Corey E. Tarwater

Young females pay higher costs of reproduction in a short-lived bird

Theory predicts a trade-off between current reproduction and future reproduction or survival. Nevertheless, costs of reproduction are often not found owing to heterogeneity in environment or individuals, or to studies not evaluating multiple costs or reproductive metrics that could influence costs. Detecting costs of reproduction is further complicated by the fact that they may increase with age if physiological condition declines in older individuals, or decrease with age if younger individuals are less efficient at acquiring resources than older individuals. We used a 37-year study to evaluate costs of reproduction in song sparrows (Melospiza melodia). Our results support theoretical expectations for short-lived species by demonstrating costs of reproduction on future survival, but not on future reproduction. We examined two metrics of reproductive allocation—reproductive effort and termination of breeding—and found that only higher reproductive effort increased costs. Thus, testing of multiple allocation metrics may be necessary because results may not be coincident between metrics. Lastly, we observed that younger females paid higher costs of reproduction than older females. Although older female sparrows senesced, they had lower costs of reproduction than younger females who may be less able to acquire food or high-quality mates. By taking into account variation among individuals and examining multiple metrics, our study provides strong support for costs of reproduction and a decrease in costs with age.Significance statementOne premise of life history theory is that reproduction is costly, but evidence of such costs remains mixed. Mixed results may arise because resources are not always limiting for all individuals or in all environments, leading to temporal, spatial, and individual heterogeneity in trade-offs between current reproduction and future reproduction or survival that can be hard to detect. Costs may also vary with age, depending on how resource acquisition and allocation vary with age. We used a 37-year study of female song sparrows (Melospiza melodia) to control statistically for individual and environmental heterogeneity and test multiple cost metrics. We demonstrate marked costs of reproduction on future survival, particularly in young females.

Social information cascades influence the formation of mixed-species foraging aggregations of ant-following birds in the Neotropics

Animals frequently make decisions based on social information obtained from other animals, which can influence interspecific interactions and affect individual fitness. For example, animals eavesdrop on other animals to find profitable food resources, yet the types of cues they use and how these cues influence decisions to approach a resource remain poorly understood. In tropical systems, arthropods inadvertently flushed by army ant, Eciton burchellii, swarms are an important food resource for many bird species, which form mixed-species foraging aggregations at swarms. Competition at swarms is intense and birds vocalize to defend foraging areas, inadvertently producing acoustic social information about the swarms location. Eavesdropping birds may use these acoustic cues, which provide information about the bird aggregation (i.e. species participating in the aggregation, the size of the aggregation and/or diversity of the aggregation) to assess potential benefits (food resources) and costs (competition for food) of joining an aggregation. To test this hypothesis, we used an acoustic playback experiment to simulate aggregations of birds foraging at ant swarms and we measured community-wide and guild-specific responses of forest birds to playbacks. We included three types of acoustic social information in playbacks that potentially interact to affect an eavesdropping birds probability of attraction to a swarm: (1) aggregation size, (2) aggregation species richness and (3) degree of specialization on ant swarms for food of birds vocalizing in the aggregation (hereafter ‘dependency’). Using Bayesian generalized linear mixed models, we found that playbacks of obligate ant-following species elicited greater community-wide responses (i.e. attracted more individuals and species) to simulated aggregations compared to playbacks of other, less dependent guilds. We also found that interactions between dependency, species richness and aggregation size influenced the overall community response to playbacks and that species from one guild generally responded to the guild above them (i.e. from less to more specialized). Our results suggest that species evaluate multiple types of acoustic cues representing the costs and benefits of foraging in a mixed-species aggregation at a swarm. We hypothesize that species change from information receivers to information producers upon joining a swarm, ultimately producing an information cascade that further affects the dynamics of feeding aggregations at swarms.

Examination of context-dependent effects of natal traits on lifetime reproductive success using a long-term study of a temperate songbird

ABSTRACT Identifying the causes of individual variation in fitness should improve predictions about population dynamics and responses of populations to environmental change. Precise predictions may require long-term studies to parameterize models when the fitness of individual phenotypes depends on environmental conditions. We used a 37-yr study of a resident Song Sparrow (Melospiza melodia) population to identify traits that predicted individual variation in female lifetime reproductive success and to test for context dependence in trait–fitness relationships. Specifically, we asked how individual inbreeding coefficient, maternal age, and a suite of natal morphological traits influenced 2 components of lifetime reproductive success: (1) the probability of surviving to breed, and (2) the lifetime number of offspring produced, given that a female bred locally. We then tested whether population density influenced trait–fitness relationships. We found that differences in natal traits had life-long impacts on female fitness. Lower maternal age, a higher inbreeding coefficient, later laying date, lower nestling body condition, and longer tarsi were all negatively related to lifetime reproductive success. Maternal age and the inbreeding coefficient influenced both components of lifetime reproductive success, whereas other factors only influenced one. Therefore, traits that predict the probability of surviving to breed locally may differ from those that predict the number of offspring produced. We also observed larger effects of the inbreeding coefficient on fitness in years of low population density, which were often preceded by cool winters. Our findings demonstrate that natal traits and the environment experienced early in a birds life can have life-long effects on individual fitness, primarily independent of population density.

