…strong aggregative behavior overrides vegetation characteristics to produce the highly clumped distribution of Least Flycatchers. Least Flycatchers in our forest are always aggregated, suggesting a strong social bond. We have sought in vain for consistent or measurable vegetation differences within occupied clumps of flycatchers versus adjacent unoccupied areas. … The significantly clumped dispersion of territories, in addition to their relatively small size, suggests to us that proximity of neighbors (i.e., sociality) is an important, if not essential, dimension of Least Flycatcher ecology.
Sherry and Holmes (Sherry and Holmes 1985)
Least Flycatchers, one of the smallest and most common Empidonaces, are known for their propensity to form overt clusters of territories during the breeding season. The Least Flycatcher breeds in deciduous or mixed forests and is readily identified by its characteristic “chebec” song. In the spring, males sing incessantly and establish small territories in highly dense clusters that resemble classical leks. Extra-territorial forays by males and females often result in aggressive chases and fights during peak female fertility. Once pairs form, females weave their nest from fine grasses, placing it in the crotch of a small tree or shrub, or saddling it on a limb or large branch. Recent advances in our understanding of Least Flycatcher ‘sociality' and explanations for clustering have helped foster a newly emerging perspective of habitat selection in territorial animals within the context of social behavior.
The Least Flycatcher typically lays four-egg clutches over most of its range. Neither predation nor the ability of adults to hatch and feed enlarged broods seem to limit its clutch size. Its eggs hatch asynchronously over a period of one to three days, but the pattern of hatching asynchrony does not appear to be an adaptation to reduce nest failure caused by predators. On their territories or near their nests, Least Flycatchers are very aggressive to heterospecific intruders. Where this species overlaps with the American Redstart (Setophaga ruticilla), for example, it may exclude adult redstarts from mutually preferred habitat. Such aggression may also partially explain why the Brown-headed Cowbird (Molothrus ater) parasitizes this flycatcher only infrequently, despite the apparent suitability of flycatchers as hosts and the abundance of cowbirds throughout its breeding range.
Extensive banding studies of Least Flycatchers have provided one of the first examples in North America of differences in the timing of molt and migration between adult and hatch-year birds (Hussell et al. 1967, Hussell 1980, Hussell 1981, Sealy and Biermann 1983). Unlike most Passerines, adult Least Flycatchers migrate to their wintering grounds before molting, whereas young molt before and during autumn migration. Why this pattern has developed is unclear, but it may result from strong selection on adults for early arrival and establishment of territories on the wintering grounds.