Every season seems to end the same for me, here in South Florida. The end of martin season coincides with the start of our rainy season. Each morning our colony would get a faithful dozen or so visitors that would sit on the housing and chatter at any remaining nests. Late in the morning they would leave to only to return again the next morning. But then when a few days of poor/rainy weather persists, they are gone. I wrote a post last year about how it is thought that birds are sensitive to changes in barometric pressures and the weather , could avoid bad weather by delaying migration to an area of poor weather or vice versa. I’ll repost it here:
According to Melvin L. Kreithen and William T. Keeton of the Division of Biological Sciences, Langmuir Laboratory, Cornell University in Ithaca, New York,(23 October 1973) Homing pigeons were able to to detect air pressure changes. As purple martin landlords can tell you, a purple martins homing ability is at the very least equal to that of a homing pigeon. So the correlation is fair.
By any account, my season is over here in South Florida and all martins have left.
The factors that affect a birds migration are complex and not completely understood. Click this article for Neotropical Migratory Bird Basics from the Smithsonian National Zoological Park. If you want to learn more about Neotropical Migratory Birds read this article on Birds Built-in Barometer.
Not that my birds need bad weather to end their party, migratory birds know when it is time to go based on known factors such as the length of day and for some types of birds, even star patterns. There is nothing to be done for purple martins (or any other migratory bird-for that matter) that linger on. Some folks will tell you to lower or remove housing, but don’t bother. Just like the old wives tales that persist that tell hummingbird aficionados to remove hummingbird feeders to push hummers to migrate, nothing needs to be done.
So keep the feeders full, leave the housing up till you feel like it. Birds have been migrating for a long time and the only thing we need to do for them is support them, by way of a beak full of nectar, a belly full of seed or perhaps a dry place to sleep at night.