Differential Migration is a fascinating subject and is appreciable by observing the migration of Least Flycatchers. Migrant adults precede immature birds by about a month (Hussell et al. 1967). Over a three year study period conducted at Long Point Bird Observatory from 1966-1968, 90% of adult Least Flycatchers occurred between 11 July and 13 August. Peak migration of immatures occurred during the last days of August. Incredibly, peak migration of immatures averaged 38 days later than adults during the 1966-1968 study period (Hussell 1981). Ratty and faded adults present little identification challenge to those relatively well-versed in Empidonax ID, but immatures are a whole other story. “Vested” undersides with greenish-brown to the flanks and creamy yellow tones to the belly; rich tones throughout the upperside; and, in some cases, these features are in concert with a rather full, almond-shaped eyering. Add a cascade of sunlight piercing through the canopy above, and you have yourself a Yellow-bellied Flycatcher 😉
Photo: A first basic (immature) Least Flycatcher. Shirleys Bay, Ottawa, Ontario. 4 September 2015.
Hussell, D.J.T., T. Davis and R.D. Montgomerie. 1967. Differential migration of adult and immature Least Flycatchers. Birding-Banding 38:61-66. https://sora.unm.edu/sites/default/files/journals/jfo/v038n01/p0061-p0066.pdf
Hussell, D.J.T. 1981. Migrations of the Least Flycatcher in southern Ontario. Journal of Field Ornithology 52: 97-111. https://sora.unm.edu/sites/default/files/journals/jfo/v052n02/p0097-p0111.pdf