Alex E. Jahn

Conserving migratory land birds in the New World: Do we know enough?

Migratory bird needs must be met during four phases of the year: breeding season, fall migration, wintering, and spring migration; thus, management may be needed during all four phases. The bulk of research and management has focused on the breeding season, although several issues remain unsettled, including the spatial extent of habitat influences on fitness and the importance of habitat on the breeding grounds used after breeding. Although detailed investigations have shed light on the ecology and population dynamics of a few avian species, knowledge is sketchy for most species. Replication of comprehensive studies is needed for multiple species across a range of areas, Information deficiencies are even greater during the wintering season, when birds require sites that provide security and food resources needed for survival and developing nutrient reserves for spring migration and, possibly, reproduction. Research is needed on many species simply to identify geographic distributions, wintering sites, habitat use, and basic ecology. Studies are complicated, however, by the mobility of birds and by sexual segregation during winter. Stable-isotope methodology has offered an opportunity to identify linkages between breeding and wintering sites, which facilitates understanding the complete annual cycle of birds. The twice-annual migrations are the poorest-understood events in a birds life. Migration has always been a risky undertaking, with such anthropogenic features as tall buildings, towers, and wind generators adding to the risk. Species such as woodland specialists migrating through eastern North America have numerous options for pausing during migration to replenish nutrients, but some species depend on limited stopover locations. Research needs for migration include identifying pathways and timetables of migration, quality and distribution of habitats, threats posed by towers and other tall structures, and any bottlenecks for migration. Issues such as human population growth, acid deposition, climate change, and exotic diseases are global concerns with uncertain consequences to migratory birds and even less-certain remedies. Despite enormous gaps in our understanding of these birds, research, much of it occurring in the past 30 years, has provided sufficient information to make intelligent conservation efforts but needs to expand to handle future challenges.

Migration Timing and Wintering Areas of Three Species of Flycatchers (Tyrannus) Breeding in the Great Plains of North America

ABSTRACT. Descriptions of intra- and interspecific variation in migratory patterns of closely related species are rare yet valuable because they can help assess how differences in ecology and life-history strategies drive the evolution of migration. We report data on timing and location of migration routes and wintering areas, and on migratory speed and phenology, of Eastern Kingbirds (Tyrannus tyrannus) from Nebraska and Oklahoma and of Western Kingbirds (T. verticalis) and Scissor-tailed Flycatchers (T. forficatus) from Oklahoma. Eastern Kingbirds primarily departed the breeding site in September, migrating to the Amazon Basin (Bolivia and Brazil), >6,400 km from their breeding site, then used a second wintering site in northwestern South America (Colombia, Ecuador, and Peru) before returning to the breeding site in April. Western Kingbirds left Oklahoma in late July, migrating >1,400 km to northwestern Mexico, then to central Mexico and finally to Central America before returning to Oklahoma in April. Scissor-tailed Flycatchers departed Oklahoma mainly in mid-October, migrating to Central America (El Salvador, Guatemala, Honduras, and Nicaragua), ∼2,600 km from the breeding site, remaining there until early April before returning to Oklahoma. Timing of migration appears to be tightly linked to molt. Early departure of Western Kingbirds from the breeding site appears to be timed so that they molt in the Sonoran Desert region during the monsoon, whereas Scissor-tailed Flycatchers remain at their breeding site to complete molt in late summer, when insect prey are abundant. Eastern Kingbirds delay molt until reaching South America where, possibly, abundant fruit supports molt.

Long-Distance Bird Migration within South America Revealed by Light-Level Geolocators

ABSTRACT. Little is known about the timing of migration, migration routes, and migratory connectivity of most of the >230 species of birds that breed at south temperate latitudes of South America and then migrate toward the tropics to overwinter. We used light-level geolocators to track the migration of 3 male and 3 female Fork-tailed Flycatchers (Tyrannus savana) captured on their breeding territories in Argentina. All birds initiated fall migration between late January and late February, and migrated 45 to 66 km day-1 in a northwesterly direction through central South America to either one or two wintering areas. Five individuals first spent several weeks (in April and May) in western Amazonia (mainly Peru, northwestern Brazil, and southern Colombia) before moving east to spend the rest of the non-breeding season in central Venezuela and northern Brazil. One individual occupied primarily one wintering area in eastern Colombia, northwestern Brazil, and southwestern Venezuela. Fall migration took approximately 7–12 weeks to complete and covered a distance of 2,888–4,105 km. We did not analyze spring migration data because of broad overlap with the austral spring equinox. These results are the first data on wintering locations, migration timing, and routes of individual migrant passerine birds that breed in South America. Given the general lack of similar data for practically all migratory birds that breed in South America, geolocator technology has the potential to revolutionize our understanding of how birds migrate—and the threats they face—on South Americas rapidly changing landscape.

