Population abundance and trends of Saltmarsh (Ammodramus caudacutus) and Nelson’s (A. nelsoni) Sparrows: influence of sea levels and precipitation

Abstract

Evidence of biological responses to climate change continues to grow. Long-term monitoring programs are critical in documenting these changes as well as identifying the primary stressors that may influence a species’ ability to adapt to changing climate. Eastern North American salt marshes support the greatest number of endemic salt marsh vertebrates globally, two of which are sympatric from southern Maine to northern Massachusetts, USA. Saltmarsh Sparrows (Ammodramus caudacutus), listed ‘vulnerable’ by the International Union for Conservation of Nature (IUCN), have a restricted global breeding range that occurs in salt marshes from Maine to Virginia, USA. Nelson’s Sparrows (Ammodramus nelsoni) breed in salt marshes from Massachusetts north to the Canadian Maritime Provinces and west to the prairie pothole regions of central Canada. These taxa hybridize in sympatry which may affect how these taxa respond to changing habitat quality and availability caused by climate change. We present the first estimates of the effects of sea level rise, breeding season precipitation, and salt marsh patch size on the abundance and population trends for three groups: (1) Saltmarsh Sparrows, (2) Nelson’s Sparrows, and (3) all Sharp-tailed Sparrows [the combined population of both species including hybrids]. We used 14 years of population monitoring data (2000–2013) from nine saltmarshes within the Rachel Carson National Wildlife Refuge, Maine, USA. We detected a declining trend for Saltmarsh Sparrow (i.e., significant decline, but not significantly more than 5 % per year), stable trends for Nelson’s Sparrows and for all Sharp-tailed Sparrows (i.e., no significant increase or decrease over the time period). Abundances for the three sparrow groups varied among years and marsh units. Drier years with relatively low mean sea levels had the greatest abundances. Breeding season precipitation negatively influenced population trends for Saltmarsh and Nelson’s Sparrows and mean sea level had a negative effect on Saltmarsh Sparrow population trends. Our results indicate that Saltmarsh Sparrow, the species most specialized to salt marshes, has declined which may be indicative of broader, regional patterns. The negative relationships of mean sea level and precipitation with Saltmarsh Sparrow population trends suggest that the negative effects of increasing nest flooding may be having demographic-level effects on this local population. Analyses of other salt marsh bird long-term monitoring programs are warranted to determine if this pattern is consistent in other portions of the Saltmarsh Sparrow range.