Where did all my birds go?

Readers over the years have reported sudden changes in the numbers of the usually common birds they’ve had at their feeders or in their yards, including Black-capped Chickadees, American Goldfinches, Blue Jays, Purple Finches, and even crows.
Among the factors that can change what shows up in your yard are:

• The neighborhood food supply. Feeders are only a fraction of the food source for birds. A change in the natural food supply can make a lot of difference in their local bird populations. For instance, last season’s acorn crop was apparently not a large one hereabouts. The mast, the annual production of tree nuts in a region, can vary from year to year, and a low mast year can drastically affect populations of Blue Jays as well as mammals, such as squirrels and chipmunks. Blue Jays are major consumers of acorns (and may plant more oak trees than do squirrels, which get all the credit). Wild Turkeys are also big acorn-eaters; wonder how they are doing.
• Neighborhood habitat. Changes in the places where birds nest, roost and feed can vary the population. For example, some birds like evergreens for roosting and nesting. If someone cut down, or disease killed, a noticeable number of evergreens in the area, certain species might become less common.
• Predator changes. A hawk or perhaps even an owl deciding to nest in the middle of an area popular with song birds would discourage the songbirds from hanging around. Even something as seemingly innocuous as a family cat – and worse, feral cats – can affect the bird population in a small area. (The number of birds killed by pet cats each year would astound most people.)
• Man-made environmental changes. Obviously, major sources of noise, light, or air pollution can affect local bird populations. Set up a rock crusher next to a stand of trees that have sheltered birds for years, and you will undoubtedly chase them all away. But less obvious changes can occur. For example, spraying trees for caterpillars may kill many of the insects the birds feed on, forcing them to look elsewhere.
• Disease. Of course, disease can affect local and regional populations of birds. We saw just recently many reports of Pine Siskins dying, probably from Salmonellosis. West Nile virus, to which corvids seem more susceptible to than many other species, may be lowering the jay population.

Bird populations fluctuate naturally, mostly due to changes in food supplies. However, any change is of interest to ornithologists, and that’s why the Cornell Lab of Ornithology has set up programs like Project Feeder Watch, in which “citizen scientists” – you and I – regularly report what we see out our windows each cold-weather season to the professional scientists in Ithaca. To find out more about Project Feeder Watch, visit birds.cornell.edu/pfw/

Sick siskins

We've gotten many reports of Pine Siskins this winter and early spring. The incredible irruption of these northern birds into southern territories is like none in recent years, and siskins are still here as of April 7.

But this big crowd of birds has brought with it a problem – aside from breaking our birdseed budgets with their gluttonous consumption. Siskins may be prone to disease.

Kathy Cory of South Salem, N.Y. tells us, “I noticed a lethargic siskin in the yard this morning, which perished soon after. That's the second dead one in the yard this week, and I remembered seeing another sickly one a few weeks ago.

“I have learned that they are very susceptible to salmonella, possibly from dirty bird feeders. Since all of us have had very busy bird feeders this winter, perhaps we need to be more diligent than usual about cleaning them. Could it be the feeders or is there another force at work out there?”

On the Connecticut bird hotline, sick siskins have been a hot topic.

Julie Keefer of Lyme said there were “a lot of reports of siskins dying in North Carolina this winter and I think it was pretty much a mystery as to why.” She wonders whether the ones succumbing here caught their fatal disease in the South – the ones we are seeing now may be migrating north and are not necessarily the same birds that were at our feeders in winter.

Paul Carrier, a Harwinton naturalist and wildlife artist who found dead siskins in his yard recently, said, “As we feed the birds from the same feeders continually in one spot all winter, it is not natural. The accumulation of husks and spillage under the feeders will eventually become a breeding ground for molds and disease, especially as it gets warmer. We all must clean up under the feeders as much as we can now that it’s warmer out.”

Paul added, “thistle is a very fast decomposing seed (husks), especially when wet. These I believe are the culprit to the sickness in these siskins, especially when they eat them from the ground. When it is cold, the seeds don't grow molds and such. But when wet and warm, they become instant breeding grounds for disaster!”

Cornell Lab of Ornithology’s Project Feeder Watch has been getting many similar reports. “As often happens in birds that feed and roost in tight flocks, there have been outbreaks of salmonella reported in some Pine Siskin flocks,” Cornell said. “Salmonellosis is caused by a bacteria... It is a common cause of mortality in feeder birds, but the symptoms are not always obvious. Sick birds may appear thin, fluffed up, and may have swollen eyelids. They are often lethargic and easy to approach. Some infected birds may show no outward symptoms but are carriers of the disease and can spread the infection to other birds.”

Salmonellosis is mainly transmitted by fecal contamination of food and water by sick birds, though it can also be transmitted by bird-to-bird contact, Cornell says. Occasionally, outbreaks cause “significant mortality.”

Cornell says, “Clean your feeders about once every two weeks, more often during times of heavy use. For best results wash your feeder thoroughly in soapy water, then soak or rinse it in a solution of one part bleach to nine parts water. Dry the feeder thoroughly before refilling.”

It adds, “Remember to rake the ground below your feeder to prevent accumulation of waste. Moldy or spoiled food is unhealthy, not only for birds but for your outside pets. Bird food scattered on the ground also can attract rodents. Consider moving your feeders periodically to limit the accumulation of waste.”

One hotline participant said sunflower chips, which siskins love, have the advantage of not having hulls to get moldy and diseased on the ground.