Category Archives: Research and Monitoring

Field Notes: Southern Breeding Birds Are Moving North

“Whe-peet!” Hearing the explosive, snappy squeak of an Acadian Flycatcher at a Mass Audubon sanctuary would have been a huge surprise, were it not for the species’ ongoing shift northward into Massachusetts. Stumbling on this denizen of the American South used to be a downright rare occurrence here, but the northern edge of its summer range has advanced in fits and starts since the early 2000s.

When this particular bird was observed defending a territory at a sanctuary in Central Mass this summer, it was the first time it had been recorded as a likely breeder at a Mass Audubon property. Yet breaking the news in the Bird Conservation Department’s offices elicited mild enthusiasm and a hint of fatalism, with reactions ranging from “Cool!” to, “yeah, they’re comin’.”

Along with a few dozen other species, it seems this once-scarce visitor is on track to become a regular summer resident in a growing part of the state.

As Climate Changes, So Do Bird Ranges

Data from Mass Audubon’s first and second Breeding Bird Atlases showed an increase in breeding records of Acadian Flycatcher between 1974–2011.

Acadian Flycatchers are a naturally inconspicuous species, but other birds have made more dramatic entrances into Massachusetts. Red-bellied Woodpeckers are a loud, gaudy species of wet southeastern forests that have become downright common throughout southern New England. Northern Cardinals delighted birders in the middle of the 20th century as their brilliant reds and muted oranges became common sights in suburban yards and city parks.

A Red-bellied Woodpecker at a backyard feeder—in the dead of winter! Photo by Christine McCormack.

All of these range shifts have been thoroughly documented by scientists as well as casual birders. The most comprehensive effort to document these changes is coordinated by the National Ecological Observatory Network (NEON). Mass Audubon is NEON’s partner for bird data in New England, and every summer, our staff contribute bird censuses to NEON from across the region.

NEON treats birds as one piece of a vast puzzle: by studying how long-term ecological trends line up with each other, the project aims to parse out the causes and consequences of environmental change. Read more about our work with NEON in this blog post!

The Role Of Ecological Monitoring

Range shifts represent more than a curiosity to ornithologists. Rather, they are part of larger ecological disruptions caused by a warming climate and other human-caused factors like agricultural intensification, urbanization, and invasive species.

While a few species adapt to these changes and even benefit from them, they do spell trouble in the grand scheme of things. Niches go unfilled as some species’ ranges shift away from habitats they were once well-adapted to, leaving their home ecosystems in flux.  Other species’ ranges are limited by physical factors like elevation, or by the distributions of their competitors or their food source. Birds with finely-tuned ecological roles struggle to adapt to changing conditions, most bird species’ populations decline.

This makes keeping tabs on bird populations critical.

Conservationists first establish which species are declining or adapting (and why, and how) in order to target habitats to create or manage and prioritize species for legal protection.

This leads to concrete action, like advocating for the state to list acutely declining species as Endangered, or creating young-forest habitat at wildlife sanctuaries– all pieces of planning for a future with brave new ecological realities.

Join the Avian Collision Team’s Second Season

Here’s an easy way for anyone living or working in Boston to help migratory birds: help monitor window collisions!

An Indigo Bunting lies stiffly among litter, hours after striking an office building window in Boston. (Photo: ACT)

Mass Audubon is seeking new volunteers for the fall season of the Avian Collision Team (ACT). ACT is an initiative to collect data on bird–building collisions, and to rescue injured birds.

This spring, the team of birders, conservationists, and other concerned citizens observed 115 birds across 38 species affected by window strikes. This fall, and in coming seasons, we need to keep up the momentum and grow our dataset.

The Problem

Window collisions are an under–appreciated source of bird mortality in the US, causing several hundred million casualties annually.

Birds struggle to distinguish reflections from reality, and often strike glass windows that reflect the sky or nearby greenery. City lights also confuse night-migrating birds, which use the stars to navigate, and which often land near sources of light pollution. Many window strikes occur as birds try to re-orient in the morning, after being drawn in to an unfamiliar concrete jungle.

