Songbirds in the Salt Marsh: Living on the Edge

By Amy Schwarzer

Two subspecies of little brown songbirds, the Worthington’s marsh wren (Cistothorus palustris griseus) and the MacGillivray’s seaside sparrow (Ammodramus maritimus macgillivraii), call the salt marshes of northeast Florida home. Both subspecies are salt marsh obligates, confined to the marshes near the mouths of the region’s rivers. These birds, which used to range from the state line to Volusia County, have seen their distributions contract until the most recent surveys in 2000-2001 only found breeding individuals north of the St. John’s River. The range contractions led to a state listing of Threatened for the Worthington’s marsh wren, while the MacGillivray’s seaside sparrow has been petitioned for federal listing.

In 2014-15 we conducted point counts both north and south of the St. John’s River to estimate occupancy rates and abundances of both species. We found no signs of repatriation into the previously abandoned southern areas, but no further range contractions north of the river. Both species preferred higher elevation patches in the saltier smooth cordgrass marshes over the neighboring brackish black needle rush marshes, and had increased occupancy rates and abundances farther from upland edges.

From 2015-2017, we monitored 996 wren nests and 123 sparrow nests at seven study plots to determine which habitat and nest features affected nest survival rates.  Study plots were picked based on our point count data and represented high, medium, and low densities of wrens and sparrows. Most singing male sparrows appeared to be unpaired at all but one site, which suggests that there is a sex ratio imbalance in the region. Both wrens and sparrows experienced high rates of nest loss, with evidence pointing to predation as the main cause. Yet daily high tide height most strongly predicted the probability of nest failure for both species, though we saw limited evidence of nest flooding for sparrows and even less for wrens, which nest comparatively higher off the ground in taller grasses.  It may be that extreme high tides concentrate predators in the higher elevation areas of the marsh where the birds tend to nest.

In the last component of our study, we radio-tagged and tracked 50 wren fledglings to look at post-fledging survival. Post-fledging survival was also low compared to similar songbird species, though fledglings that were heavier at the time of tagging survived much better than lighter birds.  The causes of fledgling mortality are unknown, but we confirmed at least one predation event when we tracked one of our transmitters to the belly of a corn snake!

Though analyses of the project’s data are on-going, it has become increasingly clear that these subspecies have a tenuous grasp on survival in northeast Florida, with both low nest survival and low fledgling survival.  While the birds are not yet losing many nests to flooding, they seem sandwiched between the uplands and the rising seas, with high predator concentrations suppressing their reproductive potential.  We intend to synthesize our count and demographic data to identify habitat features that best support wrens and sparrows and to share this information with local managers, hopefully leading to management and restoration efforts that will alleviate some of the pressures these little brown birds face.