Monday, April 10, 2006

Pelicans and Silversides

One summer gig I always enjoyed was as a teaching/sail-charter captain down in the British Virgin Islands. The 3-month job was to sail a 50-foot catamaran loaded up with high school kids, who had signed up for a summer of sailing, diving, marine biology and fun. So I went to the program’s website to get in the mood.

That’s when I saw a photo of a pelican and started to daydream roughly as follows:

I remember on one of these jobs a few years back, watching pelicans feed while anchored near Guana Island. I noticed - actually it’s hard not to - that they had a very specific and cyclic pattern. Sitting on the deck of “Gone Native” I observed (and filmed), squadrons of pelicans, flying in perfect formation. Groups of 2 to 20 would circle around in an undeniable “follow the leader” pattern. The lead bird was doing most of the hunting. It would frequently hesitate in mid flap, as if lining up prey. The rest would mirror the leader exactly, right down to the wing beat. Only when the timing was perfect would the leader commit to the classic pelican dive bomb, hitting the water at about 80 mph. Each successive bird mimicked the motions faithfully with the sole difference being that it would impact a fraction of a second later, and almost EXACTLY 2 feet away from the one just before it. The effect was like watching machine gun bullets fired from the deck of a warship as they aimed for a low-flying kamikaze.

See Pelican Video:

I’ve watched a lot of birds. And I’ve watched pelicans feed in different parts of the world. But this is the only place I’ve ever seen this habit practiced with such predictable precision. In Florida for example, I've watched pelicans fly in formation, but have only ever observed them to hunt/dive bomb independently.

Curiosity kicked in. Why do BVI Pelicans exhibit this peculiar behavior?

I’m sure there is a marine biologist or ornithologist out there who has studied the cooperative feeding behavior of pelicans. If you are (or know one) please let me know. But to me this is a completely original idea.

I think it has to less to do with geography and more with prey.

In the Caribbean there is an abundance of small fish called silversides. These little minnow-sized fish roam the shallows, in gigantic dense schools.

At some of the more remote anchorages, “bait balls” form and can dominate entire cove. A bait ball is a special event that happens when huge schools of silversides converge. From above it’s an insanely noisy scene with birds squawking and fish splashing all about (the last clip of the pelican movie). Pelicans and other seabirds such as gannets, gulls, and boobies, make diving assaults. Underneath, massive predatory fish like tarpon dart around with jaws agape, sending the fish boiling up to the surface. When not in an active bait ball things are a bit more calm but still disorienting.
See Silversides Video:

One time, while diving below a school of silversides, I looked up and noticed that the only space clear of fish, was a vertical tube in the path of my exhaled bubbles. Apparently the silversides don’t like bubbles. Maybe they think it’s a predator and are scattering to get away.

It was this radial scattering behavior that got me thinking about pelican feeding behavior. To do a little nonscientific experiment, I devised a simple “Rock Drop Test.”

See Rock Test Video:

I think Pelicans have learned that they can get more silversides per dive if they time and space themselves to the impact of a lead bird.

See diagrams.

Get It?

It could be a wonderful example of how complex, seemingly “intelligent” behaviors can evolve. How things evolve is a topic for another day.

And all it took was a little bird watching, daydreaming, and science.

PS. I had an opportunity to dive into an intensely active bait ball once and the experience blew my mind (unfortunately I did not have a camera). From below it is even more dramatic than above. Clouds of black, swirl around in the inches squeezed between the reef and the surface. At times you are enveloped in a vortex of fish not knowing which way is up. Vertigo sets in and you feel like you are tumbling through space because of the chaotic swarms of fish. Every few minutes the motion becomes even more panicked. Tarpon jaws flash by your face and these big fish actually bump into you. It feels like a brother’s punch.

If you ever get the opportunity to dive or even snorkel into one you should jump right it. It’s amazing. Check out this video.

1 comment:

Anonymous said...

Well you have more guts than I do Rich. The video that i saw (i think on National Geographic)of a bait ball was created by dolphins. The dolphins circling the bait ball while communicating back and forth attracted a ton of shark that circle and attacked from below. It also brougt in pinnipeds and a bird that i cant recall the name of. If you get the chance to see the video its amazing to watch. Guess your looking to add to the list of your crazy adventures.