Birds don’t fit well into many of the boxes we’ve created for them, but flyways do have some merit. A “flyway” is a rough route of travel used by birds during spring and fall migrations. In North America, there are four migratory regions recognized: Pacific, Central, Mississippi, and Atlantic Flyways. Though these divisions were created in part to establish units for conservation and management, they were also created with natural migratory travel routes in mind (National Audubon Society 2021, US Fish and Wildlife Service 2021). Millions of birds travel these paths each year, moving between wintering and breeding grounds and vice versa. Many of the species winter thousands of miles from where they breed and are termed Neotropical migrants. Neotropical meaning “new tropics”, as in tropics of the New World: Central and South America. This is where they winter and spring is when they move through North America in full force, working their way to breeding grounds across the continent.
The four migration regions are centered on major land features that promote migration in some way, from food abundance to uninhibited paths of travel. This is easy to see along the Mississippi Flyway, which encompasses the immense Mississippi River Delta, extending from the Gulf Coast to northern Minnesota. It’s a major thoroughfare for Neotropical migrants that make landfall along the central Gulf Coast, after an exhausting and dangerous flight across the Gulf of Mexico. The Mississippi Delta provides habitats that are ideal foraging stops for migrant birds of all kinds: songbirds, waterfowl, wading birds, etcetera. There are also no major land features to inhibit movement. Take it from someone who grew up in the Delta, it’s almost nothing but flat! From the coast to northern forests, there’s hardly a feature over 1000 feet in elevation the whole way (National Audubon Society 2021).
Spring in the heart of the Mississippi Flyway is one of my favorite events to witness. By early May, travel along the Flyway Highway is at its peak in the region. Warm fronts from the south carry migrants north, while lingering cold fronts attempt to hinder their movements. When these two forces are at odds, migrants may be left stranded waiting for better conditions. On the right day, birding can be so fast-paced that it leaves your head spinning, only to be rivalled by birding in the tropics. Choosing to visit the right mix of habitats can easily yield close to a hundred species or more. For this text, though, I’ll focus on migrant songbirds, primarily those travelling through the Delta’s forested habitats.
Warblers. Every birder dreams of them all the year through. These most coveted of spring migrants are like Nature’s crown jewels, many of which are fast-moving travelers along the Mississippi Flyway Highway. Their taxonomic family, Parulidae, is incredibly diverse and includes over 100 species in 18 genera. Of these species, a little over half are permanent residents in the tropics. Most of the remaining species are neotropical migrants, wintering in the tropics and breeding in North America. An elite handful are permanent residents in North America, migrating comparatively short distances (Winkler et al. 2020). As with many large bird families that are closely related, their taxonomy seems to gets shuffled around every few decades. Currently, most species are in genus Setophaga, which includes many of the common warblers you might expect to see during spring migration in North America. Although, many of the most common are in smaller genera, including some that are monotypic, meaning they’re the only members of their genus! Here are some of their stories.
Car windows down, but not yet stopped, a high-frequency “seet, seet, seet, seet, seet…” came from a nearby tree. Blackpoll Warbler (Setophaga striata). What a first bird of the day! Though not uncommon, Blackpolls aren’t our run-of-the-mill warbler in Arkansas; a good omen. I searched the trees, eventually finding it in the lower branches of a great hickory. This Blackpoll was a male, overall a matrix of white with heavy black streaking. It’s black cap and white cheeks stood in high contrast to each other. A closer view yielded yellow edges to flight feathers and—whoa—those feet! Bright orange. My local enjoyment of Blackpoll Warbler is fleeting. In fact, Arkansas is at the fringe of their migration route. Rather than cross the Gulf, most Blackpoll Warblers migrate from South America via the eastern Caribbean and make landfall in eastern Florida. This part of the country is but a stop along the way to their breeding grounds in the boreal forest—what a vague designation! While in the boreal, they center on habitats with black spruce and tamarack trees, and subalpine zones (DeLuca et al. 2020).
