Do Shrubs Push Out Grassland Birds?

The PLJV partnership has been using the Integrated Monitoring in Bird Conservation Regions (IMBCR) program to monitor grassland bird populations since 2016, with most of that sampling designed to measure birds across average conditions. However, shrub encroachment is becoming more and more of an issue, one that many of our conservation partners are wanting to address. Woody plants, like juniper and mesquite, have always been a feature of grasslands and woodland savannas in the plains, but they were kept in check by frequent fires. Now, areas that were once prairies are quickly becoming shrublands, and even woodlands. This is an issue that just about every state in the JV is dealing with.

“Because we’ve only been sampling birds in grasslands, we can’t really say how grassland birds respond to woody encroachment,” explains PLJV Biologist Kyle Taylor. “Nor can we say how birds respond to common treatments used to remove woody plants in grasslands, such as mechanical removal or prescribed burns.”

To address this issue and identify the thresholds of shrub cover that lead to the exclusion of common grassland birds, PLJV devoted about one third of the 2019 IMBCR sampling points to areas with varying amounts of woody shrubs, like the location pictured above. The bird sampling was conducted by Bird Conservancy of the Rockies’ highly-trained field crews and included points within encroachment hotspots in New Mexico, Texas, and Oklahoma.

“This year’s survey locations have been an interesting departure from the places we have visited over the past three field seasons,” says Brittany Woiderski, Bird Conservancy crew leader. “I expect we’ll see higher counts of shrub-loving birds in the data this year—species like Ash-throated Flycatcher, Painted Bunting, Pyrrhuloxia, Curve-billed Thrasher, and possibly Ladder-backed and Golden-fronted Woodpeckers.”

The presence of shrubs on the landscape certainly changes the quality of habitat in regards to grassland dependent bird species. However, each species has very specific habitat preferences. For example, Cassin’s Sparrow, the most frequently detected sparrow species in the PLJV region, prefer a small amount of shrub cover on the landscape, whereas Horned Larks prefer open ground and short grasses. Shrubs also provide more perches and, therefore, more opportunities for birds like Loggerhead Shrike and Mississippi Kite to predate upon smaller grassland species.

“Ultimately, we think some species will benefit from woody encroachment and some species will lose habitat,” notes Taylor. “Better identifying which birds will be winners or losers, and where habitat conditions are changing the fastest, will inform where we should be targeting conservation delivery work to restore grasslands in the future.”

Photo courtesy of Jill Wussow.

Posted: July 24, 2019