Patterns of partial avian migration in northern and southern temperate latitudes of the New World

Abstract We describe partial migration of passerine birds across temperate latitudes in the New World. Owing to lower climatic seasonality near the coast of North America, the proportion of partial migrants at high latitudes in North America should be lower when excluding coastal records. We detected a 10% decrease in the proportion of partially migratory species at high latitudes when excluding species recorded only coastally at those latitudes. We also expected a smaller proportion of partial migrants in inland North America compared with South America. However, at high latitudes (i.e. >39°) we found a similar proportion of partially migratory species but a different taxonomic makeup between continents. Within the Tyrannidae (New World flycatchers), we evaluated the latitude at which species winter in North and South America. The mean latitude at which partially migratory New World flycatchers winter in temperate South America (30.3°S.s.d.5.6)is not significantly different from the mean latitude in inland North America (30.4°N, s.d. 2.8). Partial migration of birds in the New World may be under different constraints, depending on factors such as habitat occupancy and distance of a population from the coast.

Patterns of austral bird migration in the Bolivian Chaco

Abstract Austral migrant birds were banded during austral fall (March–April) and spring (October–November) in the Chaco of Santa Cruz, Bolivia. The Tyrannidae was the most common family captured in terms of number of species and total individuals. The difference in capture numbers between the austral spring and fall seasons was the greatest for the Fuscous Flycatcher (Cnemotriccus fuscatus), although most species exhibited little change among seasons. In terms of numbers of captures across each season, no apparent trends in migratory timing were evident. The Creamy-bellied Thrush (Turdus amaurochalinus) exhibited similar fat class distributions among seasons. Within-season trends showed that in fall Creamy-bellied Thrushes with lower fat levels arrived at the study site earlier than fatter birds. Most migrant species captured were not sexually dimorphic or in breeding condition, so sexing was generally not possible. Our lack of knowledge on habitat use and migratory routes of austral migrants in the Chaco, combined with the current high rate of habitat destruction in the region, makes an understanding of the migratory patterns of these birds important.

Morphological and Genetic Variation Between Migratory and Non-migratory Tropical Kingbirds During Spring Migration in Central South America

Abstract We attempted to distinguish spring passage migrant Tropical Kingbirds (Tyrannus melancholicus) from resident conspecifics where they overlap in South America. Migrant males at our Bolivian study site had significantly less tail feather molt and longer wing chords than resident males. Migrant females had significantly longer wing chords, less flight feather molt, and less flight feather wear than resident females. We found no evidence of genetic population differentiation between migrants and residents. We also compared wing chords of migrants and residents to those of breeding kingbirds in breeding populations further south. Wing chords of migrants were more similar to those of breeders from further south than to those of breeders at our study site. An ability to distinguish migrant from resident conspecifics will be critical to understanding migrant winter ecology, migratory routes, and connectivity of migratory populations in South America.

First Documented Migration of Individual White-Crested Elaenias (Elaenia albiceps chilensis) in South America

ABSTRACT Few details are available on the migration (rates, routes, dates) of Neotropical austral migrant birds, which breed and migrate wholly within South America. Only one long-distance austral migrant breeds in the South American temperate forest biome: the White-crested Elaenia (Elaenia albiceps chilensis). However, the migratory dates, routes, and wintering locations are poorly known. During the austral summers of 2011–2013, we attached light level geolocators to breeding White-crested Elaenias at the world’s southernmost forests, on Navarino Island, Chile. The duration of fall migration of three Elaenias to the Amazonian wintering grounds was 64–96 days, while spring migration was 45–60 days. The average distance between breeding and wintering grounds was 5,932 km, which constitutes the longest migration of a Neotropical austral migrant studied to date. A better understanding of the annual cycle of Elaenias could offer new opportunities to examine the evolution of migration and population regulatio...