Emily B. Cohen

A call for full annual cycle research in animal ecology

For vertebrates, annual cycles are organized into a series of breeding and non-breeding periods that vary in duration and location but are inextricably linked biologically. Here, we show that our understanding of the fundamental ecology of four vertebrate classes has been limited by a severe breeding season research bias and that studies of individual and population-level responses to natural and anthropogenic change would benefit from a full annual cycle perspective. Recent emergence of new analytical and technological tools for studying individual and population-level animal movement could help reverse this bias. To improve understanding of species biology and reverse the population declines of many vertebrate species, a concerted effort to move beyond single season research is vital.

SURVIVAL, HABITAT USE, AND MOVEMENTS OF FLEDGLING WHITE-THROATED ROBINS (TURDUS ASSIMILIS) IN A COSTA RICAN AGRICULTURAL LANDSCAPE

Abstract We used radiotelemetry to study behavior of White-throated Robins (Turdus assimilis) during the postfledging dependent period. The study was conducted in a mixed agricultural and forested landscape in southern Costa Rica from March through August of 2001 and 2002. A transmitter was attached to one fledgling per brood (n = 53). Each bird was located daily prior to dispersal. We compared survivorship, habitat use, and movements of fledglings from (1) nests in coffee plantations and (2) nests in cattle pastures. The probability of surviving the first three weeks out of the nest was 0.67 ± 0.07 (SE) for fledglings from nests in all habitats, 0.58 ± 0.10 for fledglings from nests in coffee, and 0.74 ± 0.26 for fledglings from nests in pasture. Fledglings from nests in pasture left their nesting habitat at younger ages than did those from nests in coffee, and most birds from both habitats moved into forest when they left their nesting habitat. Pasture was rarely used during the postfledging period, whereas coffee plantations were used extensively. Fledglings that remained in agricultural habitats (coffee or pasture) were less likely to survive until dispersal than were those that moved into forested areas. Average daily distances from the nest gradually increased until fledglings dispersed away from the natal area, always into forest, and were not different for birds from pasture or coffee. White-throated Robins can nest successfully in agricultural habitats, but use of forest positively influenced survivorship of young during the postfledging dependent period.

Experimental Evidence for the Interplay of Exogenous and Endogenous Factors on the Movement Ecology of a Migrating Songbird

Movement patterns during songbird migration remain poorly understood despite their expected fitness consequences in terms of survival, energetic condition and timing of migration that will carry over to subsequent phases of the annual cycle. We took an experimental approach to test hypotheses regarding the influence of habitat, energetic condition, time of season and sex on the hour-by-hour, local movement decisions of a songbird during spring stopover. To simulate arrival of nocturnal migrants at unfamiliar stopover sites, we translocated and continuously tracked migratory red-eyed vireos (Vireo olivaceus) throughout spring stopover with and without energetic reserves that were released in two replicates of three forested habitat types. Migrants moved the most upon release, during which time they selected habitat characterized by greater food abundance and higher foraging attack rates. Presumably under pressure to replenish fuel stores necessary to continue migration in a timely fashion, migrants released in poorer energetic condition moved faster and further than migrants in better condition and the same pattern was true for migrants released late in spring relative to those released earlier. However, a migrants energetic condition had less influence on their behavior when they were in poor quality habitat. Movement did not differ between sexes. Our study illustrates the importance of quickly finding suitable habitat at each stopover site, especially for energetically constrained migrants later in the season. If an initial period prior to foraging were necessary at each stop along a migrants journey, non-foraging periods would cumulatively result in a significant energetic and time cost to migration. However, we suggest behavior during stopover is not solely a function of underlying resource distributions but is a complex response to a combination of endogenous and exogenous factors.

Avian migrants facilitate invasions of neotropical ticks and tick-borne pathogens into the United States.

ABSTRACT Migratory birds have the potential to transport exotic vectors and pathogens of human and animal health importance across vast distances. We systematically examined birds that recently migrated to the United States from the Neotropics for ticks. We screened both ticks and birds for tick-borne pathogens, including Rickettsia species and Borrelia burgdorferi. Over two spring seasons (2013 and 2014), 3.56% of birds (n = 3,844) representing 42.35% of the species examined (n = 85) were infested by ticks. Ground-foraging birds with reduced fuel stores were most commonly infested. Eight tick species were identified, including seven in the genus Amblyomma, of which only Amblyomma maculatum/Amblyomma triste is known to be established in the United States. Most ticks on birds (67%) were neotropical species with ranges in Central and South America. Additionally, a single Ixodes genus tick was detected. A total of 29% of the ticks (n = 137) and no avian blood samples (n = 100) were positive for infection with Rickettsia species, including Rickettsia parkeri, an emerging cause of spotted fever in humans in the southern United States, a species in the group of Rickettsia monacensis, and uncharacterized species and endosymbionts of unknown pathogenicity. No avian tick or blood samples tested positive for B. burgdorferi, the etiologic agent of Lyme disease. An extrapolation of our findings suggests that anywhere from 4 to 39 million exotic neotropical ticks are transported to the United States annually on migratory songbirds, with uncertain consequences for human and animal health if the current barriers to their establishment and spread are overcome.

