Bats & Migration
Where bats live is dependent on the availability of food. When the food supply declines, usually due to weather, bats have two options. They can hibernate to pass through the low or non-existent food supply period, or migrate to a place with a more abundant food supply. In some areas, bats will also do a combination of both.
Migration involves two parts: movement from one location when food is scarce and the return to that same location when the abundance of food returns. There is no clear distinction between migrating bats and hibernating bats. Some bat species, like the silver-haired bat, migrate and hibernate. The same is true for red bats. Red bats migrate from the northern portion of their range and hibernate in the southern portion of their range. Most temperate bat colonies start migrating in September.
The Mexican free-tailed bat (Tadaria brasiliensis) ranges from Oregon to south Mexico. Bats in the northern California/Oregon region are non-migratory, spending inclement weather in torpor or hibernation. Bats of this species in the eastern Nevada, western Arizona and Colorado regions are non-migratory as well, however do not hibernate. Mexican free-tails from southeastern Utah and southwestern Colorado migrate to western Mexico. The most well known colonies, like Carlsbad Cavern and the caves in Texas and southeastern United States migrate to eastern Mexico.
Some Myotis species also migrate to winter roosts traveling over shorter distances. Their migration is not necessarily in latitudinal direction. These bats may travel in any direction depending on the location of the hibernaculum. Little brown bats (Myotis lucifigus) migrate from 200 km to 800 km in distance between summer and winter roosts. Gray bats (Myotis grisescens) migrate several hundred square kilometers from northern Arkansas hibernaculum to Kansas, Missouri and Oklahoma. Most records of big brown bat migration distances are less than 40 km in distance, however there have been exceptional distances of 230 km recorded. Tolerance of colder weather may be one reason for the short distances.
In general, tree roosting bats are migratory since trees do not provide enough shelter for the winter. Hoary bats from all areas migrate more equatorially and are found below 37 degrees latitude during the winter. Both Lasiurus and Lasionycteris species are sometimes found in migrating groups, sometimes accompanied by migratory birds.
Long distance flights consume a lot of energy. Bats that migrate lose about 0.5 g per 100 km traveled. Navigation must be accurate and flight efficient. Migrating bats are known to use vision, echolocation and the sun as orientation and may use other factors as well, but these have not been studied. Not only are the energy demands of migration costly, other threats to the bat's survival are more likely during migration. Adverse weather, higher chance of predation and disease are all factors that the bat may face. Accidents during migration such as running into buildings with wind gusts also happen. Another threat to migratory bats is pesticides. Pesticides are stored in the body fat over the course of the summer as the bats eats insects exposed to pesticides. When the body fat is burned during migration, resins are released into the bloodstream and may cause illness or death.