Study finds Marin Swainson’s thrush winters in Jalisco


When avian ecologist Renée Cormier sat down in her Palomarin Field Station office on Monday to talk about the implications of a recently published study she co-authored on Swainson’s thrushes, she didn’t tell a woe-is-the-species story.

Swainson’s thrushes aren’t officially endangered or threatened, but they are highly dependent on riparian habitats. The songbirds act as ecological thermometers of their locales, and Ms. Cormier and three Point Blue Conservation Science colleagues found exciting readings.

Published in the ornithological journal The Auk, the study determined for the first time where exactly the Marin County population of these songbirds winter—in the western Mexican state of Jalisco, about 1,600 miles south. 

This corresponding narrowness of breeding and winter locations is called migratory connectivity, making the Swainson’s thrushes more vulnerable to changes in habitat in either region and making Marin and Jalisco kind of ecological pen pals for the songbirds’ corresponding homes.

Ms. Cormier explained the significance of the sites’ geographic ties from her field station office—her hair loosely pulled back, her face free of makeup, notes from a 4 a.m. owl survey scrawled on her hand. (“Stopping to pull out the notebook when running toward a calling owl in the dark is not that convenient.”) She radiated satisfaction with her work, as one who considers climate change and the fragility of her subjects but who bookends office hours creeping outside pre-sunrise and post-sunset, playing recorded hoots in hopes that a live owl replies and reveals its location. 

Monday morning’s dense fog kept Point Blue staff from unfurling the mist nets that catch birds and barred any Swainson’s thrush sightings. The nets work like a spiderweb with pockets: a soft mesh that stretches about as wide as a volleyball net and is undetectable to a bird fixated on a distant oak. Ms. Cormier unwound a corner to demonstrate how birds pop into the net and slip into its fold, but she froze when a nearby Swainson’s began its upward spiraling trill. 

The thrush study started in the summer breeding season in 2010 and entailed strapping a 0.9-gram geolocator tag to 35 birds caught in mist nets that were set up at Palomarin and at Muddy Hollow in Point Reyes National Seashore, as well as in Marin County Open Space District’s Bolinas Lagoon Preserve. 

The tags weigh about 3 percent of the songbirds’ body mass, looping around their legs and lying on their backs like little $165-a-piece knapsacks. The leg harnesses are adjusted so each bird can gain or lose weight.

“When they first get it, they’ll do a little bit of preening and figuring out what’s back there,” Ms. Cormier said. “But they eventually will ignore the tag.”

Staff also outfit each bird with a numbered aluminum band issued by the United States Geological Survey. Weighing less than a tenth of a gram, the band twirls around a bird’s leg but doesn’t slide past its foot.

The geolocator tag has an internal clock and a tiny stalk that sticks up between a bird’s feathers, recording light intensity every two minutes. That combination of time and daylight helps researchers determine sunrise, sunset and a bird’s latitude and longitude with about 100 miles’ accuracy, Ms. Cormier said. 

The data arrived when 11 of the tagged birds flew back into the mist nets in summer 2011, the 12th and final bird arriving last summer. 

Retrieval of the tag goes fast. Researchers blow briskly all over the bird, parting feathers to determine if it’s a male or female in breeding condition, if it’s molting, and its level of body fat; they measure the wing, weigh the bird and let it go.

“We’re a large being with big eyes, so it’s probably like having an encounter with a predator for them,” Ms. Cormier said. “That’s why we try to minimize handling time and do everything really quickly and efficiently, so that we keep the wild birds wild and get them on their way as quickly as possible.”

Of the approximate 34,000 Swainson’s thrushes banded in California since 1933, the 12 caught by Point Blue are the first to definitively link the birds between breeding and wintering grounds. 

Previously, the Geological Survey had recorded just one Swainson’s caught in California and recovered in Mexico—banded in Big Sur in 1997 and found in Chiapas in 2001.

“With the geolocator tags, finally, we’re able to pinpoint where these birds are going,” Ms. Cormier said. “That’ll help us in the future better assess what might be causing population declines or other changes in the populations if we know where they’re starting from and where they’re going to and even where they’re stopping over along the way.”

Those 12 birds’ pricey little backpacks made the link, filling in part of a much larger migration narrative, according to broad-scale landscape ecologist Philip Nott of Forest Knolls, who has worked with The Institute for Bird Populations in Point Reyes Station.

“Working on a rare species, you don’t have many data points,” Dr. Nott said. “When you’re working with something as common as the Swainson’s thrush and that’s so easy to work with, you can use all these techniques to put the whole story together. Like fruit flies, they’re easy to study. There are lots of them around. ”

While thousands of Swainson’s breed across the U.S. and Canada, population rises and falls serve as a greater ecological bellwether, especially in terms of riparian health. 

A viable habitat is crucial for fledglings at the end of the summer. The 30 percent of Swainson’s thrushes that survive their first year live an average of five years, according to research ecologist Charles van Riper III of the Geological Survey. 

And Swainson’s thrushes are crucial for a viable riparian habitat. Deforestation or reduced habitat in Mexico could force birds to winter longer in order to get enough energy to return north, according to Dr. van Riper. Faced with habitat loss, animals don’t pack, he said. Displaced animals can’t mimic the “false density” of human social systems, relying on importing resources.

And Swainson’s provide crucial support for riparian habitats, though the contribution is hard to quantify. The songbirds subsist on berries and insects, digesting and dispersing native seeds in an organic fertilizer. Pebbles and stones that accidentally make it down the birds’ beaks pass through their digestive system and break a seed’s hard shell, allowing it to germinate.

“What’s the value of something in an ecosystem?” Dr. van Riper said. “If I went to an ornithologist, they would say invaluable. But there’s no dollar figure on it.” 

Dr. Nott can’t say for certain how the songbirds know how to get from one seasonal habitat to the other. There’s evidence of magnetic particles that allow the birds to know north from south, he said. Traveling by night, the birds must use the stars, too. 

Dr. van Riper seconded magnetic fields and stars, mentioning wind and visual cues, as well. A lot of it’s innate, he said.

In her office, Ms. Cormier couldn’t exactly say, either. 

“They’re not necessarily flocking up and migrating there together,” she said. “It’s just how they’re wired essentially to know where to migrate. There are a lot of mysteries around migration. Right now, we’re just starting to unlock that mystery.”