Large‐scale spatial analysis of ringing and re‐encounter data to infer movement patterns: A review including methodological perspectives

Summary A major aim of bird ringing is to provide information about the migration and movements of bird populations. However, in comparison with demographic studies, little research has been devoted to improving quantitative inferences through large-scale spatial analyses. This represents a serious knowledge gap because robust information on geographical linkages of migratory populations throughout the annual cycle is necessary to understand the ecology and evolution of migrants and for the conservation and management of populations. Here, we review recent developments and emerging opportunities for the quantitative study of movements of bird populations based on marked birds. Large-scale spatial analyses of ringing data need to account for spatiotemporal variation in re-encounter probability and the complexity of movement processes, including variability among individuals and populations in migration direction and distance. We identify seven recent studies that used quantitative methods for large-scale spatial analyses of ringing and re-encounter data gathered by national ringing centres. In most cases, numbers ringed and recovered in a series of source and destination areas were used to derive estimates of the proportion of each source population travelling to each destination area. Where recovery data were sparse, precision was improved by incorporating information on re-encounter probabilities of similar species. When numbers ringed were not available, inferences could sometimes be drawn based on local recapture data from the source areas. Studies to date illustrate that analyses of these large-scale ringing data sets can provide robust quantitative inferences. Further work is needed to develop these modelling approaches and to test their sensitivity to key assumptions using both real and simulated data. Data for all birds that were marked, not only those re-encountered, are often inaccessible and should be computerised in parallel with analytical developments. Further, there is great potential for the formal combination of re-encounter data with information from additional data sources such as counts and detailed movement data from radiotracking or data loggers. Because data from bird ringing operations cover long periods of time and exist in large quantities, they hold great promise for inferring spatiotemporal migration patterns, including changes in relation to climate, land use change and other environmental drivers.

Habitat use of adult White-throated Robins during the breeding season in a mosaic landscape in Costa Rica

Abstract Most work on the effects of land-cover change on tropical birds has focused on forest-interior birds because these species are assumed to be the most severely affected by forest loss. However, even species that use human-altered habitat types may be severely affected by forest loss. White-throated Robins (Turdus assimilis) frequently nest in coffee and pasture in southern Costa Rica, although several lines of evidence suggest the species could not exist solely in agricultural habitat. We used radio-telemetry to examine the area and proportion of different habitat types used by adult White-throated Robins. Mean area of use for the tracking period was 0.26 km2 and varied from 0.07 to 0.58 km2. No robins had areas of use solely in either primary forest or agricultural cover, although individuals were initially captured in both primary forest and a coffee plantation. All areas of use contained some forest, pasture, and coffee or regenerating forest. The mean percentages of the habitat types used by the birds were fairly equal with 30% forest, 33% pasture, and 38% coffee or regenerating forest. Our results, combined with other evidence, indicate that the White-throated Robin uses multiple habitat types but that it may require some threshold level of forest to sustain viable populations.

El Niño-Southern Oscillation is linked to decreased energetic condition in long-distance migrants

Predicting how migratory animals respond to changing climatic conditions requires knowledge of how climatic events affect each phase of the annual cycle and how those effects carry-over to subsequent phases. We utilized a 17-year migration dataset to examine how El Niño-Southern Oscillation climatic events in geographically different regions of the Western hemisphere carry-over to impact the stopover biology of several intercontinental migratory bird species. We found that migratory birds that over-wintered in South America experienced significantly drier environments during El Niño years, as reflected by reduced Normalized Difference Vegetation Index (NDVI) values, and arrived at stopover sites in reduced energetic condition during spring migration. During El Niño years migrants were also more likely to stopover immediately along the northern Gulf coast of the southeastern U.S. after crossing the Gulf of Mexico in small suboptimal forest patches where food resources are lower and migrant density often greater than larger more contiguous forests further inland. In contrast, NDVI values did not differ between El Niño and La Niña years in Caribbean-Central America, and we found no difference in energetic condition or use of coastal habitats for migrants en route from Caribbean-Central America wintering areas. Birds over-wintering in both regions had consistent median arrival dates along the northern Gulf coast, suggesting that there is a strong drive for birds to maintain their time program regardless of their overall condition. We provide strong evidence that not only is the stopover biology of migratory landbirds influenced by events during the previous phase of their life-cycle, but where migratory birds over-winter determines how vulnerable they are to global climatic cycles. Increased frequency and intensity of ENSO events over the coming decades, as predicted by climatic models, may disproportionately influence long-distance migrants over-wintering in South America.

