Vertical stratification is not limited to forests and seas. It is also found in saitmarshes, such as those found along the Long Island coastline. Salt marshes have interior pools, mud flats, salt pannes, low marsh, high marsh and, in some cases, unique interior islands of maritime scrub. As with forests and seas, plants and animals found in each zone are specially adapted to their zones and, in fact, depend on them for survival. As a side note, the excavation of ditches in salt-marshes for mosquito control purposes robs the marsh of much of its zonation, an issue we have examined in this column in the past.
Saltmarsh zonation and ecology are intimately linked to the daily and monthly tidal cycles. In particular, the low marsh typically floods on a daily basis during each high tide. The high marsh, on the other hand, floods only during the monthly spring tide, or about once every 28 days. In
our area, the low and high marsh zones are easily distinguished, as the low marsh is dominated by the grass, Spartina alterniflora, while the high marsh is dominated by Spartina patens. Learning to identify these grass species is the key to walking out on the saltmarsh while keeping one’s feet dry.
Let's examine the ramifications of saltmarsh zonation on the ecology of one of our most delightful birds. The Saltmarsh Sharp-tailed Sparrow, my favorite sparrow, nests locally in our salt-marshes. Incidentally, this is the longest common name of any North American bird. Each spring, after bird migration has largely ended, I make it a point to visit the saltmarsh at Gardiner County Park, specifically to observe these sparrows, an experience I have also previously discussed in this column. In late May and early June they are relatively easy to observe, as they are more interested in what is known as the “copulatory chase” than they are in avoiding potential danger.
The Saltmarsh Sharp-tailed Sparrow makes its nest on the ground in the saltmarsh. That would be a seemingly doomed strategy in a habitat that regularly and predictably floods. For example, the Marsh Wren, which also nests in saltmarshes locally, builds nests that are suspended in the vegetation, up above the ground.
In spite of a seemingly perilous nesting behavior, the sparrows are highly successful. In a beautiful example of behavioral evolution, the Saltmarsh Sharp-tailed Sparrow has developed two strategies to cope with the perils of tidal inundation. First, they nest in the high marsh zone, which, barring storms, only floods once per month. Their second strategy was only recently documented. As demonstrated by a nicely conceived study by Shriver, et al., and reported in the April, 2007, issue of The Auk, Saltmarsh Sharp-tailed Sparrows begin nesting within three days after spring tide. This maximizes their “dry time” and allows them to fledge their chicks prior to the next flood. This is one of very few documented examples of a bird timing its reproductive behavior to coincide with the lunar cycle.
It seems that my favorite sparrows have borrowed a trick common in various marine invertebrates and turtles to adapt themselves to this dynamic ecosystem. That’s just one more feather in their cap!