Mourning Warbler

Caterpillars, beetles and their larvae, spiders, and other insect larvae make up the bulk of the Mourning Warbler's diet during the breeding season. The species also takes mayflies opportunistically. One of its more distinctive foraging habits is the removal of legs and wings from larger insects before swallowing them — the bird catches the prey, picks off the appendages, discards them, and swallows the body whole.

Foraging takes place almost entirely within 3 m of the ground, and most often at 1.5–1.8 m (5–6 ft). The bird gleans prey from branches and leaves, hops along the ground to search leaf litter, and makes short aerial sallies to catch flying insects. It forages alone rather than joining mixed-species flocks, and is most active at dawn and dusk.

On the wintering grounds in tropical lowlands, the diet broadens considerably. Mourning Warblers consume the protein bodies (Müllerian bodies) produced at the leaf-bases of young Cecropia trees — a plant-derived food source unusual for a warbler. Fruit consumption also increases during migration and on the wintering grounds, with birds pecking at fruit while it still hangs from the branch. Wintering birds occasionally follow army ant swarms to capture flushed insects, a behaviour that supplements their usual gleaning.

The Mourning Warbler is a skulking bird that spends most of its time low in dense shrubs and thickets, rarely venturing above 3 m (10 ft) from the ground. It hops rather than walks — a key behavioural distinction from the Connecticut Warbler, which walks deliberately across the ground. When disturbed near the nest, both sexes perform a broken-wing distraction display, fluttering weakly away as though injured, then scurrying mouse-like along the ground for 6–8 m before flying to safety.

Males are highly territorial during the breeding season. Territory establishment involves frequent singing from low perches, with males working their way gradually higher into shrubs and small trees. Close-range encounters between rival males are intense: birds bob vigorously, flip their wings and fan their tails, and exchange rapid short 'tschrip' notes before resorting to chasing. Territories average approximately 0.7 ha (1.75 acres) in extent. Near dusk, males perform a distinctive flight song — rising steeply, then dropping rapidly back to earth while singing.

Outside the breeding season, Mourning Warblers are largely solitary and do not join mixed-species flocks, unlike many other warblers. Foraging activity peaks at dawn and dusk. On the wintering grounds in Central and South America, birds occasionally follow army ant swarms, capturing insects flushed by the ants — an opportunistic behaviour shared with several other Neotropical migrants.

The species is a 'fugitive' in ecological terms: because its preferred early-successional scrub becomes unsuitable after just 7–10 years as trees overtop the shrubs, individuals must periodically abandon established territories and locate new disturbed habitat elsewhere in the landscape. This constant movement makes local populations highly dynamic.

The primary song is a loud, bright, rolling warble of repeated two-syllable phrases, typically rendered as 'chirry-chirry-chirry-chorry-chorry' or 'churry-churry-churry-chur-or.' The second set of phrases drops slightly in pitch and volume compared to the first, giving the song a characteristic descending quality. The overall effect is rich and burry — carrying well through dense scrub.

Only males sing the primary song, using it to advertise territory and attract mates. Singing begins approximately half an hour before sunrise, declines after mid-morning, resumes in early evening, and ends shortly after dusk. Frequency decreases as the nesting cycle progresses but typically continues until after the young have left the nest.

Near dusk, males also perform a flight song — similar to the primary song but incorporating chip notes — delivered as the bird flies steeply upward and then drops rapidly back to earth. This display is most common early in the breeding season.

The main call is a sharp, buzzy 'chit' or 'chip.' A second, higher-pitched call is also given. The flight call — useful for identifying nocturnal migrants — is a thin, sharp 'svit.' During close-range territorial encounters, males give rapid 'tschrip' notes while bobbing and flipping wings and tail.

The Mourning Warbler is one of very few warbler species known to have distinct regional song dialects, called regiolects. Research by Jay Pitocchelli of Saint Anselm College identified four stable regiolects across the breeding range: a Western regiolect (northeastern British Columbia across the Prairie Provinces into the Great Lakes region), an Eastern regiolect (central Ontario to the Maritimes and south through the Appalachians), a Nova Scotia regiolect, and a Newfoundland regiolect. A 36-year study (1983–2019) found these boundaries have remained geographically consistent across 12–13 generations — an unusual degree of stability driven by strong content bias in song learning rather than random copying.

The Mourning Warbler's flight is typical of small, compact warblers: rapid and slightly undulating, with bursts of wingbeats interspersed with brief glides. The short, rounded wings and relatively short tail give the bird a chunky silhouette in the air, quite different from the longer-winged, more attenuated look of some other Parulidae. In direct flight between patches of cover, birds move low and fast, rarely rising more than a few metres above the vegetation.

During migration, Mourning Warblers travel at night, which is when they are most vulnerable to building and window collisions. The flight call — a thin, sharp 'svit' — is the primary way to detect nocturnal migrants overhead. The species is a circumgulf migrant, meaning it travels overland through Mexico and Texas rather than crossing the Gulf of Mexico directly, a route that adds distance but avoids the high-risk open-water crossing taken by many other warblers.

The male's dusk flight song display is one of the more distinctive aerial behaviours among North American warblers: the bird rises steeply from a low perch, sings while ascending, then drops rapidly back to earth. This display is most frequent early in the breeding season and appears to function in both territory advertisement and mate attraction. Outside of this display, the species spends very little time in open flight, preferring to move through dense cover on foot or in short low hops between shrubs.