Though the Blackpoll Warbler appeared to be alone, not far away a Carolina Chickadee called. Why, on a day that has so much potential for more exciting finds, would I clue into the presence of a Carolina Chickadee? Chickadees are just one very vocal member of the forest bird assemblage that forms mixed feeding flocks. They’re not hard to find thanks to their constant chatter, and their affinity to join feeding flocks often leads observers to other birds. This particular chickadee did just that. With the chickadee were at least fifteen other species, most of them small migrant songbirds, including the ubiquitous Yellow-rumped Warbler and much-coveted Golden-winged Warbler.
Yellow-rumped Warblers (Setophaga coronata) may associate with neotropical migrants, but they are hardly so themselves. They breed in northern and mountainous coniferous forest like many of their relatives, but rather than winter somewhere deep in the tropics, most remain in the southern half of North America. This time of year, they look completely different than their drab-gray winter selves. In the east, their snazzy breeding plumage is dark gray, black, and yellow, with a white throat. In the west, the white throat is replaced with yellow. Myrtle and Audubon’s Warblers, respectively, are two of four recognized subspecies. The others—Black-fronted and Goldman’s Warblers—are more restricted in range (Hunt and Flaspohler 2020). No matter the flavor, they all sport a bright yellow spot just above the tail, which affords them both their official name and colloquial among birders: “Butterbutt”.
Back to the birding. Among the aforementioned mixed flock was a Golden-winged Warbler (Vermivora chrysoptera), a stunning yet diminutive bird. This small warbler is quick at foraging, acrobatically maneuvering through leaves from mid-level to canopy. It’s a tough one to get eyes on, but it doesn’t take much of a glimpse to know what it is. The beautiful yellow feathers of their forehead and wing patches sit strikingly against a field of gray. A black bib and mask add to their elegant dress. I tracked it for a while, moving among the leaves that hid it from view so easily.
The Golden-winged Warbler is in trouble and the trouble is complex. To start, the species actively breeds in transitional habitats in the early stages of reforesting, like old farmland, forest clearcuts, and old burned areas. As these habitat types have declined within their breeding range, so have the warblers. To further the issue, Blue-winged Warblers (Vermivora cyanoptera)—a very closely related species—are expanding into the range of the Golden-winged. Blue-winged and Golden-winged Warblers are so closely related that they test the limits of what makes a species a species. The two actively hybridize and produce viable and recognizable offspring: dominant “Brewster’s Warbler” and recessive “Lawrence’s Warbler”. Lawrence’s is rare, but Brewster’s is common in the Northeast. While the hybrids are beautiful, they don’t help the case of the declining Golden-winged Warbler. I can’t help but consider the issue anytime I see these gorgeous species (Confer et al. 2020). I wonder what will come of them in the future?
Early in spring migration, leaves are small enough that it’s easy to see all that goes on while songbirds forage, but by early May, when migration gets really heavy, they’re difficult to keep up with. It doesn’t take a very large leaf to hide a warbler! As I tried to track the Golden-winged Warbler, other species manifested from among the leaves. A little closer to the ground, a Tennessee Warbler (Leiothylpis peregrina) showed itself by hover-gleaning insects from the underside of a leaf. Hover-gleaning is an interesting behavior that some small songbirds engage in. Exactly as it sounds, they hover around leaves or twigs and pick tiny invertebrates from their undersides. This individual was a female of yellow-gray plumage. Nearby, a male sang and I soon spotted it. Some songbirds don’t readily vocalize during migration, but the Tennessee Warbler is most willing to talk. Like the chickadee, it’s staccato song has led me to many a mixed flock. It starts off a series of slow “dit, dit, dit, chip, chip, chip”, then becomes fast, as if the bird begins calm and ends quite annoyed. The subtle browns, grays, and yellows of the Tennessee Warbler follow suit with other members of its genus. Half of them occur in the arid west and one, the Colima Warbler, is practically endemic to the Sierra Madre Oriental range of northern Mexico (Rimmer and McFarland 2020).
Often, the first songbirds I see in a mixed flock are those like Tennessee or Yellow-rumped Warbler that make themselves obvious in some way; the former for being noisy and the latter for being abundant. The longer you stare, though, the more interesting things can get. Among the flock, both occasionally singing, were two species that I repeatedly have trouble differentiating by sound. The first, Chestnut-sided Warbler (Setophaga pensylvanica), is present in nearly every migrant flock on my end of the Mississippi Flyway. Their “very, very pleased to meet cha!” is boisterous and loud, but sounds all too similar to their travelling partner, American Redstart (Setophaga ruticilla), and it’s most common song. To further complicate matters, the American Redstart has a vocal repertoire with multiple songs that don’t sound similar.
