A new study from the American Ornithological Society has found that harsh winter conditions negatively impact birds that are forced to leave their homes in northern forests and wander far from their normal ranges to find food. The researchers used data from citizen scientists to show how these winter movements, known as “irruptions,” significantly reduce the size of breeding populations the following summer.
Irruptions bring seldom-seen boreal birds to the south in large numbers, which is a treat for birdwatchers. On the other hand, little is known about how the birds are affected by these journeys.
The current study was focused on Red-breasted Nuthatches, a useful species to study because they consistently return to the same core breeding areas after massive irruptions. This makes it possible to compare their breeding populations from one year to the next.
Erica Dunn of Environment Canada analyzed more than fifty years of records from Ontario’s Long Point Bird Observatory (LPBO) against citizen science data from Project FeederWatch, the Christmas Bird Count, and eBird. She confirmed that fall irruptions at Long Point are a good indicator of the state of nuthatches across North America in any given year.
Next, Dunn used Breeding Bird Survey data to examine the numbers of nuthatches in summers following major irruptions. The study revealed that breeding population density noticeably dropped following winters where nuthatches had wandered farther than usual.
This is the first study to show a clear link between the extent of winter irruption and population density during the following breeding season. The researchers believe that the stress of travel, exposure to predators, and the desperation to find food in unfamiliar places may be taking a toll on the birds.
“This paper actually had its genesis over 30 years ago, when I was running LPBO’s Ontario Bird Feeder Survey and noticed that feeder watchers were reporting more nuthatches in winters following large fall irruptions at Long Point. When the biggest irruption ever at LPBO occurred in 2012, I was inspired to use their fifty-plus years of data to investigate that old observation in more detail,” said Dunn.
“It was truly a project without a particular goal or hypothesis – I simply had a great dataset and wanted to see what I could learn from it. Citizen science data are great for this kind of exploration, because the datasets are so large and are freely available to anyone who wants to work with them.”
The study is published in the journal The Auk: Ornithological Advances.