No Refueling Stopovers for Migrating Bats

Migratory birds make refueling stops at one or more locations along their route. These stopover sites are critical; they provide food rich environments where birds can rapidly regain depleted fat stores before continuing their long treks. Likewise, several species of North American vespertilionid bats make long, north-south migrations each year. Silver-haired bats (Lasionycteris noctivagans) are a case in point. They migrate several hundred to a thousand kilometers each year. Previous research suggested that silver-haired bats make stopovers before attempting to cross Lake Erie. Are these stopovers used for refueling before the next stage of the journey?

To answer this question, a team of Canadian scientists lead by Liam McGuire captured 79 silver-haired bats at their stopover site at Long Point Ontario on the northern shore of Lake Erie. The researchers capture newly arrived bats, measured their body fat composition using quantitative magnetic resonance scanning, and fitted 30 of the bats with tiny radio transmitters (Figure 1).

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Figure 1. A roosting silver-haired bat (Lasionycteris noctivagans) with a tiny Lotek radio transmitter glued to its back. (From www.Lotek.com)

A series of radio towers (Figure 2) allowed the researchers to track the movements of the radio-tagged bats prior to their departure. The majority of bats were captured at dawn suggesting that they had just arrive at Long Point after flying all night from regions further to the north. The radio-tagged bats roosted in trees or on man-made structures during the day. While some tagged bats spent the following night foraging, most departed the night following their capture. Seven bats stayed two days, but rain the following night prevented departure for six of those bats (the bats prefer not to travel on rainy nights). The short stopover duration suggests that refueling was not the primary reason for stopping. Because these bats do not migrate during the day, it is likely that the stopover is more a temporary refuge allowing the bats to roost and enter torpor during the day.

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Figure 2. Map of Long Point, Ontario showing the telemetry towers (red triangles). (From McGuire et al., 2012)
Additionally, when the bats departed Long Point, half headed straight across Lake Erie (a minimum distance of 38 km at Long Point). The remaining bats departed along the shore.

The body composition analysis was used to simulate migration characteristics for this species. Assuming a fight speed of 9 meters per second, the bats cover roughly 250 to 300 km per night. At this rate, the bats would arrive at their southern range in 5 to 6 nights. Interestingly, silver-haired bats arrive at Long Point with roughly 19% body fat, plenty of fuel to complete their migration without refueling. Thus, migrating bats appear to use daily torpor to conserve energy and therefore do not need to refuel during stopovers – a strategy very different from many migrating birds.

Reference
McGuire, L., Guglielmo, C., Mackenzie, S., & Taylor, P. (2012). Migratory stopover in the long-distance migrant silver-haired bat, Lasionycteris noctivagans Journal of Animal Ecology, 81 (2), 377-385 DOI: 10.1111/j.1365-2656.2011.01912.x