Amy carefully lifts the threads of the mist net over the bird’s white speckled wings (briefly exposing its yellow “wing pits”) slipping them over its striped head, and carefully avoiding its stout beak. The bird is freed from the net and Amy, a permitted bird bander, holds in her hand not just a Black-headed Grosbeak, but a big opportunity. In addition to the chance to collect valuable demographic data about the grosbeak, such as age and sex, advances in tracking technology and genetics give bird banders like Amy an opportunity to learn vital information about migratory bird species.

A new study published in the journal Ecology and Evolution, by IBP scientists and colleagues from Point Blue Conservation Science and the National Park Service, integrates MAPS demographic data, individual movement data collected using GPS tracking devices, and climate data to understand the population dynamics of widely-distributed, migratory species- the Black-headed Grosbeak. Because migratory species spend their lives in two (or more) places, conserving them requires understanding their population dynamics and environmental stressors in multiple locations across their annual migratory cycle.

The Monitoring Avian Productivity and Survivorship (MAPS) and the Monitoreo de Sobrvivencia Invernal (MoSI, a.k.a. “Monitoring Overwintering Survival”) bird monitoring programs are large, long-running networks of bird banding stations that follow standardized protocols to age birds as precisely as possible using molt, plumage and other characteristics. Age data collected by these programs allow scientists to estimate important population measurements like productivity (number of young produced), recruitment (how many breeding adults are added to the population) and survivorship (how long do individuals survive). The MAPS program operates in North America on the breeding grounds of Neotropical migrant birds and the MoSI program operates on their wintering grounds from Mexico to South America and the Caribbean.

To conserve migratory species, its important to understand “migratory connectivity”- how breeding and wintering populations relate to each other. In other words, where in the wintering range do specific breeding populations overwinter. In an ideal world, MoSI banders would recapture birds banded by MAPS banders thus connecting the dots between breeding populations and their wintering grounds. But in the real world, the odds of that are akin to winning a 40 million dollar lottery jackpot. But as both the size and cost of GPS and other tracking devices decreases, our ability to use them to track bird movements expands. In addition, advances in genetic analysis allow scientists to link breeding and wintering populations using DNA extracted from a single feather collected by bird banders, as in the very successful Bird Genoscape Project.

This Black-headed Grosbeak study examined two breeding populations of grosbeaks: those breeding along the coast and Central Valley of California and those breeding in the Sierra Nevada. The researchers used demographic data from 44 MAPS stations operated by IBP, Point Blue, the National Park Service, and independent operators, and was done with support from the March Conservation Fund and Yosemite Conservancy. Researchers fitted 33 adult Black-headed Grosbeaks with archival GPS tags, which do not transmit data- instead the data must be downloaded directly from the recovered device. Archival tags are smaller and lighter than tags that transmit, which is advantageous for use on songbirds; however, they require recapturing the bird to access the data about its movements. Four of the tagged grosbeaks were recaptured and in August all four moved from their breeding grounds in California to molting grounds in northwest Mexico where they remained until October when they moved to their wintering grounds hundreds of miles farther south in Mexico.

The researchers then developed a new statistical model to analyze the effects of climate variables, such as drought and timing of rainfall on Black-headed Grosbeak populations. They showed that the signal of climate variation was clearly evident in vegetation characteristics, such as the timing of green-up and amount of primary productivity across the annual cycle. Effects of climate on the bird populations were less clear. However, as expected, survival tended to be highest in years with relatively cool, wet winters. Recruitment of birds in the coastal California population was also positively associated with cool, wet winters and with relatively early rainfall on molting grounds. Lead author and IBP Research Ecologist Jim Saracco says that the basic approach taken in the study can be extended to many other bird populations and species:

Saracco and his co-authors emphasize that the MAPS program (and the MoSI program) are ideal platforms for investigating migratory connectivity using tracking, genetic or other technology:

A bird in the hand, like Amy’s grosbeak, is an opportunity that we should make the most of. The MAPS and MoSI programs train and encourage banders to use molt, plumage and other criteria to determine age as accurately as possible. This rigorous data collection allows scientists to estimate population metrics that are critical for understanding and conserving bird populations. As techniques and technology for studying migratory connectivity advance we should all be looking for ways we can use them to get even more vital information out of the birds in our hands.