Natural History

From Wired.com:

In the stomachs of tiger sharks in the Gulf of Mexico, researchers are finding something unusual: land-faring migratory birds.

Sharks are known to eat seafaring birds, but land birds such as woodpeckers, meadowlarks, swallows and tanagers are unexpected.

“We’re the first to look this exhaustively at the diet of tiger sharks, as far as I know, and this certainly seems surprising,” said fisheries ecologist Marcus Drymon, leader of an ongoing tiger shark diet study at the Dauphin Island Sea Lab in Alabama.

The American Bird Conservancy thinks the research suggests oil platforms are to blame for dropping migratory birds into Gulf waters. Night-flying birds are known to get trapped in bright light sources, including offshore oil platforms and the 9/11 memorial lights. More than 6,000 illuminated platforms that pepper the Gulf could become giant nighttime bird lures, causing birds to circle in confusion until they’re exhausted, drop into the water and become shark food.

“Unfortunately, we don’t have good baseline data to know that,” Drymon said. “We don’t know if migratory birds are normal things for them to eat or not. There’s no data on tiger shark diet from 100 or 50 or even 30 years ago.”

Migratory land birds in the U.S. typically overwinter in South America and return in the spring or summer. Each leg of the migration is a non-stop flight covering hundreds or even thousands of miles. Threats range from storms and airborne predators to exhaustion and malnutrition.

The birds stay on course in part by using moonlight and starlight to calibrate internal compasses. Human light pollution can interfere with that ancient system, and poses an evolutionarily unique challenge that researchers have just started to study.

These are all interesting theories, but there are a few possible problems with interpreting these facts in this fashion.

First of all, I don’t know how clearly we know that birds use the moon and starlight to migrate.  It has been one of the theories postulated, but I don’t think we have any kind of definitive evidence of how birds migrate.

Tiger sharks are called the garbage men of the sea for a very simple reason:  they will eat anything.  The ones around Hawaii have been found with mongooses in their stomachs.  The small Indian mongoose that was introduced from Jamaica is not an aquatic animal, but whenever there are heavy rains in Hawaii, some mongooses get flushed out to see, where the tiger sharks pick them off.

And anyone who has ever seen Jaws can remember the scene where Hooper dissects a large tiger shark that had been caught by a mob of bounty fishermen.  When Hooper opens up the tiger shark’s stomach, he finds a Louisiana license plate.

The discovery of what appear to be rather large numbers of land-based migratory birds in the stomachs of tiger sharks is really interesting, and it does raise an interesting hypothesis.

Are oil rigs causing the problem with the collapse of so many migratory birds?

Counting the number of birds in tiger shark stomachs is not a very good measure for the reasons I’ve just described.

Further, we don’t know how many land-based birds are normally found inside tiger sharks.

To do the experiment, we’d have to get some analysis from tiger sharks that are found in seas without oil rigs and that exist along major migratory routes for birds.

I don’t know of any regions that meet those criteria.

So it’s an interesting hypothesis, but it is going to require a lot more study to be in any way definitive.