For the Birds Radio Program: Rats, revised
This past December, the first animal I saw in the twilight of early dawn during the Duluth Christmas Bird Count was not a bird—it was a rat, feeding below a feeder at my favorite Duluth backyard feeding station. Rats don’t survive long in Duluth neighborhoods where houses are well insulated. There’s a large population of rats in downtown Duluth, and grain elevators in the harbor attract and sustain them, but overall, rats aren’t something we Northlanders think about much. My friend with the bird feeders said she’d close down the feeders for a week or so—that’s really all it takes to send them away.
But rats are a huge problem in the tropics, and not just for people. Ornithologists have long known of the problems they pose for ground-nesting birds on islands, where without serious rat control efforts, many bird species would be extinct. But rats are a problem well above the ground, too. Jason Townsend, an ornithologist at the State University of New York, was doing research on Bicknell’s Thrush in the Dominican Republic when he got a real eye-opener. Every day just before sunset, he’d notice rats up in the treetops, often backlit by the glow a lovely sunset. He and colleagues from the Cornell Lab of Ornithology and colleges in Vermont and the Dominican Republic were tracking 53 Bicknell’s Thrushes via radio transmitter, and during the course of the study, rats killed 5 of the birds—a full 9.4 percent. None of the other birds died or were lost during the course of the study, so the researchers concluded that rat predation of roosting thrushes is a significant cause of mortality for the species.
Rats concentrate in broadleaf cloud forests where most thrushes live. But on one study area where there was also a stand of longleaf pine, many of the thrushes flew some distance from their daytime habitat to sleep in the pines, where rats are far less numerous. Apparently some of the birds are adapting to the rats by avoiding their main nocturnal stomping grounds.
Songbirds are surprisingly helpless when roosting at night. Many species, including Bicknell’s Thrush, are night migrants, but they still have poor night vision. On takeoff during migration, they fly directly toward the moon or stars. This instinct is a sound one because there can’t be any obstructions between them and any stars they can actually see, so they won’t crash into branches. Birds feel barometric pressure, and when they’ve reached a safe altitude, they travel horizontally, using the stars for navigation.
But except during migration, these birds are helpless in the dark. When I was rehabbing a bird who was stressed by handling, I provided any hands-on treatments at night. My husband or one of my kids would hold a flashlight, and we never had trouble approaching the bird and picking it up and working on it in the darkness. It makes sense that roosting birds hold tight during most night disturbances—if they took off more easily, they’d be crashing helter-skelter into branches and one another, as that flock of red-winged blackbirds did when fireworks went off at midnight in Arkansas last month. Bicknell’s Thrushes apparently hang onto their roost branch as rats approach until it’s too late.
The researchers were tracking just this one rare and declining species. It’s scary to think about all the other birds that rats on Hispaniola kill every night, and sad to realize that this is yet another way we humans are causing the decline of wild birds. Norway and black rats have been following us since ancient times. They reached the Caribbean Islands, where there are surprisingly few natural predators, about 500 years ago.
The world is ever-changing, and people have made many changes for the better, restoring habitat and protecting endangered species-even if it is habitat we messed up in the first place, and species we endangered. I often feel a hopelessness when I discover yet another bad thing is happening out there thanks to us. But knowledge is the only way we can do anything at all. I hope we can start attacking this problem with the vigor people so often use to tackle far more selfish issues.