How Songbirds End Up in a Tiger Shark’s Stomach
August 2, 2019
Forbes — A couple of months ago, it was reported that juvenile tiger sharks along the Mississippi and Alabama coasts are eating birds—birds that live in people’s backyards, such as wrens, swallows, doves, and woodpeckers. How did those terrestrial bird species end up as a tiger shark’s lunch?
American researchers proposed that the birds were migrating, became exhausted and fell into the sea before reaching land—due to inclement weather. But they did not explain why migrating birds are falling from the sky and into the sea immediately after starting their journey—a trek for which they usually prepare carefully, accumulating ample fat reserves, strategically timed.
Leave it to Prof. Reuven Yosef, an award-winning orinthologist in the Department of Life Sciences at Ben-Gurion University of the Negev, who is a particularly observant bird watcher and biologist.
Prof. Yosef observed that in the Negev European bee-eaters, Merops apiaster, migrate through the Eilat bottleneck every spring and autumn. (The Eilat bottleneck is a land bridge where three continents—Asia, Europe and sub-Saharan Africa—meet.) He noted that the greatest density of bee-eaters pass northward through this area in May and June. This massive movement of bee-eaters coincides with the peak heat waves of early summer when ambient temperatures can range from 18 to 48oC (65 to 120oF).
“During these heat waves I observed the birds diving into shallow salinas when temperatures were above 43oC at the Eilat Bird Sanctuary,” Prof. Yosef writes in Ecology, the journal from the Ecological Society of America.
“Birdwatchers also reported similar observations from the beaches of Eilat, wherein non-pelagic birds were observed to dive in and fly out of the waters of the Red Sea.”
But some of the heat-stressed birds did not make it out of the water, so they had struggled back to land, using their wings as oars, whereas the lucky ones would dry their waterlogged plumage before flying away.
“On days when I was not present to rescue some of these birds, 117 carcasses were found,” Yosef writes. “Such birds in the open sea would not be able to find a refuge if they were unable to take off after the plunge in the water.”
So these backyard birds along the Gulf of Mexico may well have been trying to cool off during hot weather by diving into the sea. Some of them, like those poor water-logged bee-eaters in the Eilat desert, were unsuccessful in becoming airborne again, and were eaten by young tiger sharks before they could reach land.
Prof. Yosef states that diving into the sea is a “little-known thermoregulatory behavior of non-aquatic birds [that] may result in a portion of them drowning in the sea,” and recommends that shark researchers “correlate their data with […] inclement weather, especially heat waves.”