Rail Predation

In the Northern California section of the ABA's North American Birds 59(2) was the following text: "A Yellow Rail was seen by a throng of observers at Tomales Bay, Marin during the 10 Jan high tide (EDG, WEH, m.ob.). Several Black Rails were seen here 12 Dec and 10 Jan as well, with most being eaten by Great Blue Herons and Great Egrets (EDG, m.ob.)."

I was among the throng. Waldo's Dike is a man-made structure stretching across the south end of Tomales Bay. During extreme high winter tides, the entire marsh in front of the dike is flooded, forcing rails to seek cover on higher ground. They do so largely on the eastern edge of the salt marsh and on the dike itself. On 10 Jan, 2005, the dike was peopled from end to end (with birders). Rails came (fourteen Black, I believe, 1 Yellow, several Virginia). Perhaps some were deterred, one cannot be sure. Heron and Egret predation occurred largely along the eastern edge of the marsh, with Black Rails and Virginia Rails seeming to be the main prey. Most of the rails on the dike had taken refuge under cover, but at least two Black Rails were seen foraging along the edge of the dike, very close to people. This photograph shows one of them. None were eaten by Ardeids. One Black Rail and one Virginia Rail were briefly but pretty thoroughly disturbed, however, by a birder who was trying to get closer looks at them.

When I returned to the Dike on 7 Feb, I saw a notice posted by the National Park Service, requesting birders to remain near the western end of the dike during extreme high tides to avoid disturbing rails seeking refuge. There was a ranger present to be certain all of the birders were aware of this request. They more or less complied; most of the dike was unpeopled. Although some predation of rails occurred along the eastern edge of the marsh, the tide was not sufficiently high to drive rails onto the dike. I noticed that the eastern half of the dike was occupied from time to time by foraging herons or egrets. None had been there on 10 Jan.

Here is a question for the ABA Ethics Committee: Assuming that people deter the herons and egrets, should we people the dike, or should we leave it entirely to the birds? My conversation with the ranger convinced me that the Park Service would mandate that it be left to the birds-- allowing predation to take place without interference from humans. Of course humans already have interfered, by building the dike. Every bird seeking refuge there must expose itself by crossing a wide, man-made channel on its northern side. Advantage Ardeids.

[A year or two after the preceding caption was written, the Park Service resolved the issue-- they removed Waldo's Dike.]

Last updated 03/07/11