Estimating migratory connectivity of birds when re-encounter probabilities are heterogeneous

Understanding the biology and conducting effective conservation of migratory species requires an understanding of migratory connectivity – the geographic linkages of populations between stages of the annual cycle. Unfortunately, for most species, we are lacking such information. The North American Bird Banding Laboratory (BBL) houses an extensive database of marking, recaptures and recoveries, and such data could provide migratory connectivity information for many species. To date, however, few species have been analyzed for migratory connectivity largely because heterogeneous re-encounter probabilities make interpretation problematic. We accounted for regional variation in re-encounter probabilities by borrowing information across species and by using effort covariates on recapture and recovery probabilities in a multistate capture–recapture and recovery model. The effort covariates were derived from recaptures and recoveries of species within the same regions. We estimated the migratory connectivity for three tern species breeding in North America and over-wintering in the tropics, common (Sterna hirundo), roseate (Sterna dougallii), and Caspian terns (Hydroprogne caspia). For western breeding terns, model-derived estimates of migratory connectivity differed considerably from those derived directly from the proportions of re-encounters. Conversely, for eastern breeding terns, estimates were merely refined by the inclusion of re-encounter probabilities. In general, eastern breeding terns were strongly connected to eastern South America, and western breeding terns were strongly linked to the more western parts of the nonbreeding range under both models. Through simulation, we found this approach is likely useful for many species in the BBL database, although precision improved with higher re-encounter probabilities and stronger migratory connectivity. We describe an approach to deal with the inherent biases in BBL banding and re-encounter data to demonstrate that this large dataset is a valuable source of information about the migratory connectivity of the birds of North America.

Effects of landscape composition and configuration on migrating songbirds: inference from an individual-based model

The behavior of long-distance migrants during stopover is constrained by the need to quickly and safely replenish energetic reserves. Replenishing fuel stores at stopover sites requires adjusting to unfamiliar landscapes with little to no information about the distribution of resources. Despite their critical importance to the success of songbird migration, the effects of landscape composition and configuration on fuel deposition rates (FDR [g/d]), the currency of migration, has not been tested empirically. Our objectives were to understand the effects of heterogeneous landscapes on FDR of forest-dwelling songbirds during spring migration. The results of field experiments were used to parameterize a spatially explicit, individual-based model of forest songbird movement and resulting FDR. Further field experiments were used to validate the results from the individual-based model. In simulation experiments, we altered a Gulf South landscape in a factorial design to predict the effects of future patterns under different scenarios of land use change in which the abundance of high-quality hardwood habitat and the spatial aggregation of habitat varied. Simulated FDR decreased as the amount of hardwood in the landscape decreased from 41% to 22% to 12%. Further, migrants that arrived in higher-quality habitat types gained more mass. Counter to our expectations, FDR was higher with lower spatial aggregation of habitat. Differences in refueling rates may be most influenced by whether or not an individual experiences an initial searching cost after landing in poor-quality habitat. Therefore, quickly locating habitat with sufficient food resources at each stopover may be the most important factor determining a successful migration. Our findings provide empirical evidence for the argument that hardwood forest cover is a primary determinant of the quality of a stopover site in this region. This study represents the first effort to empirically quantify FDRs based on the configuration of landscapes.

How do en route events around the Gulf of Mexico influence migratory landbird populations

ABSTRACT Habitats around the Gulf of Mexico (GOM) provide critical resources for Nearctic–Neotropical migratory landbirds, the majority of which travel across or around the GOM every spring and fall as they migrate between temperate breeding grounds in North America and tropical wintering grounds in the Caribbean and Central and South America. At the same time, ecosystems in the GOM are changing rapidly, with unknown consequences for migratory landbird populations, many of which are experiencing population declines. In general, the extent to which events encountered en route limit migratory bird populations is not well understood. At the same time, information from weather surveillance radar, stable isotopes, tracking, eBird, and genetic datasets is increasingly available to address many of the unanswered questions about bird populations that migrate through stopover and airspace habitats in the GOM. We review the state of the science and identify key research needs to understand the impacts of en route events around the GOM region on populations of intercontinental landbird migrants that breed in North America, including: (1) distribution, timing, and habitat associations; (2) habitat characteristics and quality; (3) migratory connectivity; and (4) threats to and current conservation status of airspace and stopover habitats. Finally, we also call for the development of unified and comprehensive long-term monitoring guidelines and international partnerships to advance our understanding of the role of habitats around the GOM in supporting migratory landbird populations moving between temperate breeding grounds and wintering grounds in Mexico, Central and South America, and the Caribbean.