Seasonal feathered visitors are showing up to be counted

Now that neotropical migrant birds are settled in and established on their wintering grounds in México and Central America, the new season of Monitoreo de Supervivencia Invernal can get underway. This program seeks to determine if survival in the birds’ wintering habitats is a limiting factor for a number of species that have been identified as declining in North America.

Last year there were about 30 monitoring stations operated in Central America, according to coordinator Leticia Andino, who works for the conservation organization Salvanatura in El Salvador. Overall management for the program is at the
Institute for Bird Populations at Point Reyes Station, California.

This summer tanager was probably born somewhere in the southeast United States and migrated to Costa Rica for the winter.

ant-effort mist netting is the method for population monitoring used traditionally for breeding bird studies but now applied to wintering populations. Many migrant birds set up territories in Costa Rica just as on their breeding grounds. Birds are captured in mist nets, which are vertical nets with fine nylon mesh that are nearly invisible in the shade.

When birds fly into the net they are tangled and can be extracted and marked with colored or numbered leg bands. Each bird receives its own unique designation, and information is recorded as to species, age, sex, physical condition, and other details.

When these birds are recaptured during the same or later seasons, statistical analysis seeks to determine survival rates over the time span in question.

These techniques were developed in Europe and North America to look at breeding success of mostly resident species, and they have not been much applied to tropical birds. Results of a study from Sarapiquí were recently published in Zeledonia, the journal of the Costa Rican Ornithological Society.

Some banding stations in Costa Rica operate almost year-round, with focus on migrants in the northern winter months and residents during the rainy season, when most breed. Two established banding operations on the Caribbean coast are at Tortuguero and Kèköldi.

Long-term studies focused on the same areas and bird populations can reveal some astounding facts along with the dry data.

One male Swainson’s thrush appeared 10 consecutive years at a banding station in the Oregon Cascades. Given that population seems to winter mostly in southern Mexico, researchers calculated that this one-ounce bird flew about 35,000 miles in its lifetime.

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