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“Backyard birds” are being eaten by young tiger sharks in the Gulf of Mexico. But how did these terrestrial birds end up in the sea in the first place?
A couple months ago, it was reported that juvenile tiger sharks along the Mississippi and Alabama coasts are eating birds (ref). But the surprise was the birds found in shark stomachs were not gulls or pelicans or other sorts of seabirds, but instead, they were birds that live in people’s backyards, such as wrens, swallows, doves and woodpeckers (Figure 1). How did those terrestrial bird species end up as a tiger shark’s lunch? This led the authors of that study to propose that the birds were migrating, became exhausted and fell into the sea before reaching land.
But the details of these observations were peculiar (Figure 2).
“Surprisingly, 11 of the 13 interactions we documented took place in the fall, during the initial portion of the birds’ southward migration,” the authors wrote in their paper (ref).
The authors suggested that inclement weather events could force the migrating birds into the sea, resulting in mass mortality. Such mass mortalities provide unique scavenging opportunities for juvenile tiger sharks (ref). They went on to propose that female tiger sharks may specifically give birth their young (tiger sharks produce live young) in areas, such as the northern Gulf of Mexico, that may have a high probability of a “windfall of nutrients from the sky” (migrating birds).
However, the authors of the study did not explain why migrating birds are falling from the sky and into the sea immediately after starting their journey. It is well-documented that migratory birds prepare carefully so they have the best chance of surviving their migratory journeys: for example, the authors note that “once migratory birds accumulate ample fat reserves, they strategically time their fall departure to coincide with favorable (i.e., southward) winds following cold fronts, which are more prevalent in late fall (after 24 September).”
Thus, the details and the timing of these events is all wrong, which makes an observant person wonder whether avian migration and the relative abundance of hungry sharks could instead be a false correlation?
How did those “backyard birds” fall into the sea into the first place?
The answer to this question has been proposed by a particularly observant bird watcher and biologist, Reuven Yosef, a professor at Ben-Gurion University of the Negev. He noticed local sharks had non-pelagic birds in their stomachs too, wondered about this and subsequently investigated it over several years.
Every year, Professor Yosef observed European bee-eaters, Merops apiaster, that 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 (65-120oF).
“During these heat waves I observed the birds diving into shallow Salinas when temperatures were above 43oC at the Eilat Bird Sanctuary,” Professor Yosef writes (ref). “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 struggle back to land, using their wings as oars, where 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,” Professor Yosef writes (ref). He then notes: “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.
Professor 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.”
Sources:
Reuven Yosef (2019). Tiger Sharks Eat Songbirds: Comment, Ecology, published online on 27 July 2019 ahead of print | doi:10.1002/ecy.2846
J. M. Drymon, K. Feldheim, A. M. V. Fournier, E. A. Seubert, A. E. Jefferson, A. M. Kroetz, and S. P. Powers (2019). Tiger sharks eat songbirds: scavenging a windfall of nutrients from the sky, Ecology, e02728 | doi:10.1002/ecy.2728
How Did Songbirds End Up In A Shark's Stomach? | @GrrlScientist
