William E. Davis, Jr.
Fig. 1. The Sanderlings are having trouble with the large waves that lift the seaweed that they are walking upon. Note the breaking wave. The turnstones are mostly foraging in the beach wrack and from a floating board. All photographs by the author.
I had several opportunities during March and April 2020 to watch shorebirds foraging on an ephemeral substrate of floating seaweed. My observations began at 1:15 pm on March 15, a mildly breezy day after several windy days, when I saw 100-plus shorebirds roosting on the dock at our oceanside rental house on Long Beach Drive, Big Pine Key, Florida. At 1:20 pm, many of the shorebirds flew down the beach to the beach wrack two houses down from us. About 50 Ruddy Turnstones (Arenaria interpres) foraged mainly in the wrack but a few turnstones and 50-plus Sanderlings (Calidris alba) walked around on and foraged in a belt of floating seaweed. The belt rose and fell with the incoming waves and the birds moved up and down as well as a foot or so inshore or offshore along with it. At the shore's edge, many birds on the floating seaweed had to raise or flap their wings to maintain their balance when the waves hit shore (Figure 1). At nearby Spanish Harbor on Big Pine Key, high tide was 2:09 pm, so it was nearly high tide. The wind was coming in off the water..
The following day, the high tide was at 3:14 pm. At 1:20 pm, 50-plus turnstones and Sanderlings were foraging on the floating seaweed patch, which had moved much closer to my house. It was less extensive, occupying an estimated 60 feet of shoreline. At 3:30 pm, I saw 60-plus Sanderlings and a few turnstones foraging on the floating seaweed patch and they were still there at 5pm. By then, the turnstones were concentrated on a floating, seaweed-covered board in the patch (Figure 2). The shorebirds were still foraging at 5:35 pm, when I ended observations for the day. This had been another day of low wind.
Fig. 2. The Ruddy Turnstones are bunched together foraging from a floating board.
High tide on March 17 was at 4:38 pm. At 5:08 pm, approximately 50 Sanderlings foraged on floating seaweed that was in nearly the same location as the previous day. A few Ruddy Turnstones on floating seaweed near shore were moved about whenever a wave shifted the seaweed they were on. The Sanderlings foraged farther away from shore than did the turnstones. The reason is probably related to differences in weight between the species. In winter, Sanderlings average 1.8 ounces. whereas turnstones are much heavier, averaging 4.1 ounces. Thus, Sanderlings could more easily be supported by the floating seaweed..
For the next few days it was windy and the floating seaweed washed up and became beach wrack. I collected samples of the floating seaweed; its three major components were sargassum (Sargassum sp.), turtle grass (Thalassia sp.), and manatee grass (Sringodium sp.). I was unable to determine what organisms the shorebirds were feeding upon. From the concentration of the birds on the floating seaweed and the persistence of foraging during major segments of the tide cycle, the food must have been abundant.
By March 20, all the resident wintering shorebirds had disappeared, probably on their way north to their breeding grounds. On March 28, we had an influx of migrating birds including Least Terns (Sternula antillarum), and a large flock of shorebirds that consisted of mostly Turnstones and Sanderlings but included species that had not been part of the resident wintering flock such as Least Sandpipers (Calidris minutilla).
High tide was at 11:50 am on March 29. That morning at 11:17 am, several hundred shorebirds alighted on the dock, creating quite a stir among the roosting terns. At 11:25 am, I estimated that there were 200–300 shorebirds foraging, mostly in the wet areas of the wrack. I approached and at 11:35 am, the entire flock flew off but more than a hundred Sanderlings and a few Turnstones soon returned. The birds moved closer to my yard and were not foraging in the large area of floating seaweed they had used earlier. They did, however, use a smaller patch that was somewhat sheltered from wave action by protruding rocks. The wind was strong, causing the waves to move the floating seaweed considerably. At about 12:30 pm, after leaving briefly, the shorebirds returned and 100-plus alighted on the floating seaweed. The wind increased in strength and the birds were moved several feet by incoming waves. By 12:45 pm, the waves forced most of the Sanderlings onto the beach wrack and the high winds seriously reduced the patch. These patches are indeed ephemeral. By 4:00 pm, the patch was essentially converted to beach wrack and all the birds had flownOn April 1, I saw some Least Sandpipers foraging on floating seaweed behind my house. The lightest of our shorebird species at two-thirds to slightly over one ounce, Least Sandpipers had no trouble walking about on the floating seaweed. On April 16 at about 6:30 pm, I noticed Least Sandpipers foraging on some floating seaweed in a shallow pool among the rocks. Least Sandpipers tend to forage at the edge of shorebird groups and the floating seaweed is a substrate that heavier shorebirds tend to avoid (Figure 3). On April 19 in the early afternoon when the tide was fairly high, the area of floating seaweed near the dock in my backyard was populated with foraging Sanderlings (Figure 4).
Fig. 3. Least Sandpipers are the only shorebirds foraging on the floating seaweed.
Fig. 4. About 70 Sanderlings forage on the floating seaweed patch by the dock.
Apparently, wind and tide conditions determine the appearance and disappearance of the ephemeral floating seaweed foraging substrate. Very windy days tend to convert floating seaweed to beach wrack. On less windy days, the patches of floating seaweed form during the high tide portion of the tide cycle. As the tide recedes it leaves the floating seaweed as a coating on the tide-exposed shore. The next tide comes in and floats the seaweed, once again creating the floating patches. All of my observations of shorebirds foraging on these patches occurred within a half hour before or within three hours after high tide. This further suggests that the formation of these patches and their use by foraging birds is heavily influenced by wind velocity and tide cycle.
In Birds of the World (formerly Birds of North America), I did not find any reference to Sanderlings (Macwhirter et. al. 2020), Ruddy Turnstones (Nettleship 2020), or Least Sandpipers (Nebel and Cooper 2020), foraging on floating vegetation. This floating seaweed foraging substrate may be advantageous for the lighter shorebirds such as Sanderlings and Least Sandpipers because it virtually eliminates competition from larger sandpiper species that cannot, because of their weight, use it.
- Macwhirter, R. B., P. Austin-Smith Jr., and D. E. Kroodsma. 2020. Sanderling (Calidris alba), Version 1.0 in Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, New York. https://doi.org/10.2173/bow.sander.01 Accessed June 7, 2020.
- Nebel, S., and J. M. Cooper. 2020. Least Sandpiper (Calidris minutilla), Version 1.0. In Birds of the World (A. F. Poole, editor). Cornell Lab of Ornithology, Ithaca, New York. https://doi.org/10.2173/bow.leasan.01 Accessed June 7, 2020.
- Nettleship, D. N. 2020. Ruddy Turnstone (Arenaria interpres). Version 1.0. In Birds of the World (S. M. Billerman, Editor). Cornell Lab of Ornithology, Ithaca, New York. https://doi.org/10.2173/bow.rudtur.01 Accessed June 7, 2020