U. ILLINOIS (US)—When forests are fragmented, snakes win and birds lose, new research shows.
That finding is based on data from radio transmitters implanted in rat snakes combined with information from more than 300 bird nests that were located and their fate tracked. Details appear in the journal Ecological Applications.
A fragmented forest is one that is cut down in a way that leaves relatively small, isolated patches of forest remnants with more perimeters, which are beneficial to snakes.
“Snakes really like that interface between the open and closed habitat, whether it’s an edge where the forest opens onto a wetland or a rock outcrop or a manmade habitat such as a hay field. So, if your priority is to conserve the birds, you’ll want to preserve unfragmented forest habitat,” says Patrick Weatherhead, professor of natural resources and environmental sciences at the University of Illinois.
Rat snakes, which in eastern North America are the top predator to nesting birds, go into the forest to feed, then return to the edges to regulate their body temperature, breed, and shed their skin. Knowledge of habitat preferences of rat snakes is starting to explain why forest fragmentation usually results in increased nest predation for forest birds.
“Clearly, a lot of the time they are on the edges they’re not actively hunting, because nests on the edges were not at greater risk from the snakes than the nests on the interior of the forest,” Weatherhead explains.
With fragmentation, “It’s not just that you’ve lost habitat, but the smaller chunks you’re left with aren’t as good for a variety of wildlife,” Weatherhead explains. “The smaller fragmented areas attract birds but they don’t do very well there. They’ve been called ecological traps.”
Weatherhead says the message is, if you’re going to clear land for agriculture or other development, try to avoid breaking the forest into bits and pieces.
In addition to the data from radio transmitters and the 300 nests involved in the study, a number of additional nests were monitored with video cameras to document the predators.
Rat snakes are common in eastern North America from Texas in the west to the Florida Keys in the east and all the way up to southern Ontario.
“Everywhere there have been camera studies, as long as it’s in wooded or semi-wooded habitat, rat snakes emerge as the single most important predator. They’re common throughout the range, and they’re really good at finding bird nests,” Weatherhead says.
The miniature video cameras make identification of nest predators possible because they can record activity at the nests around the clock.
How do snakes on the ground see nests up in trees? Weatherhead says the evidence is circumstantial.
“Snake predation is much higher on nests where the young are being fed than when the eggs are being incubated. There’s a lot more parental activity when the young are being fed than when the eggs are being incubated. The limited evidence available all seems to point to the snakes observing the parents flying back and forth to the same place, an indicator to the snake that there’s a nest there.
“There are anecdotes of people watching a nest and noticing a rat snake watching that same nest, and from the snake’s head movements it was obviously tracking the movement of the adult pairs back and forth.” In one case, the snake was then observed going to the nest and eating the young.
Rat snakes get their name because they are primarily predators of small mammals. “But rat snakes are very opportunistic,” Weatherhead says. “I have a picture of a rat snake eating a full-grown squirrel. So that’s a mouthful.
“They’re generalists both in terms of the mammals they eat and in terms of the birds that they prey on. They’ll take whatever birds they encounter, and because they’re such good climbers, they can get to both low nests and high nests. They can climb just about any kind of tree. They eat bird eggs, fledglings, and sometimes they’ll even get the mom if she’s sitting on the eggs.”
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