Saltmarsh Sparrows Surviving Virginia’s Winter Weather

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CCB field technician, Riley Strasbaugh, trudges through a black needlerush marsh

CCB and Saltmarsh Bird Surveys

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Published by Center for Conservation Biology at March 29, 2023

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saltmarsh sparrow

A frozen marsh on the eastern shore of Virginia. Photo by Zak Poulton, a biologist with The Nature Conservancy.

By: Chance Hines
3/29/23

For the sparrows of the marsh, the worst of winter has passed. In Virginia, the 2022-2023 winter was relatively warm, but temperatures did approach single digits in coastal areas during late December. That cold weather system brought plenty of wind and ice to marshes where these birds make their home. Blustery conditions like those could mean death to birds that are unable to cope with the weather.

In order to survive harsh winters, these birds must find a patch of marsh that provides the basic requirements in life, food and shelter. The lower elevations of suitable marshes provide areas for sparrows to forage on seeds gleaned near the muddy surface and the higher portions provides safe places to wait out high tides that inundate the lower marsh twice per day, as well as access to areas that can provide shelter during the harshest winter weather.

CCB Biologist Laura Duval and volunteers Nick Newberry and Andrew Rapp banding marsh sparrows among shrubs at a high tide roost on Fisherman Island. Photograph by CCB biologist Chance Hines.

CCB Biologist Laura Duval taking morphometric measurements on saltmarsh sparrow after capturing at a high tide roost at Bulls Marsh. Photograph by USFWS biologist Pam Denmon.

Biologists from The Center for Conservation Biology and the United States Fish and Wildlife Service have used the higher portions of marshes to capture birds roosting during high tides for the past three winters at the Eastern Shore of Virginia National Wildlife Refuge. The focal species for this study is the saltmarsh sparrow, a taxon that is likely to be listed on the threatened and endangered list in coming years. Researchers estimate that the species is declining 9% annually and could be extinct by 2035 if the current trajectory continues. Like many other terrestrial species, sea level rise is presenting a hurdle to their existence and researchers throughout their range are rushing to address various issues that may be confronting the species.

Tidal flooding associated with increasingly high tides is lasting longer and occurring more frequently. Tidal flooding during the summer breeding season is thought to be the primary driver for the documented portion of the saltmarsh sparrow’s decline. However, negative effects resulting from greater severity of tidal flooding may not be limited to the summer breeding season for birds that use the saltmarsh. Higher tides may also have a negative effect during winter when the highest tides are often associated with strong winter storms that bring along frigid temperatures and howling winds.

Figure from the Saltmarsh Sparrow Conservation Plan produced by the Atlantic Coast Joint Venture showing population projections under various scenarios.

Seeds from saltmarsh cordgrass are the primary food for marsh sparrows during winter. These seeds are produced during late summer and fall from seed heads throughout winter to the marsh mud below where sparrows forage. Seeds dehisce more rapidly during harsher winters and in marshes that are more exposed to wind. Photograph by CCB biologist Chance Hines.

The highest winter tides can flood even peak elevations on many of the marsh islands along Virginia’s coast, compelling birds that typically use these lower lying island marshes to flee to marshes where suitable roosting habitat is available. The marshes with the most available roosting habitat are those along the mainland edges and larger barrier islands. The marshes surveyed as part of this project are all areas adjacent to higher ground with larger shrubs and trees that offer roosting habitat and serve to reduce exposure to wind that scours seeds from plants.

Photograph of a small marsh island completely frozen over during the winter of 2018. The photograph was captured with an unmanned aerial vehicle by The Nature Conservancy biologist, Zak Poulton.

Mist nets set in a high tide roost location a few days after a January 2022 snowstorm at Bulls Marsh. More saltmarsh sparrows were captured following the snowstorm than any other trapping session that winter, possibly because birds sought the shelter they could not find on nearby marsh islands that had frozen over. Photograph by CCB biologist Chance Hines.

Capture data seems to support the idea that birds move to these more protected marshes in harsher winters. More birds are captured at these sites at the end of a harsh winter than at the beginning, possibly because many birds that typically use lower lying marsh islands are forced to the mainland study sites. During the warmest winters, the number of captures declines the most throughout the season at these inland sites, possibly because birds are able to stay on the smaller marsh islands. Now that temperatures are warming, invertebrate prey items are becoming more abundant, helping to fuel these species as they prepare for their annual northward spring migrations. While the birds bustle to secure their place on their breeding grounds throughout the North Atlantic, CCB will be taking a closer look into the data to sort out how our findings can be best utilized to help inform management decisions for these vulnerable birds.