The secret life of the Black-headed Grosbeak includes a food-filled fall retreat in northwest Mexico free of the demands of breeding and migration.
By Rex Graham
The songbirds’ nearly 2-month break takes place in a region of northern Mexico that turns green in late-summer and early-fall from seasonal monsoons.
Black-headed Grosbeak ‘molt migration’
New bird-tracking research has verified that the Sonora, Mexico is a major continental hotspot for Black-headed Grosbeaks, Bullock’s Orioles, Painted Buntings and other songbirds. They and take “molt migrations” to Sonora to increase their food intake as new feathers push out the old ones.
Researchers with the Institute for Bird Populations at Point Reyes Station, California, and Yosemite National Park in California reported in 2016 that after Yosemite-breeding Black-headed Grosbeaks leave the park, they flew 1,300 km (800 mi) directly to Sonora.
Molt migrations were discovered long ago in ducks, geese and other waterbirds. Most waterfowl molt close to their breeding territories.
Migration mystery solved
Black-headed Grosbeak parents fly south with worn feathers after breeding season. Their molting locations had been a mystery. The California scientists solved it with relatively inexpensive Global Positioning System (GPS) receiving devices.
They attached the 1-gm electronic devices to a leg of each grosbeak. The devices wake up every 2-6 weeks, record the GPS position and switch off. The GPS device, leg harness and leg bands weigh less than 2 g, or the equivalent of a long-distance runner carrying a laptop in a backpack. Each bird must be recaptured to download the data.
Kingbirds, Tanagers, Orioles & Buntings
The receivers clearly revealed what observations hadn’t been able to confirm – Black-headed Grosbeaks fly hundreds of km south to molt. Their molting layovers take place from mid-August to mid-October in Sonora and the surrounding region. Western Kingbirds and Western Tanagers molt at the same time in the same area.
Bullock’s Orioles use a similar molt migration to Sonora. “There is a broad range of moult locations within the Mexican monsoon region for birds of the same breeding population,” the researcher said in 2015 in the Journal of Ornithology.
Post-breeding Painted Buntings have been tracked from southwestern Oklahoma to Sonora to molt. With new feathers, the buntings fly to Central America to spend the winter. The buntings’ molt migration strategy is based on “crossing a large region of relatively low primary productivity to reach the North American monsoon region,” researchers in Oklahoma said in 2015 in the journal Behavioral Ecology.
Importance of migratory stopovers
Songbirds that breed in Europe and western Asia often stop in Israel to molt before continuing to breeding grounds in Africa.
“Bird conservation efforts have generally focused on breeding or wintering areas, but stopover sites used for molting could be at least as important for some species,” the California scientists studying grosbeaks aid in their 2016 paper in the Journal of Field Ornithology.
Molting consumes a huge amount of energy. Songbirds must become eating machines, consuming as much as 50% more food than usual. The food consumed in the breeding area has a different atomic signature than the molting-area food. Scientists can measure the changing signatures along the length of a bird’s newly molted flight feather, as if they were inspecting the stamps on a traveler’s passport. The atomic passport stamps can confirm satellite-tracking results.
For Black-headed Grosbeaks, they somehow know that beginning in August, monsoon rains transform Southeast Arizona and northwest Sonora into a food bonanza.
Second spring festival
The Tucson Audubon Society calls August its “second spring.” Local bird species breed, sometimes for the second time of the year.
The Southeast Arizona Birding Festival in August offers some of the best summer birding in the U.S. The festival features local species plus the challenge of spotting songbirds that have migrated south and Mexican species that have moved north. Black-headed Grosbeaks are often found with Painted Redstarts and other songbirds along high-elevation streams in the “Sky Island” mountains of southern Arizona and northern Sonora.
Molting grosbeaks are not always easy to find because they hide in the new plant growth. At the same time, they feed on a rich bounty of insects and seeds. Not all grosbeaks travel together. “The propensity for molt migration may vary across populations of a species,” the California scientists said.
Observing a songbird’s molt may not on the to-do list of most birdwatchers. Like scraping, sanding and slowly re-painting a house, a molting bird can look messy. But feather refurbishing is critically important to all birds.
Perching songbirds lose a few primary feathers at a time. They’re not as vulnerable as waterfowl, which lose all their primary feathers simultaneously. Molting songbirds can usually fly well enough to escape most predators.
Grosbeaks’ taste for Monarchs
The Arizona and Sonora monsoon seasons lasts about 73 to 80 days, but before it ends the grosbeaks are gone. Species-distribution maps in most North American bird guides crop-out where they go next: southern Mexico where Monarch Butterflies roost in 19 well known hibernation colonies.
Most of the butterflies hibernate within the Monarch Butterfly Biosphere Reserve, a protected area the size of San Francisco, California. More than 200,000 ecotourists visit the reserve annually.
The annual Monarch migration has captured the imagination of the world. The hibernating butterflies return north in March. They lay eggs on milkweeds in Texas and other southern states. The caterpillars are the first of a 4-generation process that culminates with migration back to Mexico.
North America’s most iconic migratory insect transforms fir trees from green to black, orange and white – the colors of their wings. These conspicuous potential meals are high in fat and protein. They might look like easy meals, but milkweed-derived cardenolides toxins in their wings, thoraxes and abdomens protect them from most predators. After a bird’s first vomit-inducing meal, Monarchs’ “aposematic coloration” greatly discourages a second.
In a famous 1950s experiment, Jane Brower, a Yale University biologist, fed Monarchs to 4 Florida Scrub Jays. They learned their lesson after eating just one: each bird quickly vomited. Later, the mere sight of a Monarch made some birds nauseated.
When jays with no experience with Monarchs were fed similarly colored Viceroys, they ate them – but unenthusiastically. The birds preferred more tasty butterflies. Chemists later discovered that Viceroys also contain a toxin, but it’s present at lower levels than the toxin in Monarchs.
Two species of mice in southern Mexico can eat Monarchs with no ill effects. Black-headed Grosbeaks and Scott’s and Black-backed Orioles use a clever trick. They remove the toxin-filled wings and abdomens and use their pointed bills to withdraw the butterflies’ nutritious wing muscles.
Female grosbeaks are confronted with a romantic dilemma: migrate north in the spring to mate, or breed with a local, non-migratory male. “There is potential for wintering birds to linger year-round and start breeding within the winter distribution,” U.S. researchers said in the journal Molecular Phylogenetics and Evolution.