State of the Birds 2017: Case Study - Saltmarsh Sparrow
Drowning in the Rising Seas
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The case of the saltmarsh sparrow demonstrates the extreme vulnerability of some coastal species to sea level rise, as well as the unintended effects of development on our shorelines.
As sea level rises, increasingly wetter salt marshes can produce drastic consequences for marsh-nesting birds such as saltmarsh sparrows. Their nests are in the high marsh, but even there they are in danger of being flooded by storms and the highest tides. The nesting season is already challenging for most birds, but for the birds that nest in salt marshes, there is the additional challenge of living within just a few vertical inches of tideline. Additionally, the grasses in the high marsh are mostly saltmarsh hay (Spartina patens), not the cordgrass (Spartina alterniflora) of the low marsh; the line between the two species of grasses is determined by the frequency and depth of tidal inundation at high tide. As marshes become wetter due to rising sea level, the amount of high-marsh habitat will continue to shrink.
In salt marshes, the height of the tide varies, with predictable and unpredictable factors influencing the height. The most predictable factor is the phase of the moon: During full and new moons, the tide is higher; these high tides, which occur twice a month, are called spring tides. Other less-predictable factors that bring higher-than-normal tides, such as storms and wind, can occur at any time.
It takes about 26 days for a saltmarsh sparrow to go through egg laying, incubation, hatching, and fledging. Therefore, if a female saltmarsh sparrow lays eggs immediately after a spring tide has passed, her eggs and chicks will have to survive only one more spring tide before the young birds are ready to leave the nest. This precarious timetable does have some built-in safeguards: the structure of the nest allows the eggs to float up during an extreme tide and then settle back down when the tide drops. But the very young chicks can’t climb up the slender grass fronds to escape a very high tide. So if the tide is high enough to inundate a nest with chicks, then the very young chicks will drown. If flooding happens too often, even older chicks are at risk.
In addition to the increased flooding caused by sea level rise, human development on marshes tends to increase the risk of flooding. Marshes in developed areas are often traversed by roads and railroad beds, which almost always restrict the natural flow of tides. It is possible that this restriction decreases the amount of sediment that would otherwise flow into the marshes, thus reducing the marshes’ natural ability to build up. With a lower marsh, flooding events become catastrophic.
saltmarsh sparrows can still be found in the salt marshes of Massachusetts, but their population is declining precipitously. Recently a multi-partner and multi-state research project, Saltmarsh Habitat and Avian Research Project (SHARP), was formed to study salt marsh-nesting birds, and saltmarsh sparrows are one of the focal species of this project (tidalmarshbirds.org). Researchers at SHARP have done extensive work to evaluate the status of tidal marsh-nesting birds, and the evidence is unambiguous—the future is grim for saltmarsh sparrows. The researchers have documented an alarming population decline of about 9% per year along the Atlantic coast—the only place in the world where this species lives.
The compounding rate of decline is difficult to imagine. SHARP researchers put it this way—it's as if three out of four saltmarsh sparrows have died and not been replaced since the 1990s. Without a massive intervention, it is likely that saltmarsh sparrows will be extinct within 50 years.