Project Details

The program runs from August 24–October 28 in downtown Boston. Volunteers need to sign up for 1-4 weekly shifts, Saturday–Tuesday, that can take place between 6-9am. Most shifts last around 30-60 minutes.

Volunteers walk predetermined routes through downtown Boston to photograph or collect deceased specimens, fill out data sheets, and occasionally rescue live birds. We’ll be holding volunteer trainings on August 11, 18, and 22.

Carrying out ACT surveys can be an eye-opening experience, between watching the city as it’s waking up, discovering seemingly out-of-place warblers, buntings, and vireos, and occasionally saving the life of an errant, injured migrant. And once you’ve found your first few birds, a collector’s instinct sometimes kicks in, making the search all the more engaging. It’s like birding with a twist– a sense of urgency, purpose, and sometimes, a touch of sadness.

If this sounds compelling, sign up here!

Joan's Video

Joan Walsh: In the field at Great Gull Island

Ever wonder what we do in the field? Check out this video of Bertrand Chair, Joan Walsh, marking Roseate Tern nests on Great Gull Island, NY. Jeff Collins, Director of Conservation Science at Mass Audubon, asked Joan to share what a day in the field looks like on Great Gull Island. Joan has worked on Great Gull Island off and on for 39 years. The island is home to 18,000 Common Terns and 3,000 federally endangered Roseate Terns.

When she reaches a Roseate Tern nest box, if there is a nest, she marks it with GPS coordinates and records data on eggs and nestlings. Roseate Terns prefer to nest under vegetation or in man-made nest boxes. The work seen in this video was part of Joan’s 5-day effort to mark nests.

Important: It is illegal to approach or disturb a nest without a permit. Joan works under a permit to research Roseate Terns and Common Terns.

*Video footage was sped up when Joan is doing the nest box marking to fit more content in 1 minute. Joan is not that speedy!

Studying Forest Structure At Elm Hill

Our bird conservation staff spent the past week collecting data at Elm Hill Wildlife Sanctuary, our demonstration site for Foresters for the Birds. Since we’re using this site to show how responsible forest management can enrich bird habitat, we need before-and-after data to compare changes in vegetation and bird diversity.

Birds See Forests For The Trees

The physical structure of a forest directly affects which birds are found there. The amount of vegetation near the forest floor (or “understory”) changes whether or not the forest can host a whole suite species that nest near the ground, like Ovenbirds and Black-throated Blue Warblers. Forests with open midstories (the layer of vegetation between 5–50 feet off the ground) attract flycatching birds like Eastern Wood-Pewees, but dense midstories appeal to Wood Thrushes and Canada Warblers.

The goal of our work at Elm Hill is to demonstrate how every forest species has its own habitat preferences, and how thoughtful land management can create habitat for declining species. Since three quarters of Massachusetts’ forests are in private hands, it’s critical to make these lands as hospitable as possible to wildlife.

Collecting Vegetation Data

A data sheet we use for recording information about trees and forest structure at Elm Hill.

Before we alter any habitat at Elm Hill, we’re recording these factors at sites where we’ve previously done bird surveys:

  • Total woody biomass (i.e., the average size and number of trees in a given area)
  • Tree species makeup (i.e., which trees are there, and how many of each)
  • Canopy density (i.e., the amount of cover provided by leaves in the treetops)
  • Sapling density (i.e., the number of young trees from around 1–6 feet tall)
  • Coarse woody debris (i.e., the number of logs and slash piles on the ground)

We outline a 400-square-meter plot with ropes at every site to make sure we collect data from the same area of land each time. This works pretty well until somebody tangles the ropes:

Jeff tangled the ropes. It definitely was not me.

While measuring trees, logs, and saplings is straightforward, you might wonder how researchers measure canopy density— and the instrument for this is quite cool. A spherical densiometer (pictured below) condenses a wide view of the canopy into a small image (much like a fisheye camera lens) with a grid over it. By estimating the percentage of each grid square occupied by leaves or trunks (and adding them up, and taking readings in each cardinal direction), we have a standardized and simple way of measuring canopy density. This also works as a proxy for determining how much light reaches the forest floor.

