Researchers studying avian space use, dispersal, survival, and migration select tracking tags that optimize tag size (dimensions) and mass relative to the focal species to minimize risk to birds as they move and fly. Here we argue that antenna length and transmitter color are also critical tag design parameters that must be considered. Based on a review of the songbird tracking literature and unpublished cases, we found most cases of entanglement, which can result in bird injuries and death, occurred with longer antennas (>13 cm). However, such cases cannot infer rates of mortality or survival, nor may they be representative of entanglements across antenna lengths and studies using radio tags. Additionally, white or brightly colored transmitters that contrast with plumage, particularly of cryptically colored females and juveniles, may increase visibility to predators and result in higher mortality, or alternatively, cause a female to eject a nestling by mistaking the tag for a fecal sac or foreign object. Consequently, we call on researchers to report and publish cases of antenna entanglement, and for journal editors to encourage and applaud such reports, such that data can be compiled to inform guidelines on tag antenna lengths for different species. Future research is also needed to examine the impacts of tag color on parental behavior and survival of tagged birds. Meanwhile, researchers should strive to ensure that their tag color does not contrast markedly with avian plumages, while also taking care not to alter (i.e., do not color over) areas required for harness attachment, activating and deactivating tags. We hope such efforts will enable researchers and manufacturers to make more informed decisions about tracking tag design, particularly with respect to balancing the benefits of longer antennas, which provide greater power and detection distance, with risks of bird entanglement and mortality.

Among stages of avian ontogeny, the act of nest departure or fledging is an abrupt transition into a new environment and a major leap toward independence for offspring. In altricial birds, the timing (i.e. time of day) of fledging is notable in that many species tend to fledge early in the morning. Past studies have proposed nest predation as a key factor driving birds to fledge earlier in the morning (the ‘survival hypothesis’), whereby offspring avoid peak times of nest predation that occur later in the day. A natural extension of this hypothesis is the predation of offspring post‐fledging, whereby offspring are also timing their fledging with future survival prospects outside of the nest. However, few studies have investigated fledging behaviour in the context of both nesting and post‐fledging predation. To help fill this knowledge gap, we investigated factors driving the timing and duration of fledging across six songbird species in the context of offspring predation: daily nest mortality, post‐fledging mortality and diel patterns of nest predation risk. We found that > 60% of songbirds fledged early in the morning, whereas the peaks in nest predation risk occurred several hours post‐fledging. Furthermore, species under greater risk of nest predation fledged earlier in the day and in closer succession to their siblings. Parameters of post‐fledging mortality were poor predictors of fledging timing, but individuals from broods of species under higher risk of post‐fledging mortality fledged in closer succession to their siblings. These results provide evidence in support of the survival hypothesis, and suggest that songbirds fledge in the morning to avoid peak times of nest predation risk that occur later in the day (~ 8 h after civil dawn). Such results corroborate past research highlighting predation on dependent offspring as a key factor driving variation in life histories across animal taxa; however, estimates of post‐fledging mortality suggest that nest predation alone does not fully explain variation in fledging behaviour among species. Future research is therefore needed to investigate the contribution of other factors, such as energetics, parent–offspring conflict and diel patterns of post‐fledging survival, which may help to mediate diel patterns of fledging within and among songbird species.

Avian eggs develop outside of the female body, and therefore embryonic development is subject to multiple internal (physiological) and external (ecological) factors. Embryonic developmental rate has important consequences for survival. Within species, embryos that develop too quickly often experience deformities, disorders, or mortality, while embryos that develop slowly risk inviability and increase the time they are exposed to various sources of mortality in the nest. These contrasting forces may lead to interspecific variation in developmental rates. We investigated potential factors affecting embryonic heart rate (EHR), a proxy of development, across 14 passerine species in the field. More specifically, we investigated if nest predation risk, clutch size, seasonality, and egg volume influenced EHR. From previous research, we expected, and found, that EHR was positively associated with embryonic age and egg temperature. Species with greater nest predation risk had higher EHR, shorter incubation periods, and lower nest temperature variance. EHR increased as the season progressed and with egg volume, while EHR declined with clutch size. Bird species exhibit varying strategies to increase nestling and fledgling survival in response to predation risk, and these results suggest that variation in embryonic development may be related to species-specific differences in nest predation risk.

ABSTRACT The Red Goshawk (Erythrotriorchis radiatus) is one of Australia’s least encountered birds, is poorly known, and under serious threat following its recent extinction in one-third of its historical breeding range, and population decline in another one-third of that range. The species is now confined to the tropical savannas of northern Australia as a breeding resident yet our understanding of its diet and trophic niche within these environments is limited. Using a comprehensive dataset of prey records, we found that Red Goshawks are specialized avian predators targeting specific prey such as Rainbow Lorikeet (Trichoglossus moluccanus), Red-collared Lorikeet (Trichoglossus rubritorquis), Blue-winged Kookaburra (Dacelo leachii), and Sulphur-crested Cockatoo (Cacatua galerita). These bird-eating habits likely require extensive home ranges and may explain why the species exhibits extreme levels of reverse sexual size dimorphism. We found that females hunted larger prey than males, and that the size of prey increased throughout the breeding season likely due to the female hunting later in the breeding season. The size and composition of prey appeared to be influenced by breeding habitat, with riparian areas apparently supporting higher prey diversity and larger prey size including waterbirds. Prey selection by Red Goshawks appeared specialized and not proportional to the relative availability of birds within the environment. Fledgling sex did not determine the size of prey delivered to nests despite the significant size discrepancy between male and female young. We also found that the prey species that Red Goshawks most depend upon appeared to remain abundant in the Red Goshawks’ former range areas. Consequently, the loss of prey populations did not appear to account for the species’ significant recent decline, although habitat loss and change may inhibit its capacity to hunt these prey items.

American robins ( Turdus migratorius ) are among few hosts of parasitic brown-headed cowbirds ( Molothrus ater ) that frequently eject foreign eggs from their nests. However, we know little about some characteristics of the robins’ egg ejection process including its distance and direction. We used a novel technique (i.e., radio transmitters inserted into 3D-printed eggshells) to examine egg rejection in female robins as a function of model egg coloration (i.e., robin-mimetic blue, non-mimetic deep blue, and non-mimetic beige). Based on prior work, we predicted that female robins’ egg ejection decisions would be dependent on egg coloration. Accordingly, we found a significant effect of female identity (using nest identity as a proxy) and model egg colour on egg ejection, but neither predicted ejection distance or direction. Deep blue model eggs had shorter ejection latencies than beige eggs. Fully characterizing the egg ejection process will allow us to further understand the mechanisms and outcomes of egg ejection behaviours in host-parasite interactions.

Bergmann’s rule states that homeotherms are larger in colder climates (which occur at higher latitudes and elevations) due to thermoregulatory mechanisms. Despite being perhaps the most extensively studied biogeographical rule across all organisms, consistent mechanisms explaining which species or taxa adhere to Bergmann’s rule have been elusive. Furthermore, evidence for Bergmann’s rule in migratory animals has been mixed, and it was difficult to assess how environmental conditions across the full annual cycle impact body size until the recent miniaturization of tracking technology. Nightjars (Family Caprimulgidae), nocturnal birds with physiological and behavioral adaptations (e.g., torpor) to cope with the environmental extremes they often experience, offer a unique opportunity to elucidate the mechanisms underpinning Bergmann’s rule. Many nightjar species are strongly migratory and have large breeding ranges, offering the opportunity to look at variation in potential drivers within and across seasons of the annual cycle. Furthermore, variation in migration strategy within the family provides an opportunity to separate adaptations for migration strategy from adaptations for thermal tolerance. In this study, we use cross-continental data from three species of nightjars (Common nighthawk, Eastern whip-poor-will, and European nightjar) to assess 1) whether migratory species in this clade adheres to Bergmann’s rule, 2) which environmental factors are the best predictors of body size, and 3) the extent to which environmental conditions across the full annual cycle determine body size. For each species, we use breeding and winter location data from GPS tags to compare competing hypotheses explaining variation in body size: temperature regulation, productivity, and seasonality (during both the breeding and wintering periods), and migration distance. We found that Common nighthawk and Eastern whip-poor-will exhibit Bergmannian patterns in body size while European nightjar does not, although the spread of tag deployment sites on the breeding grounds was minimal for the European nightjar. Predictor variables associated with nightjar breeding locations more often explained body size than did variables on the wintering grounds. Surprisingly, models representing the geography hypothesis were best represented among important models in our final data set. Latitude and longitude correlated strongly with environmental variables and migratory distance; thus, these geographical variables offer a composite variable of sorts, summarizing many factors that likely influence body size in nightjars. Leveraging multi-species and cross-continental data across the full annual cycle, along with global environmental data, can provide insight into long-standing questions and will be important for understanding the generalizability of Bergmann’s rule.

Predation or brood parasitism risks can change the behaviors and reproductive decisions in many parental animals. For oviparous species, mothers can mitigate their reproductive success in at least three ways: (1) by avoiding nest sites with high predation or parasitism risks, (2) through hormonal maternal effects that developmentally prime offspring for survival in risky environments, or (3) by investing less in reproduction when predation or parasitism risks are high. Here, we tested if perceived predation and parasitism risks can induce any of these behavioral or physiological responses by exposing female red-winged blackbirds (Agelaius phoeniceus) to playbacks of two major nest threats, a predator (Cooper's hawk, Accipiter cooperii) and an obligate brood parasite (brown-headed cowbird; Molothrus ater), as well as two controls (harmless Eastern meadowlark, Sturnella magna; and silence). We found that female blackbirds did not avoid nesting at sites treated with predator or brood parasite playbacks, nor were females more likely to abandon nesting attempts at these sites. Egg size and yolk hormone profiles, which are common proxies for maternal investment in oviparous species, were statistically similar across treatment sites. Instead, we found intraclutch variation in yolk steroid hormone profiles: concentrations of three progestogens (pregnanedione, 17α-hydroxypregnenolone, and deoxycorticosterone) and two androgens (testosterone and androstenedione) were higher in third-laid than first-laid eggs. Our study largely confirms previous findings of consistent intraclutch yolk hormone variation in this species, in birds in general, and in other oviparous lineages, but uniquely reports on several yolk steroid hormones largely overlooked in the literature on hormone-mediated maternal effects.

Nearctic Neotropical migratory songbirds incur the highest mortality during migration. En-route, songbirds rely on a network of stopover sites to rest, refuel, and/or seek refuge during poor weather. Conservation strategies prioritize protection of sites that best meet these needs. However, the specific function of a stopover site is expected to vary in relation to factors, such as geographic location, surrounding landscape, and weather. To identify sites with the highest conservation value for migratory songbirds, a conceptual framework was independently developed to classify sites into three functional categories based on their geographic and landscape features: fire escapes, convenience stores, and full-service hotels. The few attempts to empirically validate this framework have focused on temperate stopover sites. We evaluated the framework by testing the hypothesis that a site's geographic and landscape characteristics can predict its function. We used capture and radio-tracking data at an island and mainland site in the Yucatan Peninsula, Mexico to quantify mean and variance in daily capture rate, body condition, stopover duration, and departure behavior during autumn, with a focus on four species: Swainson's Thrush (Catharus ustulatus), Red-eyed Vireo (Vireo olivaceus), Gray Catbird (Dumetella carolinensis), and Prothonotary Warbler (Protonotaria citrea). Our results supported our predictions that the island functions as a fire escape, providing refuge for very high numbers of birds after encountering crosswinds or headwinds over the Gulf of Mexico, and the mainland forest site serves as a full-service hotel. The framework provides valuable insight for strategic conservation planning and management of stopover sites for songbirds. We suggest future studies evaluate the framework's application to non-forest bird species. Additionally, we encourage collaborative efforts to consolidate and integrate tracking data, capture data from migration banding stations, and radar based bird density estimates across a broad geography to test the framework's ability to inform conservation planning across species' full migratory range.

Despite a large literature on the ontogeny of animal behavior, our understanding of behavioral development in young songbirds remains relatively poor. This is particularly true for the post-fledging period, where it is often difficult to follow and observe small, freely moving fledglings for extended periods of time. Fortunately, technological advances have enabled more robust examinations of songbird post-fledging behaviors. To provide greater insight into fledgling developmental and behavioral ecology, we used automated radio-telemetry systems to document post-fledging activity rates of juveniles across 9 songbird species of east-central Illinois, USA. Specifically, we sought to examine differences in the ontogeny of activity rates across species, and compare activity rates to intrinsic (e.g. morphological traits, age) and extrinsic (e.g. predation risk, brood size) factors that have the potential to influence fledgling behavior. While post-fledging activity rates differed by species, our findings show remarkable similarities in the ontogeny of fledgling activity across the avian community; all species exhibited a steady increase in activity rates with post-fledging age, with rates leveling off roughly 3 to 4 weeks post-fledging. These general patterns mirrored rates of post-fledging mortality (primarily due to predation) across the period, including the presence or absence of a post-fledging bottleneck, in which species with bottlenecks exhibit slight declines in activity rates during the first few days out of the nest. Alongside fledgling age and mortality risk, our findings also suggest that juvenile wing development and brood size at fledging mediate fledgling activity rates within and across species. Nature favors young animals to be less active when the environment is more dangerous. We show that across songbird species, recently fledged young ("fledglings") were less active during the post-fledging period when mortality risk was high. Additionally, fledglings with more developed wings and from larger broods exhibited higher post-fledging activity rates; likely due to improved locomotor (flight) abilities and from increased sibling competition over food provided by their parents.

Aim: A full annual cycle approach to conservation and understanding of regional population trends requires an understanding of migratory connectivity. We present tracking data on the eastern whip-poor-will (Antrostomus vociferus), a Neotropical migrant that has declined by 70% in recent decades. When and where populations of this species are limited throughout the annual cycle is poorly understood. Methods: We utilized data from 52 archival GPS tags from five breeding areas covering a 9.5-degree latitudinal span (similar to 1000 km) of the whip-poor-will breeding range in the summers of 2017 and 2019. We identified migratory routes and spatiotemporal bottlenecks, stopover and wintering locations, calculated migratory connectivity throughout migration and on the wintering grounds and tested predictions for three latitudinal connectivity patterns. Results: Whip-poor-wills circumvented the Gulf of Mexico, and populations across a large latitudinal gradient came together in eastern Texas in early October, resulting in decreased connectivity throughout migration. Breeding-winter migratory connectivity was low (MC = 0.22 +/- 0.12), with extensive overlap of core wintering areas in southern Mexico and Guatemala. The overlap of wintering areas by individuals from dispersed breeding latitudes suggests that whip-poor-wills most closely resemble telescopic migrants. Main conclusions: Circumventing the Gulf of Mexico influenced connectivity in the whip-poor-will, funnelling individuals into a small region in eastern Texas in migration and likely influencing breeding-winter connectivity. Thus, geographically dispersed breeding populations overlap in space and time during migration and winter, and non-breeding season conditions affecting populations (both positively and negatively) impact individuals from across the core breeding range. For example, extensive deforestation occurring in the whip-poor-will's core wintering area likely impacts individuals from all five deployment locations. We demonstrate that combining multiple indices of spatiotemporal cohesion is critical to fully understand how migratory animals are distributed in the non-breeding season.

Despite a generalist life history and a widespread distribution, the Common Grackle (Quiscalus quiscula) has declined by more than 58% since 1970. In Illinois, where this study was conducted, the current rate of decline is 7.03% annually. We hypothesized that low reproductive success in intensely agricultural areas is driving population decline. To test this, we quantified the nesting success and post-fledging survival of Common Grackles in central Illinois. Over a 2-year period, we monitored 188 nests and tracked the survival of 53 fledglings. Our estimate for nesting success of 0.622 (95% CI: 0.549-0.695) was much higher than the literature average of 0.267. Similarly, although post-fledging survival had not been estimated previously for Common Grackles, our estimate of 0.617 (95% CI: 0.471-0.764) was relatively high compared to that of other songbirds (range: 0.23-0.87). The most important factor influencing these estimates was ordinal date, which had a negative relationship with both nesting success and post-fledging survival. These results suggest that reproductive success is not the primary driver of population decline in Illinois. To expand upon the field portion of our study, we constructed a demographic model and used it to conduct a global sensitivity analysis. In our model, adult survival was the most influential demographic parameter in the context of population change. This study serves as an initial step in understanding the mechanism(s) of decline in the Common Grackle. We recommend additional research on the survival of Common Grackles, particularly in relation to persecution on the wintering grounds and exposure to agricultural chemicals.

Recently fledged birds often experience low survival as they undergo rapid changes in their behavior, (e.g., habitat use, mobility, and foraging rate). For species of conservation concern, information about the post-fledging period can be critical for effective management and conservation. We investigated the post-fledging ecology of endangered Golden-cheeked Warblers (Setophaga chrysoparia) in central Texas. Current management guidelines focus on nesting habitat even though songbird fledglings may use entirely different habitats. We radio-tracked fledglings (N = 23) to assess their habitat use, estimate daily survival, and observe age-based behavioral changes. Habitat used by fledglings was similar to adult nesting habitat and was dominated by Ashe juniper (Juniperus asheii; 75%) and oak (Quercus spp.; 21%). Apparent fledgling survival was 65%, and the estimated daily survival rate of fledglings during their first 4 weeks post-fledging was 0.985 (95% CI= 0.971-0.993). To investigate the factors associated with predation, we estimated survival while excluding mortality events due to exposure. Estimated survival was greater in areas with more junipers than oaks. As fledglings aged, use of oaks as foraging substrates increased as did their prey capture rates (nearly twice as many prey captured per hour in oaks than in junipers). Current management recommendations for nesting adult Golden-cheeked Warblers specify maintaining mature juniper-oak forests composed of similar to 70% juniper, a recommendation that also appears to accommodate fledglings. Fledgling Golden-cheeked Warblers acquire unique benefits from junipers (i.e., safety) and oaks (i.e., foraging opportunities), so management plans should include the maintenance of both junipers and oaks.

Many avian species are considered agricultural pests and cause severe and costly damage to crops worldwide. Crop producers use several methods to reduce damage by birds, with frightening devices serving among the most popular approaches. Biologically salient frightening devices, such as broadcast of conspecific alarm calls or placing predator models in fields, are considered particularly effective because they can trigger antipredator behaviors that reduce foraging activities or abundances of target birds locally. However, it remains unclear which type of biologically salient frightening device is most effective and whether some work best against certain bird species or on certain types of crops. We reviewed the published scientific literature to fill in these knowledge gaps about using biologically salient frightening devices for crop protection against avian pests. We found 48 experiments evaluating the impact(s) of biologically salient frightening devices. However, only 28 experiments statistically tested their results from which to draw conclusions. We found that alarm call playbacks, eye-spot balloons, or automated flying vehicles (UAVs) reduced avian abundance in all experiments. All experiments using alarm call playbacks, eye-spot balloons, hawk-kites, or UAVs effectively reduced crop damage. However, only experiments on passerine birds and in row crops were well-represented in these studies. We discuss potential biases in the literature that may limit our understanding of how birds respond to biologically salient frightening devices and suggest future directions that researchers might take to fill knowledge gaps.

During migration, birds must locate stopover habitats that provide sufficient resources to rest and refuel while en route to the breeding or non‐breeding area. Long‐distance migrants invariably encounter inhospitable geographic features, the edges of which are often characterized by habitat limited in food and safety. In response, they often depart in directions inconsistent with reaching their destination, presumably searching for better habitat. We used automated radio telemetry to track 442 individuals of five species to investigate the behavior of migratory birds as they departed edge habitat along the northern Gulf of Mexico coast during autumn from 2008 to 2014. Most migrants (75%) retreated inland or detoured around rather than advanced across the Gulf, but this depended on bird species and fat‐based energy stores. Most individuals in lean condition or of smaller bodied species tended to retreat or detour, rather than advance, when departing from the coast. Twenty‐one percent of all birds that departed the coast in 2013–2014 were redetected over 45 km inland, providing a unique opportunity to compare stopover duration, departure times and travel speeds between migrants that retreat away from the coast and those that continue to advance toward their destination. Individuals that retreated the coast and were redetected inland spent ~1 day on the coast before retreating inland, where they spent 11 days before resuming migration. Further when those same individuals retreated from the coast, they departed around evening civil twilight, whereas those that advanced from inland habitats departed after evening civil twilight. Travel speeds were slower for individuals retreating inland compared to those advancing towards the coast from inland habitats. The differences between retreating and advancing individuals suggest how an individual's drive to feed or fly influences behavior. Our study illustrates how the sum of individual decisions can shape habitat use, landscape‐scale movements and migration strategies.

AbstractLife history studies have established that trade‐offs between growth and survival are common both within and among species. Identifying the factor(s) that mediate this trade‐off has proven difficult, however, especially at the among‐species level. In this study, we examined a series of potentially interrelated traits in a community of temperate‐zone passerine birds to help understand the putative causes and consequences of variation in early‐life growth among species. First, we examined whether nest predation risk (a proven driver of interspecific variation in growth and development rates) was correlated with species‐level patterns of incubation duration and nestling period length. We then assessed whether proxies for growth rate covaried with mean trait covariance strength (i.e., phenotypic correlations (rp), which can be a marker of early‐life stress) among body mass, tarsus length, and wing length at fledging. Finally, we examined whether trait covariance strength at fledging was related to postfledging survival. We found that higher nest predation risk was correlated with faster skeletal growth and that our proxies for growth corresponded with increased trait covariance strength (rp), which subsequently, correlated with higher mortality in the next life stage (postfledging period). These results provide an indication that extrinsic pressures (nest predation) impact rates of growth, and that there are costs of rapid growth across species, expressed as higher mean rp and elevated postfledging mortality. The link between higher levels of trait covariance at fledging and increased mortality is unclear, but increased trait covariance strength may reflect reduced phenotypic flexibility (i.e., phenotypic canalization), which may limit an organism's capacity for coping with environmental or ecological variability.

Researchers and managers have used the tendency of some species of birds to settle near conspecifics (i.e., conspecific attraction) to help establish or reestablish species of conservation concern in targeted habitats. However, most studies of conspecific attraction as a recruitment tool have been conducted with either migratory or colonial-breeding species, and less is known about the possible importance of conspecific attraction for resident species. In 2017 and 2018 in Louisiana, we examined the possible use of conspecific attraction as a management tool for Northern Bobwhites (Colinus virginianus), game birds that are year-round residents. We used an alternating split experimental design across years where Northern Bobwhite vocalizations were broadcast at half of our study sites during the first year of our study and the other half were controls. The following year, experimental sites became controls, and controls became experimental sites. We also assessed land cover at each site to determine if variation in habitat composition at small spatial scales influenced bobwhite presence. We detected six times more Northern Bobwhites at treatment sites (sites with playback) than control sites and found a positive association between the proportion of grass cover and bobwhite presence. These results suggest that a non-migratory, resident species may select breeding locations based, at least in part, on the presence of conspecifics, and that playback of their calls could be incorporated into management plans. Playing back conspecific calls in early spring when Northern Bobwhites are likely searching for breeding locations may facilitate colonization and allow individuals to find locations where managers are attempting to restore their populations.

Evolutionary ecologists have long been interested in the adaptive significance of morphological traits across stages of animal life. In some cases, traits that are not adaptive in one life stage may be adaptive in a subsequent stage. As such, morphological traits may generate important carryover effects, whereby conditions experienced during one life-history stage influence fitness during subsequent stages. Carryover effects are particularly relevant in young animals, as early life stages are thought to be critical with respect to animal life-history evolution and population dynamics. In songbirds, pre- to post-fledging carryover effects operating within species may be critical for survival and shape life histories among species, but remain poorly understood. Among potential songbird traits, wing development and its associated flight ability may be the most important for post-fledging survival. Thus, to assess the adaptive significance of wing development for juvenile songbirds under Arnold's (Integrative and Comparative Biology, 23, 1983, 347) classic performance-morphology-fitness paradigm, we tested for pre- to post-fledging carryover effects among 20 coexisting species (nine focal species) of an avian community in east-central Illinois, USA. We found evidence for pre- to post-fledging carryover effects of wing development in all species, by which individuals with less developed wings exhibited poorer flight ability and experienced higher rates of mortality after fledging. Furthermore, our findings suggest that carryover effects operating at the species level ultimately help shape patterns of life-history variation among species. Specifically, we found that species with higher rates of nest predation had shorter nestling periods, fledged young with less developed wings and exhibited higher rates of post-fledging mortality. Our results highlight the adaptive significance of wing development as a key factor generating pre- to post-fledging carryover effects among songbirds, and demonstrate how morphological traits, locomotor performance, and age-specific survival may trade-off and interact across juvenile life stages to shape animal life histories.

Many species across taxa select habitat based on conspecific presence, known as conspecific attraction. Studies that document conspecific attraction typically provide social information (i.e., cues that indicate the presence of a given species) and then determine if a given species is more likely to settle at locations where the social information is provided compared to those locations that do not. Although the number of studies examining conspecific attraction has grown in recent years, a comprehensive review has not yet been undertaken. Here, we conducted a review of the literature and found 151 studies investigating conspecific attraction across eight taxa. We found that conspecific attraction is widespread with between 80% and 100% of studies, depending on taxa, documenting positive associations between habitat selection and the presence of conspecific cues. Conspecific attraction has been documented more frequently in bird and fish species with less attention given to invertebrate and mammal species. We use the patterns we found to (a) provide an overview of the current state of research on conspecific attraction and (b) discuss how important factors, such as cue characteristics and life history traits, may play a role in shaping conspecific attraction patterns within and across taxa.

Abstract. Feeding strategies, including cannibalism (in which an individual eats a member of the same species), are an important aspect of predator ecology. Cannibalism comprises five forms in raptors: siblicide, filicide, non-parental infanticide, conspecific strife, and conspecific scavenging. Cannibalism by raptors has been documented opportunistically for over a century, but it is unknown how frequent or widespread the behavior is. We performed the first systematic literature review and meta-analyses of the studies documenting filicide, non-parental infanticide, conspecific strife, and conspecific scavenging by raptors. We found 29 reports of these types of cannibalism; we did not review reports of siblicide due to the high frequency of the behavior, making it nearly ubiquitous among raptors. Filicide had nearly twice as many reports (n = 11, 37.9%) as any other type of cannibalism. Most reports were direct observations (n = 23, 79.3%), and nearly half the reports came from North America (n = 14, 48.3%) and approximately a third from Europe (n = 10, 34.5%). The 29 reports involved 25 raptor species from four families, with those from Accipitridae most common (n = 19, 65.5%). Cannibalism in raptors varies but most involves nestlings, which are easier to kill than adults, possibly because brood reduction can help the stronger young survive. Documented reports of cannibalism are increasing, possibly due to recent technological advancements that have increased our ability to document cannibalism and other ecological processes. Nevertheless, we encourage future reports of cannibalism from under-represented locations and for taxa that are less well studied.

Parent-offspring conflict has explained a variety of ecological phenomena across animal taxa, but its role in mediating when songbirds fledge remains controversial. Specifically, ecologists have long debated the influence of songbird parents on the age of fledging: Do parents manipulate offspring into fledging to optimize their own fitness or do offspring choose when to leave? To provide greater insight into parent-offspring conflict over fledging age in songbirds, we compared nesting and postfledging survival rates across 18 species from eight studies in the continental United States. For 12 species (67%), we found that fledging transitions offspring from comparatively safe nesting environments to more dangerous postfledging ones, resulting in a postfledging bottleneck. This raises an important question: as past research shows that offspring would benefit-improve postfledging survival-by staying in the nest longer: Why then do they fledge so early? Our findings suggest that parents manipulate offspring into fledging early for their own benefit, but at the cost of survival for each individual offspring, reflecting parent-offspring conflict. Early fledging incurred, on average, a 13.6% postfledging survival cost for each individual offspring, but parents benefitted through a 14.0% increase in the likelihood of raising at least one offspring to independence. These parental benefits were uneven across species-driven by an interaction between nest mortality risk and brood size-and predicted the age of fledging among species. Collectively, our results suggest that parent-offspring conflict and associated parental benefits explain variation in fledging age among songbird species and why postfledging bottlenecks occur.