The Burrowing Owl (Athene cunicularia) is a small bird of prey with an unusual habit of nesting and roosting underground, often in abandoned mammal burrows. Unlike most owls, this species is active during the day, possesses long, slender legs, and spends a great deal of time on the ground. Whether this unique animal migrates depends entirely on where a particular population lives, a strategy that responds directly to varying environmental conditions.
Resident Versus Migratory Populations
Burrowing Owl populations are divided into two groups based on their seasonal movements. Populations in the northernmost parts of their range, such as Canada and the northern United States, are obligate migrants, meaning they must leave their breeding grounds each year. These northern owls are compelled to undertake long-distance flights due to the severity of the winter climate.
Populations found in the southern United States, including Florida and the Southwest, as well as those throughout Mexico and Central and South America, are largely non-migratory, year-round residents. Even within some southern populations, a pattern of partial migration exists, where some individuals may make short-distance movements while others remain resident. This mix of behaviors means that a single area may host year-round resident owls and temporary migrants during the non-breeding season. Genomic studies have confirmed that these two behavioral groups are genetically distinct, with resident populations showing more localized genetic structure.
Seasonal Movements and Destinations
The migratory journey for northern Burrowing Owls begins in late summer or early fall, typically around late August or September, following the end of the breeding season. The adults and juveniles return to their northern breeding sites in the spring, generally arriving in April or May. These annual migrations cover substantial distances, with some individuals traveling thousands of kilometers.
Tracking studies have helped map these extensive movements. Owls that breed in the northern Great Plains, for example, have been recovered in wintering grounds in central Mexico. Banded owls from western regions, like Alberta and Saskatchewan, have been found overwintering in Texas and Northern Mexico. An average migratory journey for a western Burrowing Owl is estimated to be around 1,800 kilometers, though the routes themselves are still not fully known.
Juvenile owls exhibit significant dispersal and migratory movements, with some tagged individuals traveling over 1,000 kilometers from their natal sites. These movements often involve nocturnal flights.
The Biological Need to Migrate
The fundamental reason northern Burrowing Owls migrate is the seasonal collapse of their food supply. Their diet consists of a mix of large insects, such as grasshoppers and beetles, along with small mammals like voles and mice. When the northern prairies experience freezing temperatures and heavy snow cover, both the insect prey and small rodents become inaccessible or disappear entirely.
This lack of available food creates a severe energy deficit that necessitates a move to warmer climates where prey is available year-round. Migratory owls prepare for this energetically demanding journey by building up fat reserves prior to departure.
Researchers have found that genes associated with fat metabolism are significantly enriched in migratory populations. This suggests an evolutionary adaptation for efficiently storing and processing the fuel needed for long-distance flight. The environmental pressure of cold winter temperatures is the main factor driving the genetic selection for migratory behavior.
In contrast, the resident populations in the southern parts of the range do not face this severe seasonal fluctuation in food availability. Their stable access to prey allows them to remain near their nesting sites and avoid the significant risks and energy expenditure associated with migration.