WSB study: Grassland passerine nest survival goes up in flames

By Julia John

Four eggs incubate in a clay-colored sparrow nest. ©Lawrence Igl

For decades, wildlife managers have used prescribed fires to limit woody vegetation on North America’s grasslands, maintaining them much as natural fires once did. Recent research conducted on the North Dakota prairie suggests that these managed burns may promote songbird nest survival by allowing flames to keep in check trees and tall shrubs that host predators.

“Fire is one of the main formative ecological processes in the prairies,” said Todd Grant, co-author on the study published in Wildlife Society Bulletin.

In parts of northwestern North Dakota, prescribed burns are conducted every five or six years to mimic the frequency of natural fire and preserve space for herbaceous vegetation. Without these fires, Grant said, grasslands would eventually transform into woodlands, as they have in parts of the region that have undergone long-term fire suppression. In those areas, he said, past research showed that the spread of woody vegetation has lowered the density of prairie passerine nests and taken a toll on the birds’ survival by increased rates of predation and parasitism by brown-headed cowbirds (Molothrus ater).

A U.S. Fish and Wildlife Service biologist in North Dakota, Grant wanted to further investigate the impacts of woody vegetation and prescribed fire on songbird nest survival there. During breeding seasons from 2001 to 2003, he and his colleagues located and monitored 575 nests built by Savannah sparrows (Passerculus sandwichensis) and clay-colored sparrows (Spizella pallida) in the mixed-grass prairie at Des Lacs National Wildlife Refuge, near the Canadian border.

“For Savannah sparrows, we didn’t find any effect of woody vegetation on nest survival, mainly because they avoid nesting near woody plants,” Grant said, but “as patches of trees and tall shrubs increased on the landscape, clay-colored sparrow nest survival decreased.”

Based on the evidence he saw, Grant ruled out cowbirds as the culprits. Instead, he linked their drop in survival to predation by mice (Peromyscus spp.), thirteen-lined ground squirrels (Ictidomys tridecemlineatus), raccoons (Procyon lotor) and skunks, which are prevalent in the edges created by woody vegetation.

“Using fire to reduce tall woody vegetation in the grassland landscape, we can expect clay-colored sparrow nest survival to increase,” he said.

These results support previous findings that controlled fire does little harm to prairie songbirds in the long run, Grant said, since they easily recover from any short-term fire effects.

“Frequent prescribed fire appears not to be detrimental for grassland passerines because they’re adapted to historically reoccurring fires,” he said. “We can use fire as a management tool without long-term negative consequences on nest survival.”

TWS members can log into Your Membership to read this paper in the September issue of Wildlife Society Bulletin. Go to Publications and then Wildlife Society Bulletin.

Julia John is a science writer at The Wildlife Society. Contact her at jjohn@wildlife.org with any questions or comments about her article.

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