduction.