Bridget J. M. Stutchbury

Tracking Long-Distance Songbird Migration by Using Geolocators

We mapped migration routes of migratory songbirds to the Neotropics by using light-level geolocators mounted on breeding purple martins (Progne subis) and wood thrushes (Hylocichla mustelina). Wood thrushes from the same breeding population occupied winter territories within a narrow east-west band in Central America, suggesting high connectivity of breeding and wintering populations. Pace of spring migration was rapid (233 to 577 kilometers/day) except for one individual (159 kilometers/day) who took an overland route instead of crossing the Gulf of Mexico. Identifying songbird wintering areas and migration routes is critical for predicting demographic consequences of habitat loss and climate change in tropical regions.

The effect of breeding synchrony on extra-pair mating systems in songbirds

Extra-pair mating systems are now considered to be typical of songbirds because DNA fingerprinting has revealed high rates of extra-pair fertilizations (EPFs) in so many species previously thought to be monogamous. However, there is no general framework to explain the tremendous variability among songbirds in EPF rate. Here, in a comparison of EPF rates and temporal patterns of nesting among songbirds, we show that EPFs are most common in species where females breed synchronously. We argue that for both males and females, a temporal concentration of female fertility (breeding synchrony) increases the net benefits of seeking EPFs. One implication of this finding is that tropical songbirds, most of which breed asynchronously, are predicted to have low levels of extra-pair matings. Testes size is positively correlated with frequency of EPFs, and as predicted, tropical songbirds had small testes size relative to temperate zone species. The few DNA fingerprinting studies that exist for socially monogamous tropical songbirds support the correlation between breeding synchrony and EPFs. We believe that breeding synchrony is the most important factor promoting the evolution of extra-pair mating systems.

Nest Concealment and Predation in Hooded Warblers: Experimental Removal of Nest Cover

-It is commonly assumed that passerine birds with open nests reduce the risk of predation by concealing their nests. At a forest study site in northwestern Pennsylvania, about 50% of the Hooded Warbler (Wilsonia citrina) nests were depredated-the primary cause of reproductive failure. To test the hypothesis that concealed nests are less likely to be depredated than conspicuous nests, we examined seven characteristics of the warblers nest and nest microhabitat in relation to nest predation: nest visibility, vegetation density surrounding the nest, height of nest, height of nest substrate, nest-substrate species, height of cryptic dead-leaf base of nest, and proximity of nest to a microedge. Successful and depredated nests (n = 97) did not differ in any of these variables. In addition, we performed a vegetationremoval experiment in which manipulated nests (n = 15) with surrounding vegetation removed were on average 86% more visible overall than control nests (n = 15). The highly visible manipulated nests did not suffer higher predation than control nests. Contrary to our hypothesis, our results show that nest concealment is not an important factor in predator avoidance for Hooded Warblers. This outcome would be expected if nest predation is the result of nonspecialist predation that occurs by chance and is based on the location of available food in general. Received 19 September 1994, accepted 25 April 1995. THE MAJOR FACTOR influencing reproductive success of open-nesting birds is nest predation (Skutch 1949, Nice 1957, Ricklefs 1969, Gates and Gysel 1978, Martin 1992). Consequently, there should be selective pressure for birds to place their nests in sites that deter predation. It is clear that birds select nest sites nonrandomly (e.g. Peterson and Best 1985, Bekoff et al. 1987), and there is evidence that they can identify nest sites with characteristics that reduce the risk of predation (Stauffer and Best 1986, Martin and Roper 1988, Marzluff 1988, M0ller 1988). Nest concealment and characteristics of nest microhabitat, such as vegetation density and height I Present address: 3662 Kinter Hill Road, Edinboro, Pennsylvania 16412, USA. of nest from the ground, have been related to predation risk (Caccamise 1977, Murphy 1983, Martin and Roper 1988, Yahner and Scott 1988, Kelly 1993, Martin 1993) and, thus, could serve as criteria in nest-site selection. Although concealment of a nest by selecting an inconspicuous site is commonly assumed to reduce the risk of predation, there have been conflicting results in testing this hypothesis. Martin (1992) concluded in a review of 56 studies that dense foliage reduces the probability of predation by concealing a nest. However, of the 11 studies on passerines, 7 reported a negative correlation between predation and nest concealment, and 4 reported no correlation. Pertinent research, including those studies reviewed by Martin (1992), was conducted by Nice (1937), Nolan (1978), Martin and Roper

Correlates of extra-pair fertilization success in hooded warblers

Abstract We examined correlates of extra-pair fertilization (EPF) success in the hooded warbler (Wilsonia citrina), a species where females pursue extra-pair matings. The good genes hypothesis predicts that females choose extra-pair mates with morphological or behavioral traits that reflect differences in male genetic quality. EPFs were common, as 35.3% (42/119) of broods contained extra-pair young (EPY) and 26.7% (95/356) of nestlings were the result of EPFs. There was a strong skew in male EPF success which increased the variance in annual male mating success 2–3 fold. However, male morphology did not predict EPF success, as extra-pair males were not older or larger than the males they cuckolded. Likewise, there were no significant correlations between the proportion of extra-pair young in a brood and male size or age. The good genes hypothesis predicts that high-quality males will be consistently preferred as genetic mates, but the number of young sired by a male with his social mate was not consistent from one year to the next. There was a significant negative correlation between female age and proportion of EPY produced, which could result if older females obtain higher-quality social mates. We found no strong evidence that females choose extra-pair mates for good genes, but females may use behavioral rather than morphological cues to assess relative male quality.

Repeat tracking of individual songbirds reveals consistent migration timing but flexibility in route.

Tracking repeat migratory journeys of individual animals is required to assess phenotypic plasticity of individual migration behaviour in space and time. We used light-level geolocators to track the long-distance journeys of migratory songbirds (wood thrush, Hylocichla mustelina), and, for the first time, repeat journeys of individuals. We compare between- and within-individual variation in migration to examine flexibility of timing and route in spring and autumn. Date of departure from wintering sites in Central America, along with sex and age factors, explained most of the variation (71%) in arrival date at North American breeding sites. Spring migration showed high within-individual repeatability in timing, but not in route. In particular, spring departure dates of individuals were highly repeatable, with a mean difference between years of just 3 days. Autumn migration timing and routes were not repeatable. Our results provide novel evidence of low phenotypic plasticity in timing of spring migration, which may limit the ability of individuals to adjust migration schedules in response to climate change.

New Discoveries in Landbird Migration using Geolocators, and a Flight Plan for the Future

— 211 — The Auk, Vol. 130, Number 2, pages 211−222. ISSN 0004-8038, electronic ISSN 1938-4254.  2013 by The American Ornithologists’ Union. All rights reserved. Please direct all requests for permission to photocopy or reproduce article content through the University of California Press’s Rights and Permissions website, http://www.ucpressjournals. com/reprintInfo.asp. DOI: 10.1525/auk.2013.130.2.12226 1E-mail: emilymck@yorku.ca Bird migration is a spectacular natural phenomenon that has generated wonder and interest for centuries. Feats of migration inspire amazement—individual birds that weigh less than 200 g may log more than 80,000 km annually (Egevang et al. 2010), travel more than 600 km day–1 (Stutchbury et al. 2009, Akesson et al. 2012), and cross huge geographic barriers such as oceans (Bairlein et al. 2012) and inhospitable deserts (Tottrup et al. 2012b). Despite the vast geography covered during migration, many birds return to the same territories year after year. Although incredible progress has been made in our understanding of bird migration (Newton 2008), many gaps remain in our knowledge of the migration of small birds. The development of miniaturized tracking technology has produced a wave of research into the migratory behavior of small birds (Fig. 1). The inaugural application of miniaturized geolocators (or “geologgers”) on small songbirds in 2007 (Stutchbury et al. 2009) initiated a rapid increase in the number of studies of small landbird migration; there are currently more than 100 permits in North America alone for attaching geolocators to small birds. This technology has been so enthusiastically applied because it provides information critical to conservation and management of declining songbird populations (Faaborg et al. 2010a), as well as the opportunity to test long-standing hypotheses related to endogenous control mechanisms, navigation, and energetics (Robinson et al. 2010). Although more accurate devices may someday be available for tracking small birds, geolocators are currently the only option for migrants that weigh <50 g (Bridge et al. 2011). The main goal of many geolocator studies to date has been the description of little-known migratory routes and wintering sites (e.g., Beason et al. 2012, Stach et al. 2012). As this technique becomes more widely applied (both geographically within species and taxonomically across a broad spectrum of small landbirds), researchers can begin to test hypotheses about migration, nonbreeding ecology, and behavior to inform conservation measures. Many migratory species are declining; thus, a comprehensive understanding of the annual cycle is timely and important for management of species at risk. The purpose of our review is to summarize, for the first time, patterns emerging from geolocator studies. We review new data on (1) migratory connectivity, (2) migratory routes and stopovers, (3) intratropical migration of wintering birds, and (4) migration schedules. We then explore questions that can be answered with emerging geolocator studies, and provide a “flight plan” for future work as direct-tracking technology becomes increasingly smaller and more broadly applied.

Effects of post-breeding moult and energetic condition on timing of songbird migration into the tropics

Each autumn billions of songbirds migrate between the temperate zone and tropics, but little is known about how events on the breeding grounds affect migration to the tropics. Here, we use light level geolocators to track the autumn migration of wood thrushes Hylocichla mustelina and test for the first time if late moult and poor physiological condition prior to migration delays arrival on the winter territory. Late nesting thrushes postponed feather moult, and birds with less advanced moult in August were significantly farther north on 10 October while en route to the tropics. Individuals in relatively poor energetic condition in August (high β-Hydroxybutyrate, low triglyceride, narrow feather growth bars) passed into the tropics significantly later in October. However, late moult and poor pre-migratory condition did not result in late arrival on the winter territory because stopover duration was highly variable late in migration. Although carry-over effects from the winter territory to spring migration may be strong in migratory songbirds, our study suggests that high reproductive effort late in the season does not impose time constraints that delay winter territory acquisition.

Continent-wide tracking to determine migratory connectivity and tropical habitat associations of a declining aerial insectivore

North American birds that feed on flying insects are experiencing steep population declines, particularly long-distance migratory populations in the northern breeding range. We determine, for the first time, the level of migratory connectivity across the range of a songbird using direct tracking of individuals, and test whether declining northern populations have higher exposure to agricultural landscapes at their non-breeding grounds in South America. We used light-level geolocators to track purple martins, Progne subis, originating from North American breeding populations, coast-to-coast (n = 95 individuals). We show that breeding populations of the eastern subspecies, P. s. subis, that are separated by ca. 2000 km, nevertheless have almost completely overlapping non-breeding ranges in Brazil. Most (76%) P. s. subis overwintered in northern Brazil near the Amazon River, not in the agricultural landscape of southern Brazil. Individual non-breeding sites had an average of 91 per cent forest and only 4 per cent agricultural ground cover within a 50 km radius, and birds originating from declining northern breeding populations were not more exposed to agricultural landscapes than stable southern breeding populations. Our results show that differences in wintering location and habitat do not explain recent trends in breeding population declines in this species, and instead northern populations may be constrained in their ability to respond to climate change.

RECENT ADVANCES IN THE BEHAVIORAL ECOLOGY OF TROPICAL BIRDS The 2005 Margaret Morse Nice Lecture

Abstract Tropical birds offer unique opportunities to test ecological and evolutionary theory because their life history traits are so diverse and different from temperate zone models upon which most empirical studies are based. We review recent studies on the behavioral ecology of tropical birds, studies that explore new advances in this field. Life histories and their evolution remain the focus of research on tropical birds. Clutch size manipulations in two species showed that food limitation does not explain small clutch size. In antbirds, enlarged clutches decreased post-fledging survival whereas in thrushes, enlarged broods were costly due to high nest predation. Small clutches may be favored via different ultimate selective forces and shared underlying tradeoffs between the immune, metabolic, and endocrine systems in the body may account for the commonly observed ‘slow pace of life’ in tropical birds. The physiological tradeoff between testosterone and immunocompetence may explain the evolution of low testosterone levels in tropical passerines where adult survival is paramount. In contrast to life history theory, few studies have explored temperate-tropical differences in territoriality, mating systems, and song function. The idea that low breeding synchrony in tropical birds is associated with low levels of extra-pair fertilizations was supported by several new paternity studies conducted on tropical passerines. Seasonally breeding tropical birds have higher testosterone levels than tropical birds with prolonged breeding seasons, although it is unclear if this pattern is driven by mating systems per se or selection from pathogens. Recent work on relations between pair members in permanently paired tropical passerines focuses on the question of mate defense versus territorial defense and the extent of cooperation versus selfish interests in inter-sexual relations.

Extra-pair mating system of a synchronously breeding tropical songbird

Extra-pair matings are now recognized as a fundamental component of avian mating systems. However, in tropical regions where breeding is typically asynchronous socially monogamous species appear to have low sperm competition and few extrapair matings. We predicted that if breeding synchrony promotes the evolution of extra-pair matings, then tropical species that breed synchronously, like the Claycolored Robin Turdus grayi, should have abundant extra-pair young (EPY). Breeding began relatively abruptly during the dry season (Feb-Mar) in this non-migratory species, with a mean of 25% of females fertile on a given day. Overall, 53% of females produced EPY, and 38% of nestlings were EPY. Males have a long and conspicuous dawn chorus, which could be a result of intense male-male competition for extra-pair matings. Male songs are individually recognizable and males vary greatly in song output. Male song output declined significantly from the pre-nesting stage through the incubation stage, as would be expected if song is important in mate choice. Our results support the idea that breeding synchrony favors extra-pair matings, however further studies of tropical songbirds are needed to fully explore how ecology is tied to extra-pair mating systems.