A spherical densiometer, used for measuring the amount of leaves in the treetops.

Long-term Goals

After foresters have cleared the woods of invasive species and created a variety of spatial habitat types, we’ll be able to show what changes this brings to Elm Hill’s bird species.

Elm Hill contains mostly 70–90-year-old forest, like much of Massachusetts. We’ll manage parts of the sanctuary for birds that prefer young forest, which are in trouble statewide, and in other part’s we’ll try to mimic old-growth forest conditions, which would take over a century to emerge naturally. Hopefully, we can then use this site as a physical example of how foresters and landowners can improve bird habitat on the properties they manage.

 

 

50 Years Of Discovery At Great Gull Island

A Season With The Terns

I first visited Great Gull Island as a volunteer in 1980. I was an undergraduate at Southern Connecticut State College, and had friends who worked on the island. I went out for a weekend, and like many others, I never really left. Sure, I’ve gone off and worked in Georgia, on the Farallones, in Cape May, and for Mass Audubon, but there is something about this place that never leaves you.

This year, the project celebrates its 50th year of full-season tern research. I wanted to go back and indulge in a full summer once again. I arrived on April 22 for an 8-week stint, which meant that for the first time, I saw the birds arrive fresh from their wintering grounds in South America. On a cold and grey April 29 morning, burrowed deep into my winter-weight sleeping bag I heard one tern, then a second calling. I raced outdoors, camera in hand, and got a video of them drifting through the clouds, calling as they came home.

A tern and a bander on Great Gull Island. Photo by Joan Walsh.

A quick check of eBird records showed very few had been reported south of us before they came home. Had they flown over water most of the way? Was the last place they saw South America? Did they fly directly to their island? The details of the transit were important to my science brain, but the shimmering white birds falling from the grey sky, calling for their 20,000 neighbors, was one of the most powerful natural moments I have ever witnessed.

They come back slowly – just a few at first, coming in in the morning, the numbers, and noise, building each day. At first they fly in tight flocks, looking more like a group of migrating Red Knots than terns. They leave at midday to go forage, then return at night and through the early morning. Then, one day you realize they aren’t leaving, and some are even courting, then more, then more.

Their tentative residence is no longer. They are home, and they are fierce.

The Setting

Great Gull Island seems unremarkable at first: 19 acres of sand and rock, formed as so much of our coastline was by the terminal moraine of retreating glaciers 10,000 years ago. The island is tiny— about half a mile long and a quarter mile wide.

The island sits in some of the roughest waters in Long Island Sound, between Plum Island and Fisher’s Island, NY. Each outgoing tide drains Long Island Sound of 2-3 feet of water, and about half of that water comes past Great Gull. A typical tide rushes by at 5 knots; a strong tide moves even faster. The sea is often heaped into 4 foot standing whitecaps, giving the water to the east of Great Gull the title of The Race, and to the west, Plum Gut.

The currents are so strong that sandlance and small herring can’t swim well against them, making them easy prey for lurking striped bass and bluefish. The currents also put baitfish at risk from aerial predators, which is one reason Great Gull Island hosts up 30,000+ Common and Roseate Terns during the summer breeding season.

In profile, the island shows a tall “hill” to the east, a flat meadow in the center flanked by another hill, and a long meadow towards the west. Much of the island is lined with huge granite boulders, similar to those used in jetties along the rest of the coast. In the summer the Island sprouts 20 or more small white-topped towers used as blinds for watching nesting birds. This, coupled with severe signs warning “DO NOT LAND” give it an otherworldly appearance – a mysterious place clinging to the bedrock as tide rush by, and thousands of birds swirl and scream overhead.

