A Little Known Survival Strategy - Communal Cavity Roosting

From: Ken Kostka, info@purple-martin.org, Purple Martin Preservation Alliance
Date: 11/20/00
Time: 5:48:51 PM
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Martins have developed a survival strategy for coping with emergency cold-weather conditions that can occur in the spring, shortly after they have arrived at their breeding grounds or during migration. This behavior is called "communal cavity roosting." When conditions become severe, such as snow, sleet and freezing temperatures for several consecutive days, all the martins at a colony site will pack themselves into a single compartment or gourd to conserve body heat while waiting for the weather to break. As many as 30-40 martins might squeeze into one cavity for the duration of the cold-weather spell! This dense cluster of martins generates an amazing amount of warmth, allowing them to conserve energy and survive for several additional days.

But landlords should pay close attention to their colony sites during these stressful conditions, because communal cavity roosts can sometimes result in the death of all the martins in the cavity roost, even after the weather breaks. Here’s how: The last martins to pack into the cavity roost end up near the entrance hole. (Their tails will often be sticking out.) They are the most exposed to the cold and wind, and usually the first to die if severe conditions persist. If these martins near the entrance hole do die, they often entomb their surviving roost-mates by blocking the entrance/exit hole. The surviving martins are thus prevented from exiting the cavity to resume foraging when/if the weather breaks, and they starve to death. In other words, the bodies of the dead martins can block the entrance/exit hole and literally entomb their still living comrades – burying them alive!

If a landlord closely monitors the site (with binoculars) they can detect these communal cavity roosts and take action if needed (i.e., remove the dead birds). Tails sticking out of the entrance hole will often be a sign that a roost has formed in a cavity. Tree Swallows will sometimes join cavity roosts with martins.  Other species, such as wrens and bluebirds will also form their own roosts under severe conditions.

Of course, landlords can feed their martins crickets or mealworms during very cold weather, before they resort to communal cavity roosting, which is usually a last-ditch effort to survive. Go to the Emergency Feeding section of the Archives for instructions on emergency feeding. Thankfully, the severe cold-weather conditions which lead to communal cavity roosting only occur about once every 10-20 years (my guess) in any given area, and the occurrence of such conditions is more prevalent in the northern parts of their breeding range.