A mossy mystery nest in the hawthorn

December 30, 2022

The nest sits in the middle of the hawthorn in front of us, silent and inscrutable. We contemplate it, tucked securely among thorny branches, at the edge of a frozen slough near the Bitterroot River. Perhaps it contemplates us, too. It’s a flat, mossy cup, larger than my two fists. Our question today is simple enough: whose nest is this? But if we hope to find an answer, we’ll have to use all our powers of observation and imagination.

The cottonwood grove at Maclay Flat where we found the mystery nest. Smlk̓͏ʷsšná, McCauley Butte, is in the background.

Fellow naturalist Louise Weaver and I are at Maclay Flat today, a popular natural area near Missoula, Montana. The Bitterroot River, Nstetčcx͏ʷétkʷ in Salish, flows past us. Across the river from us is Smlk̓͏ʷsšná, McCauley Butte, and just upstream are the Fort Missoula Ponds. Like those ponds, Maclay Flat is a well-known spot for finding a diversity of birds. And we’re here on this blustery winter day, carrying the Peterson Field Guide to North American Bird Nests, hoping to get better at one form of naturalist sleuthing: winter nest identification.

Nests in winter provide an interesting conundrum. Among the leafless shrubs along the river, they’re easier to find now than during the summer. And we don’t have to worry about disturbing breeding birds or drawing in predators, as we would if we found an active nest in the breeding season. But by the same token, the bird that built it is long-gone now. The nest itself has weathered months of wind, snow, and rain, beginning the slow process of falling apart. To match a winter nest with the bird that built it is, needless to say, not an easy task.

Observing the nest

A close-up of the side of the nest. In addition to the mosses, note the sparse dried grasses, old pine needles, and cottonwood/aspen inner bark near the base.

We start by paying close attention to this mossy cup and its surroundings. It’s about a foot above eye level, near the middle of this thorny hawthorn (Crataegus sp.) – just high enough that neither of us can see inside. At the base of the nest, I can see some strips of fine, fibrous inner bark from an aspen or a cottonwood. The bark is jumbled together with fine, dead grasses and a few old pine needles, black-spotted with decay. Above this base, the majority of the nest cup has been built of mosses: long, brown and gold-green strands of a pleurocarpous species.

What about the inside? Although we can’t see it directly, I manage to get a glimpse of the inner nest by holding my phone over my head and blindly taking a picture. Mosses feature prominently here, too, along with a few more strips of inner bark and some fine, unidentifiable bits of plant matter. We look for a layer of mud, a notable feature in the nests of some species. None is apparent from our limited, ground-based view.

Listing the suspects

We can rule out cavity-nesters like tree swallows (here, nesting in a hole in an outbuilding).

Who built it? Looking at the eBird list of the 192 bird species that have been documented here at Maclay Flat, we can start by ruling out a lot of them. Clearly this nest doesn’t belong to a ground-nesting duck or grouse. It’s too small for any of our raptors. And it can’t belong to a woodpecker, chickadee, or tree swallow: these birds nest in tree cavities. Meanwhile, it’s too large for a sparrow, warbler, or vireo. It’s clearly not the distinctive “hanging sock” nest of an oriole. So which options does that leave?

We come up with a short list of possible candidates to check. Could it be a Steller’s jay? That seems unlikely, since they’re rarely seen here in the summer. American robin? The size seems about right, but robins line their nests with a layer of mud. What else might it be? We add black-headed grosbeak, eastern kingbird, red-winged blackbird, and gray catbird to our list of “suspects.” And what about a Swainson’s thrush?

Robins, Swainson’s thrushes, and catbirds

An American robin nest on an upturned bucket in a barn. Note the layer of mud and the fine grass lining.

It’s time to dig into the Peterson guide. I start by looking at the entry on American robin nests. If this is a robin nest, it’s certainly different from those I’ve noticed in the past, which seem to emphasize grasses instead of mosses in their construction. But the Peterson guide does list mosses among the materials robins will use. Once again, robins incorporate a layer of mud into their nests – something we weren’t able to see in this one – but the mud can be hidden by a lining of fine grasses. So could this be an unusual robin’s nest? Possibly.

Next I read up on Swainson’s thrushes. The two nest photos in the Peterson guide seem promising, showing very mossy structures. According to the guide, Swainson’s thrushes typically build their nests in “dense deciduous shrubs or conifer saplings.” That matches. And they use different nesting materials in different regions, with grasses, mosses, and twigs among the common choices. Their nests are also considerably larger than I would have guessed, the size range overlapping substantially with American robins. So a Swainson’s thrush seems like a promising possibility.

What about a gray catbird? These secretive thicket birds build nests with a thick outer layer of twigs or leaves. That doesn’t seem to match our nest. I tentatively eliminate gray catbird from our list.

Blackbirds, grosbeaks, and kingbirds

Red-winged blackbirds build neatly woven nests out of grasses, cattails, and other long fibers. They often nest in marshes, but it would certainly be possible to find one of their nests in a riverside thicket like this one. The neatly woven structure seems like a clear “no,” though. We can remove red-winged blackbird from our list.

How about a black-headed grosbeak? The height and nest placement is right for this species, but the nest itself isn’t. The Peterson guide describes black-headed grosbeak nests as “loose” and “thinly built,” going on to say that the “eggs are often visible through the nest bottom.” This isn’t a black-headed grosbeak nest.

Finally, I read up on eastern kingbirds. These birds build their nests “often on a horizontal limb, well away from [the] trunk.” These nests are surprisingly large considering the size of the bird, with a “roughly but well-built exterior.” Eastern kingbirds can use a wide range of materials for their nests, including mosses. This species seems like another possible match for our nest. The description of nest construction doesn’t quite seem to match, though. Neither does the propensity for building on a horizontal branch instead of a crotch in the middle of a shrub. But in spite of these slight inconsistencies, I don’t think we can definitively rule out a kingbird.

Whose nest was it?

So did this nest belong to a kingbird, a robin, or a Swainson’s thrush? Or perhaps some other species entirely? In the end, we can’t be sure. But getting a definite answer wasn’t the point of today’s foray. The point was to be curious, to remember the summer world of the birds here, and to see how much we could learn from one silent, mossy nest.

The winter nest, tucked in the middle of a hawthorn.

In the absence of a solid answer, I like to imagine that a Swainson’s thrush built this nest. During the summer, this place must have felt completely different. This house of moss and tree bark would have been well-hidden among the hawthorn leaves. Four brown-spotted eggs would have been well-camouflaged among the dappled light filtering down to this mossy cup. And above the nest, within the rustling of the cottonwood canopy, perhaps the song of a Swainson’s thrush floated down.

Do you have ideas about this nest? Have you found a Swainson’s thrush nest before? Or do you think some other species built this one? Leave a comment and let me know!

Wild With Nature