The Military History

The two “hills” seen in profile are anything but. They are the remains of a US Army coastal defense project. Great Gull was re-christened Fort Michie when the Army ejected the resident tern colony and began construction of a fort in 1896, which they occupied through World War 2. At one point hundreds of men lived on the island in a self-contained small town, complete with electricity, running water, and a hospital. The remains of the coastal defenses are still in evidence everywhere. Most notable are those cement “hills,” honeycombed with tunnels where armaments were stored.

These tunnels, and 6 gun emplacements, are still standing around the island. And the biggest of them all is the massive gun emplacement that held a 16 inch gun – it actually took a shell that was 16 inches in diameter. Three brick officer’s quarters also remain, and two concrete watch towers are still standing. The rest of the fort was reclaimed by the weather or demolished by 1960.

The Return of Nature

After World War 2, the Army sold the island to the American Museum of Natural History for $1, and for the most part little was done on the island for about 10 years.

In the early 1960s, the island was visited by a group of ornithologists from the American Museum of Natural History. One woman, Helen Hays, saw the island’s potential as a research site, grabbed ahold of her work, and 58 years later is still directing the research on Great Gull Island (GGI). A handful of Common and Roseate Terns had begun to colonize the retired US Army fort, and the visitors, being curious scientists, hatched a plan to return Fort Michie to its former natural glory (and name).

The Research

At GGI, Helen Hays has managed thousands of volunteers to amass one of the longest runs of known-lineage data in the world. It is not hard to trap a bird, read the band, and follow that bird’s lineage back to 1980. This is done by daily nest searches, aided now by GPS locations for all nests. A subset of the adult birds are trapped and banded, and as many Common Tern chicks are banded as possible. Roseate Terns, as federally endangered species, have stricter rules for handling. All nests are marked, but a very limited number of adults and chicks are handled.

Despite the presence of the researchers the colony has flourished. Hundreds of papers have come from the work, and each year more questions are brought to the forefront. Helen has encouraged work by independent researchers, and has mentored thousands of ecologists, despite managing one of the most complex research stations on the eastern seaboard.

Four years ago I began a series of simple observations on Common Tern. We had done them in the past, but what would a repeat show? I select a set of nests, and for two hour intervals record the time of each fish delivery to the young. I identify the fish, and estimate the size. Each year I choose between 10 and 30 nests to watch. Preliminary work with the data showed really impressive changes in the feeding rates for the nestlings. For two years we recorded fairly similar feeding rates – each nest received about 1.1 per hour. But in 2018 that rate tripled to 3.3 fish per hour per nest. These data support the anecdotal observations we had on GGI this year – there were a lot of fish!

Data like these are important for understanding the variability in chick survival, and can act as a surrogate for understanding changes in fish populations. Coupled with the long-term research on annual productivity and survival rates on the island, this helps us to flesh out what is happening in Long Island Sound from year to year, and as the waters warm due to climate change.

 

A team of researchers from Argentina that visits Great Gull every Summer. Photo by Joan Walsh.

Two Poems for Bobolinks: Dickinson and Bryant

Mass Audubon’s Arcadia Wildlife Sanctuary: a few miles from where Emily Dickinson was inspired by Bobolinks- which have returned to the property thanks to careful stewardship.

References to Bobolinks abound in poetry from 19th-century New England. Massachusetts authors drew inspiration from local birds for a host of reasons, not least because they saw local species as uniquely American subjects (as opposed to, say, the European Nightingale). Bobolinks and Meadowlarks helped distinguish their work from other English-language poets’, and perhaps more importantly, ground it in a sense of place.

Bobolinks were also particularly an familiar and evocative sight through the 1800s and into the past century. Widespread low-impact agriculture provided habitat for field-loving Bobolinks, which don’t mind living near humans as long as their nests are undisturbed. Conspicuous and bold, Bobolinks became an icon of the countryside, and a cultural touchstone for many.

Emily Dickinson, one of rural Massachusetts most-celebrated poets, took a particular liking to them. Bobolinks recurred as a motif in more than 20 of her works. Dickinson often made the birds into rowdy or joyfully anti-authoritarian figures, as here:

 

Some keep the Sabbath going to Church –
I keep it, staying at Home –
With a Bobolink for a Chorister –
And an Orchard, for a Dome –

Some keep the Sabbath in Surplice –
I, just wear my Wings –
And instead of tolling the Bell, for Church,
Our little Sexton – sings.

