Winter ecology of migratory Brazilian free-tailed bats (Tadarida brasiliensis) in bridges of the southwestern United States.

Bats are diverse and ubiquitous mammals with a worldwide distribution. In the U.S., especially the southwest, Brazilian free-tails (Tadarida brasiliensis: I. Geoffroy Saint-Hilaire, 1824) are arguably the most abundant bat in the western hemisphere. These insectivorous bats range from Nebraska to Latin America and from Oregon to North Carolina. Populations migrate frequently (every year), great distances (>1,800 km), and seasonally (south for winter). In New Mexico and Texas, migrant Brazilian free-tailed populations increase seasonally because of fecundity in local summer populations as well as migration of individuals from more northern environments in summer, to southern environments in winter. Surprisingly, apparently viable populations persist in during mid-winter months suggesting winter residency of either localized populations, northern migrants, or a combination of cryptic metapopulations. This unexpected transient stop-over or residency in the geographic middle of the migration pattern (southern New Mexico-central Texas) was puzzling and warrants scrutiny. Although T. brasiliensis is abundant and commonly researched, the ecology of wintering populations is poorly known. Research on bats has been traditionally confined to their warm-season biology with sparse information available on winter activity. The broad goals of this research on winter T. brasiliensis had four major conduits.

1. Describe basic population parameters of bridge populations such as species, abundances, and roost characteristics of bridge bats in New Mexico.
2. Determine the extent of bridge occupancy of Tadarida brasiliensis in terms of continuously occupied roosts, and year-round occupation in New Mexico and Texas.
3. Investigate winter variation in population composition, i.e. frequency and relative abundance of individuals, sex ratios, age classes, body mass, evidence of feeding, and distribution in Texas of Tadarida brasiliensis.
4. Investigate winter variation in diet composition, i.e. frequency and relative abundance of major food items in Texas Tadarida brasiliensis. This research described bridge occupancy, diet and winter ecology of T. brasiliensis and reflected on the status of these wintering populations as either transients in the midst of extended migration, or marginal laggards tolerating less than optimal conditions in lieu of completing migration. Winter presence of metapopulations continuously roosting in bridges and actively feeding were previously unknown for this opportunistic bat.