I spotted the Chestnut-sided first, it’s light coloration standing out among the greens of a forest darkened by cloud cover. Striking rusty sides and various patches of yellow make this an interesting bird to look at. While many warblers have not done well with land changes associated with humans, the Chestnut-sided has thrived in secondary growth habitats. The species was rarely reported by early ornithologists, probably because it was confined to natural secondary growth, like burned or wind-damaged woodland. Post-settlement, secondary growth forest became the norm as the east was ravaged by logging. Today, it’s one of the most abundant migrant songbird breeders in its range, outperforming many relatives that prefer rare old growth, rather than secondary growth forest (Byers et al. 2020).
Back into the trees my eyes went and I finally spot the American Redstart. Both sexes have flash patterns on their tail, which is often fanned to expose brightly-colored orange spots. Thanks to this display, I spotted the dark male as it fanned its tail, likely to scare potential prey from among the leaves. American Redstarts are unlike any other warbler. Males sport a nearly all black plumage, with Halloweenlike orange accents. The female is mostly gray with orange-yellow accents. Truly stunning to behold. Redstarts are widespread breeders that use moist deciduous forest from central Canada to the Gulf Coast in the east. Though they do breed in Arkansas, redstarts are more common on my end of the Mississippi Flyway during migration, with breeding centered on riparian uplands, rather than Delta lowlands (Sherry et al. 2020). The present redstart is not that closely related to the array of tropical redstarts (though they’re also New World warblers), and it’s certainly not related to Old World redstarts, which aren’t warblers at all!
As if it weren’t enough that Chestnut-sideds and redstarts sound alike, another similar-sounding song caught my ear, but it wasn’t quite right for the former two. After a lengthy search, a beautiful male Magnolia Warbler (Setophaga magnolia) emerged from behind a large oak leaf and sat for a sacred few seconds for me to enjoy. My heart skipped a beat. “Mags”, as they’re sometimes called, are a personal favorite despite a misleading name. The bird has nothing to do with magnolia trees, save for inadvertently landing in them during migration along the Gulf Coast. In 1810, a migrating individual was unlucky enough to stumble into such a tree in Mississippi and was subsequently shot by an ornithologist, becoming the species’ first known specimen (Dunn and Hall 2020). Regardless of whether or not the species actually has an affinity for magnolia trees, they surely make the prettiest warbler top ten…if such a list can even be made from the more than 100 radiant contestants.
Behind the Magnolia, I caught an electric flash of orange. Move over Mag, this was the Holy Grail, Blackburnian Warbler (Setophaga fusca). I dream of seeing this species every migration and will never get enough of them. Among simple black and white that covers most of the male’s body, is dizzying orange on the head and breast. Blackburnian’s are treetop specialists and move quickly through the canopy. I tracked this bird for a few minutes as it moved from treetop to treetop, but it was soon out of sight. Next stop, boreal forest!
Foraging among the sizable migrant flock were some species that, rather than being on their way to some faraway breeding ground, had already reached it. These included the tiny Northern Parula, and larger Black-and-white and Yellow-throated Warblers.
Dainty Northern Parulas (Setophaga americana) are present in many forested habitats throughout their range, especially riparian areas. Often hidden in the canopy, they are readily located by song; typically an ascending trill, though several other song types exist. Parulas spend their days foraging for insects and spiders among vegetation and often engage in hover gleaning, as I described for Tennessee Warbler. Yet again, we have a beauty on our hands. Males and females both are a palette of blue-gray, yellow, and white. To add flair, males also sport a multicolored collar of orange and dark blue. Unlike some of its longer-distance relatives, most Northern Parulas winter in the Caribbean and southern Mexico. Then there’s a name controversy. I’ve heard “parula” pronounced two ways: “pa-rue-lah” and “pear-uh-luh”. According to Birds of the World, neither is correct, with “PAR-a-la” or “PAR-ya-la” being the two options. I think they need to poll birders, because I’ve definitely never heard either of these (Moldenhauer and Regelski 2020).