God preaches, a noted Clergyman –
And the sermon is never long,
So instead of getting to Heaven, at last –
I’m going, all along.

–Emily Dickinson

 

Loosely interpreted, the poem emphasizes finding joy in nature and in the everyday. Here, the Bobolink is part of Dickinson’s everyday “Heaven” on earth; its song part of her quiet resistance to organized religion. Dickinson had studied religion in a seminary, but perhaps tellingly, dropped out after a year.

Dickinson always ascribes human qualities to the bird to illustrate a point—whether as a “Sexton” (someone who rings the bells of a church) calling her attention to beauty in nature, or in other poems, as a disruptive “Rowdy of the Meadow.”

Other poets, however, grounded poems in Bobolinks’ natural history and biology, although few connected them with complex societal themes as adroitly as Dickinson. William Cullen Bryant, for example, managed to accurately convey key points about Bobolinks’ seasonal behavior (despite leaning pretty heavily on twee personification and cutesy metaphors):

 

Merrily swinging on briar and weed,
Near to the nest of his little dame,
Over the mountain-side or mead,
Robert of Lincoln is telling his name:
Bob-o’-link, bob-o’-link,
Spink, spank, spink;
Snug and safe is that nest of ours,
Hidden among the summer flowers;
Chee, chee, chee.

Robert of Lincoln is gaily drest,
Wearing a bright black wedding-coat;
White are his shoulders, and white his crest;
Hear him call in his merry note:
Bob-o’-link, bob-o’-link,
Spink, spank, spink;
Look what a nice new coat is mine,
Sure there was never a bird so fine.
Chee, chee, chee.

Robert of Lincoln’s Quaker wife,
Pretty and quiet, with plain brown wings,
Passing at home a patient life,
Broods in the grass while her husband sings:
Bob-o’-link, bob-o’-link,
Spink, spank, spink;
Brood, kind creature; you need not fear
Thieves and robbers while I am here.
Chee, chee, chee.

Modest and shy as a nun is she;
One weak chirp is her only note,
Braggart and prince of braggarts is he,
Pouring boasts from his little throat:
Bob-o’-link, bob-o’-link,
Spink, spank, spink;
Never was I afraid of man;
Catch me cowardly knaves, if you can !
Chee, chee, chee.

Six white eggs on a bed of hay,
Flecked with purple, a pretty sight!
There as the mother sits all day,
Robert is singing with all his might:
Bob-o’-link, bob-o’-link,
Spink, spank, spink;
Nice good wife, that never goes out,
Keeping house while I frolic about.
Chee, chee, chee.

Soon as the little ones chip the shell,
Six wide mouths are open for food;
Robert of Lincoln bestirs him well,
Gathering seeds for the hungry brood.
Bob-o’-link, bob-o’-link,
Spink, spank, spink;
This new life is likely to be
Hard for a gay young fellow like me.
Chee, chee, chee.

Robert of Lincoln at length is made
Sober with work, and silent with care;
Off is his holiday garment laid,
Half forgotten that merry air:
Bob-o’-link, bob-o’-link,
Spink, spank, spink;
Nobody knows but my mate and I
Where our nest and our nestlings lie.
Chee, chee, chee.

Summer wanes; the children are grown;
Fun and frolic no more he knows;
Robert of Lincoln’s a humdrum crone;
Off he flies, and we sing as he goes :
“Bob-o’-link, bob-o’-link,
Spink, spank, spink;
When you can pipe that merry old strain,
Robert of Lincoln, come back again.
Chee, chee, chee.

– William Cullen Bryant

 

Bryant’s poem draws a parallel between the Bobolink’s behavioral changes over a breeding season and a human who is burdened with work and worry as they age. But the poem is essentially fanciful, and its goal is mainly to describe these seasonal arcs with flowery language. Still, it’s a rare poem for weaving in a significant amount of natural history.

