By: Bryan Watts
The song sparrow is likely the most polytypic species in North America if not the world. More than 50 subspecies have been proposed with nearly 30 that have been consistently recognized. Explanations for morphological variation include its large geographic range, its use of a wide range of habitats in different portions of the range and its often sedentary life history. Morphotypes generally fall into five categories based on size and plumage. One of the more interesting subspecies in eastern North America is the Atlantic song sparrow.
The Atlantic song sparrow was first described by Todd based on a series of 21 specimens collected from Smith Island and Cobb Island Virginia in the late 1800s. The form has a more gray colored back, more distinct black streaking, less red edging on the feathers and a larger bill compared to the inland form. The Atlantic song sparrow is confined to the mid-Atlantic coast from Long Island to North Carolina and is entirely associated with salt water. Birds are associated with dunes and shrub habitat on barrier islands and shrub ecotones within salt marshes where they often feed in the salt marsh.
Atlantic song sparrows have been described as abundant in coastal Virginia from the barrier islands and lower tidewater. Murray considered them to be abundant along the dunes in Norfolk and Virginia Beach during the 1930s. During the 1980s when we walked the lower 100 km of barrier islands along the Delmarva Peninsula (the same areas where the type specimen was collected) annually, Atlantic song sparrows along with yellow warblers and seaside sparrows were the most abundant singing passerines. Today their numbers are considerably diminished.
One of the most shocking results of the recent resurvey (1992-2021) of marshes along the lower Western Shore of the Chesapeake Bay was the virtual disappearance of the population within salt marshes. Surveys of the network of 186 plots in the marshes in 1992 resulted in 140 detections within 64 (34.4%) survey plots. Birds were mostly associated with the shrub (saltbush and wax myrtle) ecotone between maritime forest and the high salt marsh or shrubs (saltbush) that form on the high berm along the outer edge of marshes. By 2021 the marshes had an eerie absence of song sparrows with only 2 detections within 2 (1%) survey plots.
It is not possible from these observations to determine when this decline began or the underlying cause. One observation over the 30-year period is that the features of the marsh that song sparrows were associated with in 1992 have changed dramatically. Much of the marsh-forest ecotone supporting the sparrows in 1992 has been colonized by a band of common reed that has changed the structure of the ecotone. The other clear change has been that many of the berms that supported shrubs have been highly eroded resulting in the loss of shrub cover.
The rapid population change in this unique form in such a short period of time is concerning. Whatever the underlying causes of the decline, we need to get a better handle on the current status and distribution of the form and assess what mitigation options remain.