Below the parulas, Black-and-white Warblers (Mniotilta varia) maintained a territory. A male and female foraged near each other and the male occasionally stopped to sing its song that sounds like a rusty wheel in need of oiling. Though the song is loud, it’s also high-pitched, so is often one of the first songs my more aged birding friends can no longer hear! For a species with dichromatic plumage, I’ve always thought Black-and-whites were lookers. It helps that they’re common and easy to view, usually foraging near eye level. Black-and-whites forage like nuthatches, working up and down tree trunks and limbs with strong feet. They’re the only member of their genus, Mniotilta, and are most closely related to Prothonotary and Swainson’s Warblers (Kricher 2020).
Not far from the mixed flock that was beginning to move elsewhere, was a lonely set of pines. From it, sang a Yellow-throated Warbler (Setophaga dominica): a descending “teeew, teeew, teeew, teeew, tew, tew”. The Yellow-throated Warbler is the only continental member of genus Setophaga with such a compact year-round range. They winter further north and breed further south than their congeners, and some even remain year-round along the Gulf Coast (McKay and Hall 2020). There is little dimorphism between the plumage of males and females, and both are just lovely. They’re early to arrive in my region and their diagnostic song is something I look forward to hearing, letting it draw me from the late-winter doldrums into the hope of spring.
Not far from the pines, an ephemeral pool held water from recent rains. From it, I kept hearing a loud “chip” that told me a waterthrush must be nearby. I found the bird working its way around the edge of the pool, picking insects from the moist leaves. All the while, this drab bird wagged its tail. It was indeed a waterthrush. Not a thrush at all, but a large warbler! Both species are associated with water; one, clear streams and the other, standing water of bogs and swamps. Both species are also brown on top, white below, with dark streaks running from the throat to the flanks, and a bold white eyebrow. During migration, habitat overlap occurs, so a closer inspection was needed. Creeping nearer, I was able to get a better view of the bird. Rather than a thick eyebrow and clean white flanks, this bird had a thin, yellowed eyebrow and a slight yellowish wash to the throat and flanks. A Northern Waterthrush (Parkesia noveboracensis). This species is a migrant in the Mississippi Flyway, breeding in bogs of the boreal region. It’s congener, Louisiana Waterthrush (Parkesia motacilla), breeds along streamsides across eastern North America (Whitaker and Eaton 2020). So ended the day’s list of warblers.
In North America, most of the nearly fifty species of warblers are very similar in size. Some have slightly larger bills, while others slightly smaller that allow a bit of separation in prey size among species, but for the most part, they all feed on small invertebrates. As you might expect from such a large family, there are an array of habitat preferences that provide further separation from one another. However, many of these warblers overlap in habitat preferences, which means they are even likely to overlap in breeding territory. What comes to mind in my own experience are breeding warblers of the Ozark Highlands—Northern Parula and Cerulean Warblers in the canopy of a tree whose trunk may be used by Black-and-white Warblers, and that same tree set among a patch of understory vegetation used by Ovenbird and Worm-eating Warbler, not to mention their close neighbors the Hooded and Kentucky Warblers. What a territorial mess!
Boreal-breeding warblers are a classic example of this niche partitioning in ecology; a niche being the role an organism takes in an ecosystem. Within this idea is encompassed foraging behavior. What happens when you have different warblers within a patch of habitat, all gunning for the invertebrates? Many ecologists over the years have been interested in this very question. MacArthur (1958) sought to explore how five warbler species could seemingly inhabit the same breeding territory in mature conifer forest and not exclude each other. What he found was incredible. In cases where his study species not only overlapped in territory, but were likely to forage in the very same trees, each species foraged at slightly different places within a tree. Thus, reducing competition among the other species present and allowing each to persist without exclusion (MacArthur 1958).
Amazing! Though their relationships to one another sound mostly fine, warblers’ relationships to humans are not. Many species are sensitive to habitat change and over 20 percent of New World warblers are plagued by habitat loss and subsequent declines in abundance. Surprisingly, only two species have been lost, but further study and conservation measures will be essential to make sure others don’t share the same fate (Winkler et al. 2020). Though migration has mostly passed and the breeding season is upon us, you can still get out and enjoy the numerous breeding warblers in your region, no matter where you are in North America! Let their beauty pull you through the heat of summer to come.
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