One could say that Dickinson’s and Bryant’s poems have different goals. Dickinson uses the Bobolink as a device to illustrate the experience of finding joy and religion in nature; she ascribes human qualities to a bird to tell us something about ourselves. Bryant’s poem ascribes human qualities to a bird, but more to illustrate points about the bird itself.

Which poem do you prefer? Do you know of any contemporary poems about Bobolinks—or maybe have written one yourself? Share with us below in the comments!

You can also learn more about (currently-living) Bobolinks and how to protect them at Mass Audubon’s Bobolink Project website.

 

Conservation Success Stories: The Osprey

Ospreys are on the rebound after a troubled past. Despite a history of pesticide poisoning, persecution, and population declines, Ospreys have returned as one of the most abundant raptors of the coast. Today, the Osprey’s story stands as a testimony to the power of scientifically-informed environmental activism.

An Osprey stands watchfully on a snag over a marsh. Photo © William Freedberg 2015

DDT: A Silent Threat

Osprey numbers crashed dramatically following the widespread use of DDT, a pesticide deployed across America in the 1940s. While previous decades saw Ospreys hunted as “pests” and their wetland habitats drained for development, the introduction of DDT all but rang the death knell for the entire US Osprey population.

Nobody realized it at the time, but DDT builds up in animals’ body tissue, and persists in the environment years after being sprayed on farm fields. This spelled trouble for birds of prey: while DDT spraying rarely poisons adult birds to death, it destroys the structure of raptors’ eggshells, preventing them from reproducing.

As a result, Ospreys declined by over 90% between 1950 and 1970. When the now-famous environmentalist Rachel Carson finally named DDT as the culprit in her book Silent Spring, the discovery ignited a movement. A coalition of the National and Massachusetts Audubon societies, as well as local land trusts and nationwide advocacy groups, intervened on behalf of all species threatened by DDT. They sued the Environmental Protection Agency to ban the pesticide—and won.

A few decades later, Ospreys are almost back to their pre-DDT abundance.

A Place To Nest

The DDT ban eliminated a significant threat to Ospreys, but the bird didn’t immediately bounce back. Even in places with clean water and plenty of fish, Osprey numbers are naturally limited by the number of appropriate nest sites. They normally require a tall, dead tree at the edge of a marsh, but lacking standing trees in the open, they settle for utility poles or other problematic locations.

Artificial nest platforms are one solution. In addition to keeping Osprey nests away from telephone wires and buildings, nest platforms increase the number of Ospreys any wetland can support. With wetland edge habitats constantly losing ground to development, it’s critical to maximize the number of Osprey nesting in appropriate wetlands.

Mass Audubon Continues To Support Ospreys

Mass Audubon’s South Coast Osprey Project maintains about 100 Osprey nest platforms. The project also monitors and records data on the Osprey population, including banding and tagging several birds, and tracking their movements. The data never fails to yield exciting results— whether demonstrating Ospreys’ reliance on the spring herring migration for food, or revealing variability in Ospreys’ choice of wintering grounds (South Shore birds end up in places as far away from one another as Cuba and Bolivia).

If you love Ospreys as much as we do, consider sponsoring a nest platform!

Field Notes from the Quabbin: Moose Mamas and Magnolia Warblers

A Magnolia Warbler perches in a shrub. Photo by William Freedberg

A late-June morning at the Quabbin Reservoir:  Winter Wren songs echo from hollows and wetland thickets. Blackburnian Warblers whisper their buzzy, quiet notes from the canopy. Blue-headed Vireos squeak and argue over perches in spruce trees. These birds, like many species that prefer conifers or highland forests, are rarely seen outside of migration in eastern Massachusetts. At the Quabbin, it’s hard to miss them.

In the background there’s a constant clamor from the Quabbin’s most vocal and abundant woodland breeders: Red-eyed Vireos, Veeries, Ovenbirds, and Scarlet Tanagers.

