Area birdwatchers say they’ve started seeing fewer of some bird species, like the western meadowlark. The meadowlarks nest in grasslands and agricultural areas, and can be affected by farming activity.

MARSHALL — Migrating birds are one of the sights of fall in southwest Minnesota. But recent studies have raised questions about whether skies across the U.S. are becoming emptier.

In September, a study published in the journal Science said North America’s bird populations have fallen by nearly 3 billion since 1970. According to the study, the bird population in the United States and Canada was probably around 10.1 billion nearly half a century ago and has fallen 29% to about 7.2 billion birds, the Associated Press reported.

The study results are more complex than the media headlines they made, cautioned Bob Dunlap, a zoologist with the Minnesota Department of Natural Resources’ Minnesota Biological Survey. But while it’s hard to pinpoint exact reasons why, Dunlap said grassland bird species have been declining in western Minnesota over the past couple of decades.

Loss of native prairie habitat is one factor that can impact bird populations. Dunlap said questions have also been raised about pesticides and other man-made dangers. Another scientific study published this fall showed that neonicotinoids, a common type of pesticide, could harm birds and affect their migrations.

In the study, white-crowned sparrows who ingested neonicotinoid pesticide stopped eating and lost body fat. Birds need fat stores to help them survive while migrating, Dunlap said.

Dunlap said some other varieties of birds, like waterfowl and raptors, have seen population growth. This could be partly due to human protection, he said.

Some area birdwatchers say they have also noticed changes in the numbers and kinds of bird species found in Lyon County over the past 20 or 30 years. Some species of birds, like meadowlarks and sparrows, seem to have declining populations.

“Western meadowlarks seem to be fewer every year,” said Roger Schroeder. Schroeder, the Lyon County environmental administrator and an area resident, helps count birds for a spring breeding bird survey, and the National Audubon Christmas Bird Count.

Bird counts, like the Christmas Bird Count, can help provide some information on the numbers and species of birds present in Lyon County. Each December, volunteers conduct one-day tallies of birds they see in the areas around Marshall and Cottonwood, said Sue Morton, leader of the Marshall bird count.

Based on Christmas Bird Count data going back to 1986, Morton said overall bird populations have been fairly steady, although they fluctuate from year to year. In 1986, the Marshall count listed 38 different bird species and a total of 2,013 birds. In 2000, the count was 29 species and 611 birds. In 2018, the count was 44 species and 1,522 birds.

The 1986 Cottonwood bird count found 30 species and 1,548 birds. In 2000, the count found 34 species, and 1,605 birds. The 2018 count found 38 species and 1,757 birds.

“There are a variety of variables during a count including weather, loss of habitat, climate, experience of counters and number of counters. We are lucky to have the same experienced counters year after year,” in the Marshall and Cottonwood areas, Morton said.

While Morton said overall bird numbers seemed relatively steady, there have been changes in numbers for some species.

“The meadowlark has been in decline for several years,” Morton said. A loss of nesting habitat was one reason for the change, she said. Meadowlarks nest in grasslands and agricultural areas, but “Fields are often being planted or ditches being mowed during nesting time.”

Morton said house sparrows were another bird species that seemed to be on the decline in the area. Bird counters tallied 856 house sparrows in the Cottonwood area in 1986, but only 399 in 2018. Marshall counters found 899 house sparrows in 1986, but only 231 in 2018.

Schroeder said changes in climate and environment could be affecting some bird species seen in southwest Minnesota. Shorebirds are affected by the loss of seasonally flooded wetlands, he said. Changes in climate and environment can also expand some birds’ territory. Schroeder said over time, he’s started seeing the blue grosbeak in Lyon County, where he hadn’t encountered them before.