Black Rat Snake Predation
at a Purple Martin Colony near Pittsburgh, PA.

Ken Kostka
Purple Martin Preservation Alliance
www.purple-martin.org
Pittsburgh, PA
12/12/09

            
   On July 28th, at 1:00 PM, during a routine nest check the Lock 4 colony in Natrona, I was shocked to find a dead 36 inch long Black Rat Snake coiled inside gourd #7. It had eaten at least three martins, all of which could be seen as three distinct lumps in its body. There were several surprises associated with this discovery. First, the snake got past TWO aluminum predator guards. The first was a silver 24 inch long, 7 inch diameter guard near the bottom of the pole. The top of this guard was 44 inches above the ground, but only 30 inches above the winch. The second guard was a white 24 inch long , 15 inch diameter guard attached to the bottom of the house at the top of the pole. The snake apparently climbed over the bottom guard and squeezed through an opening between the top guard and the pole, a flaw I will correct for next season.
         I believe the snake came in at night, because it appeared to cause very little commotion in the colony. There were no signs of alarm. Had I not lowered the housing to do a nestcheck, I would have assumed all was well and possibly lost quite a few more nestlings. Most landlords report a great amount of alarm being exhibited by the martins when a snake is present in the housing. Perhaps panic only occurs when the snake climbs the pole or climbs between compartments during the day. If the snake moves only during the night, the martins might be generally unaware of its presence. Perhaps snake predation is much more common then is generally believed. Unless one is doing nest checks, it could easily go undetected.
         This snake had eaten three martins, and since all the nestlings in the colony were banded, I was able to determine that it had been in at least one other nest. Although two of the three martins were from gourd #7, the snake also had one nestling from WH-24, which was a compartment in a section of the T-14 which gourd #7 was hung from. Based on nest check records, I do not believe the snake got into any other active compartments. Luckily, this predation occurred late in the season after many of the martins had fledged.
        In addition to sealing up any cracks between the pole and the guards, landlords need to clean and wax the guards. Over time, the metal becomes gritty and easier for a snake to climb. Landlords may also wish to consider a snake netting barrier. Below, I have reprinted an article that describes a way to stop snakes using bird netting:

Huge Rat Snake Thwarted by Bird Netting

 Kent Justus  - Mt. Vernon, AR
 

Hello folks. Well, the first rate snake attempt on my martins was made the night of 06/02/01. I say attempt because it was thwarted completely. I awoke Sunday morning and went outside and immediately glanced at the martin gourd rack as I always do and immediately noticed a huge black rat snake entangled in the bird netting I had placed at the bottom of the pole under a 36" cone baffle. Last year I lost several martins to two huge black rat snakes which got around an inverted 5-gallon bucket and consumed several adult and nestling martins. I was just sick about that and decided to fortify my ground-climbing predator guards this year by installing a 36" cone baffle and the bird netting below that. The snake (over 5' long) had obviously tried to constrict up the pole and became totally entangled in the netting and was just hanging there, unable to move but still very much alive. The martins were feeding without concern while I attempted to cut the snake from the netting. While I feel that the 36" cone baffle would have prevented the rat snake from eating my martins I still feel great to have two levels of defense and it wasn't able to get by the first one. I simply taped a "skirt" of this material about 2' off the ground beneath the winch on my pole forcing any snakes to go through this before getting any higher and they end up getting themselves entangled in the fine netting. By trapping the snakes this will also let me know how many times huge rat snakes attempt to predate my colony this season and I'll share my summer total in a couple of months. The martins are safe and happy!  I got my bird netting at Wal-Mart in the garden section. It just comes in a plastic package and cost only about $5.00. It is just thin netting material with about a 1/4" gap between the threads. It is black and is used to put over fruit trees, strawberries, etc. to prevent birds from pecking the fruit. I'm not sure what you mean by an adjustable guard for the pole. I have found that if you have a serious snake problem then the only guard that has been effective nearly 100% is the 36" cone baffle. Big snakes can get around inverted 5-gallon buckets and smaller stovepipe guards. Good luck - (Photos by Thayne Muller)