Amid the birdsong, there’s the faint scratching of my pencil and clipboard. Point 4, Minute 1 // Scarlet Tanager. Male. Calling. 19 meters // Ovenbird. Unknown sex. Singing. 33 meters.  The birds are a thick during dawn chorus, and between recording each species, sex, behavior, and distance from me, it’s hard to keep up. The survey period passes quickly, and it’s time to hike to the next count site.

The data we collected that morning will eventually be used by the National Ecological Observatory Network (NEON), an organization which studies large-scale environmental change at over 70 sites across the US, and which partners with Mass Audubon to collect bird data in New England. Normally, it takes just one person to run bird surveys—but recent security concerns at the Quabbin mean that all field technicians are now accompanied by a NEON escort to drive around restricted-access roads.

My field escort that day was Jamie, a soft-spoken botanist who had just moved to New England from Appalachia. When he mentioned he hoped to see his first moose in the Quabbin, I thought it was a farfetched idea, not knowing that around 100 moose make their year-round homes there. As if to prove a point, a cow moose stepped into the road later that day as we were on our way out.

A blurry, through-the-windshield photo of the moose that wouldn’t move.

We paused before inching the car towards the moose. The moose paused, too, and took several deliberate paces towards us. We noticed a smaller second pair of ears protruding from the roadside vegetation, and realized the moose was putting itself between us and its calf. That was our cue to put the car in reverse and give it some room; we would have to wait it out if we wanted to get to our destination safely.

Rolling down the windows, we were surprised to hear a singing Magnolia Warbler, a rare breeder for Central Massachusetts. The banana-yellow male flew across the road, giving us excellent looks at his black mask and bold stripes. I broke out a granola bar and Jamie unwrapped his lunch as we watched the moose feed and listened to the warblers. The mammoth ungulate in our way eventually ambled off, but we were in no rush to leave: life at the Quabbin was good.

Part II: Chemical Clues Help Track Migratory Birds

 

Last week, we posted an article discussing how isotope ratios in a bird’s body vary depending on its past diet and geographic location. Archived in blood and hard tissue, isotopes leave a chemical record of a birds’ life history.

 

A Golden-winged Warbler on its breeding grounds in New York. Thanks to hydrogen isotope studies, we know this bird likely winters in northern Colombia. (Photo © Will Freedberg)

 

Isotope analysis has enabled a number of important discoveries about bird biology, including the following:

 

American Redstarts’ Summertime Breeding Success Depends On Winter Habitat Quality

American Redstarts feed most successfully in wet habitats, although they tolerate and occupy dry habitats as well. A team of Canadian scientists examined redstarts with varying degrees of breeding success, and used carbon isotopes to determine if there was a relationship between a redstart’s wintering grounds and the number of offspring it raised on its breeding grounds.

The ratio of Carbon to its lighter isotope, 13C, varies between wetlands and uplands because of plant metabolism: plants metabolize carbon in one of three ways, and the one most prevalent in wetland plants requires that they store more 13C. This extra 13C accumulates in the bodies of insects that feed on plants, and eventually in the redstarts that feed on insects.

The Canadian team previously showed that redstarts in wet winter habitats had more 13C in their blood cells, enabling them to identify birds on the breeding grounds that had overwintered in wet habitats. The scientists tested birds on the breeding grounds, finding that redstarts that raised more offspring in the summer also had higher concentrations of 13C in their blood, showing that they had occupied better-quality winter habitat.

By showing that wet habitats will do more for redstarts’ reproductive success than dry habitats, this study helps conservationists set priorities for protecting their wintering habitat.  More broadly, this study underscores that not all winter habitat provides what birds need to thrive during the breeding season.

