Wednesday, January 26, 2011

Cave Swallows

The Parmer Lane bridge over Lake Creek hosts a colony of Cave Swallows. Most swallows (including these) are migratory and leave central Texas for the winter. But a few Cave Swallows, including the ones under our bridge, stay all year long. In the summer they build mud nests under the bridge and breed there, foraging nearby during the day so they can feed the nestlings. During the winter at night they roost under the bridge in their old nests, but the forage elsewhere during the day. Usually the only time you can see them in the winter months is briefly in the morning when they leave the bridge and then briefly again at dusk when they return. This winter behavior has been observed in other Cave Swallow colonies as well. Why don't they stay in the immediate area during the day like they do in the summer?

Long time central Texas birder and naturalist Brush Freeman has an interesting hypothesis about that. In the winter the air can actually be warmer a few thousand feet up. He thinks these birds spend most of their winter days up in the warmer air. And maybe that's where most of the flying insects that they prey on are in the winter as well. Very interesting! Saturday morning by the bridge I saw 3 small groups of Cave Swallows. The first was at about 8:30 when I saw 5 birds flying near the bridge that soon disappeared. About 45 minutes later I saw a group of about 20 birds, and 15 or 20 minutes later I saw 4 or 5 more. These birds did not stick around either. I wonder if they were headed upwards, to spend the day in the higher and warmer air.


James said...

Fascinating. I had no idea we had wintering populations here, or that they sought warmer air high up.

Mikael Behrens said...

It's a neat idea, isn't it! According to John Arvin it's typical wintering colony behavior to only see the Cave Swallows briefly leave the bridge in the morning and then return at dusk. And that's what I'm seeing at the Parmer bridge.