Demographic consequences of invasion by a native, controphic competitor to an insular bird population

Species invasions and range shifts can lead to novel competitive interactions between historically resident and colonizing species, but the demographic consequences of such interactions remain controversial. We present results from field experiments and 45 years of demographic monitoring to test the hypothesis that the colonization of Mandarte Is., BC, Canada, by fox sparrows (Passerella iliaca) caused the long-term decline of the resident population of song sparrows (Melospiza melodia). Several lines of evidence indicate that competition with fox sparrows for winter food reduced over-winter survival in juvenile song sparrows by 48% from 1960 to 2015, enforcing population decline despite an increase in annual reproductive rate in song sparrows over the same period. Preference for locally abundant seeds presented at experimental arenas suggested complete overlap in diet in song and fox sparrows, and observations at arenas baited with commercial seed showed that fox sparrows displaced song sparrows in 91–100% of interactions in two periods during winter. In contrast, we found no evidence of interspecific competition for resources during the breeding season. Our results indicate that in the absence of marked shifts in niche dimension, range expansions by dominant competitors have the potential to cause the extirpation of historically resident species when competitive interactions between them are strong and resources not equitably partitioned.

Individual fitness and the effects of a changing climate on the cessation and length of the breeding period using a 34-year study of a temperate songbird

Studies of the phenological responses of animals to climate change typically emphasize the initiation of breeding although climatic effects on the cessation and length of the breeding period may be as or more influential of fitness. We quantified links between climate, the cessation and length of the breeding period, and individual survival and reproduction using a 34-year study of a resident song sparrow (Melospiza melodia) population subject to dramatic variation in climate. We show that the cessation and length of the breeding period varied strongly across years, and predicted female annual fecundity but not survival. Breeding period length was more influential of fecundity than initiation or cessation of breeding alone. Warmer annual temperature and drier winters and summers predicted an earlier cessation of breeding. Population density, the date breeding was initiated, a females history of breeding success, and the number of breeding attempts initiated previously also predicted the cessation of breeding annually, indicating that climatic, population, and individual factors may interact to affect breeding phenology. Linking climate projections to our model results suggests that females will both initiate and cease breeding earlier in the future; this will have opposite effects on individual reproductive rate because breeding earlier is expected to increase fecundity, whereas ceasing breeding earlier should reduce it. Identifying factors affecting the cessation and length of the breeding period in multiparous species may be essential to predicting individual fitness and population demography. Given a rich history of studies on the initiation of breeding in free-living species, re-visiting those data to estimate climatic effects on the cessation and length of breeding should improve our ability to predict the impacts of climate change on multiparous species.

Heterospecific eavesdropping in ant-following birds of the Neotropics is a learned behaviour

Animals eavesdrop on other species to obtain information about their environments. Heterospecific eavesdropping can yield tangible fitness benefits by providing valuable information about food resources and predator presence. The ability to eavesdrop may therefore be under strong selection, although extensive research on alarm-calling in avian mixed-species flocks has found only limited evidence that close association with another species could select for innate signal recognition. Nevertheless, very little is known about the evolution of eavesdropping behaviour and the mechanism of heterospecific signal recognition, particularly in other ecological contexts, such as foraging. To understand whether heterospecific eavesdropping was an innate or learned behaviour in a foraging context, we studied heterospecific signal recognition in ant-following birds of the Neotropics, which eavesdrop on vocalizations of obligate ant-following species to locate and recruit to swarms of the army ant Eciton burchellii, a profitable food resource. We used a playback experiment to compare recruitment of ant-following birds to vocalizations of two obligate species at a mainland site (where both species are present) and a nearby island site (where one species remains whereas the other went extinct approx. 40 years ago). We found that ant-following birds recruited strongly to playbacks of the obligate species present at both island and mainland sites, but the island birds did not recruit to playbacks of the absent obligate species. Our results strongly suggest that (i) ant-following birds learn to recognize heterospecific vocalizations from ecological experience and (ii) island birds no longer recognize the locally extinct obligate species after eight generations of absence from the island. Although learning appears to be the mechanism of heterospecific signal recognition in ant-following birds, more experimental tests are needed to fully understand the evolution of eavesdropping behaviour.