 

Hydrogen Isotopes Trace Golden-Winged Warblers To Wintering Grounds

 Just as carbon isotopes show if a bird has recently occupied wet or dry areas, hydrogen isotopes help approximate the latitude and elevation of a migrating bird’s origin. A heavy hydrogen isotope, deuterium, occurs in different concentrations depending on climate and weather patterns; its extra mass means it requires more energy than regular hydrogen to evaporate. So, deuterium concentrations in water vary consistently with latitude and elevation, and as birds drink and feed, their bodies build up deuterium concentrations according to their geographic location. This map shows different concentrations of deuterium collected from songbirds across the US:

 

Adapted from Hobson and Wassenaar (1996). © Springer-Verlag Berlin Heidelberg 1996

Researchers used these patterns to determine if Golden-winged Warblers that bred in similar areas in the US stuck together on their wintering grounds. The Golden-wings’ closest relative, the Blue-winged Warbler, doesn’t do this at all—a bird from Ohio is just as likely to overwinter in Honduras as it is to end up in Mexico. But certain breeding populations of Golden-winged Warbler are in much steeper decline than others, and researchers wondered if it could be because of trouble on their wintering grounds. Deuterium analyses showed that Golden-winged Warblers follow strict migratory routes: birds in Venezuela came almost exclusively from the declining population in the southern Appalachians, while birds in Central America came from the more secure Great Lakes population. These findings suggested that regional differences in population trends might be due to specific threats along migratory routes or on wintering grounds.

 

Nitrogen Isotopes Reveal Changes In Whip-Poor-Will Diet

A study of nitrogen isotopes In Whip-poor-wills showed that the quality of Whip-poor-wills’ diet has diminished over time, contributing to their decline. A heavy nitrogen isotope, 15N, increases 2-4% with every step in the food chain. As predators consume more prey items, they excrete lighter isotopes and accumulate 15N in their bodies. So, if a Whip-poor-will eats mostly large, predaceous insects, it accumulates 15N faster than a Whip-poor-will eating mostly herbivorous insects—which also tend to be less protein-rich and less nutritious. By analyzing the 15N:14N ratio in the feathers and claws of Whip-poor-will specimens in natural history museums, ornithologists found that the quality of their diet has indeed declined through the past few decades. This study was widely popularized in the news in addition to scientific journals.

 

 

Chemical Clues Help Track Migratory Birds (Part I)

Photo © Will Freedberg 2016

Where did this Scarlet Tanager spend the winter? A birder might correctly say “South America,” but where specifically? How can we know the country, the province, the latitude and longitude?

In fact, chemical signatures in feathers, blood, and muscle tissue tell the story of where a bird has been, and what it’s been eating. Over the past couple of decades, ornithologists and chemists have learned how to read into birds’ life histories by analyzing the isotopes that have built up in its body.

What’s An Isotope?

An isotope is just a lighter or heavier-than-normal atom of a given element (like hydrogen, carbon, or nitrogen).

Two kinds of particles contribute to an atom’s mass: protons and neutrons. Most atoms of any element have the same mass because they share the same number of protons and neutrons. While the number of protons in an atom define its identity and properties, adding a neutron to an atom rarely changes its behavior (other than its mass). For example, a bird will suffer no ill effects if a bird’s body contains an abnormal ratio of carbon 13 to carbon 12 (that is, “normal” carbon versus heaviercarbon with an extra neutron).

“You Are What You Eat”

Like any living beings, birds incorporate nutrients from their food into their bodies as they grow. The atoms in these nutrients, some of which will end up in a growing bird’s flesh and blood, exist in different isotopes depending on a variety of factors.  Some isotopes are more common at certain latitudes, or in certain groups of plants, or at a certain level of the food chain. So, a bird’s body will contain different proportions isotopes depending on its location and diet. By analyzing the ratio of one isotope to another in a bird’s body, scientists get an idea of where individual bird is from and what it’s been eating.

Advantages To Isotope Studies: Scale And Cost

Isotopes are particularly useful in long-term or large-scale studies. Isotope analysis ismuch cheaper and often less invasive than attaching geolocators to migratory birds, many of which are too small to carry a transmitter (meaning someone must recapture the bird to recover tracking data).

Studying isotope ratios in natural history museum specimens also provides unique historical perspective on how birds’ diets or ranges have changed over time.

In the next post, we’ll go over a few studies that have used isotopes to track long-term changes in bird diet or established where populations of breeding birds overwinter. Stay tuned!