Thursday, August 20, 2009

The abundant ocean

After participating in the American Ornithologists’ Union (AOU) conference in Philadelphia, I got the chance to do a little birding on the Delaware coast and to take a pelagic trip ( out of Lewes, Delaware. I owe a great debt of gratitude to the local birders who made this possible by taking me out and giving me rides – it is one of the things I love about birding, feeling a part of an international community.

As we left Lewes and got out beyond the breakwater a magnificent sunrise over a calm sea bode well for the day ahead. Indeed, we would be treated to some phenomenal ocean wildlife watching!

We were escorted out to sea by Laughing Gulls such as this youngster following the boat…

One of the first pelagic (i.e. open ocean) species we saw was the Wilson’s Storm-Petrel. The storm-petrels are very small seabirds, completely dwarfed by the large shearwaters and albatrosses. The Wilson’s Storm-Petrel, for example, measures 18cm from head to tail, about the size of a Barn Swallow! With their somewhat eratic flight style I think they look like tiny bats on the ocean. It always amazes me that such tiny birds can make a living out in the ocean squall!

Looking at the above photo, you should notice a few interesting features. The first is the way the feet project beyond the tail. This is because the legs are very long and is an important ID feature when separating this species from several other very similar looking Storm-Petrels. If you’re wondering why the legs are so long, well… my attempt at an answer is below….

Another thing you may notice is what looks like a bump on the bill. This is actually the specialized nostril tubes that extrude salt. They are a feature of the order Procellariiformes, which includes albatrosses, shearwaters, petrels, diving-petrels and storm-petrels. This features earns this group the nickname “tube noses”. Since they are a little hard to see on such a small bird as a Wilson’s Storm-Petrel, I made this comparison with the much larger Corey’s Shearwater to show the tubes to full effect. Notice also how the legs tuck up under the tail with the feet pointing skyward in the Wilson’s Storm-Petrel.

The Wilson’s Storm-Petrel is very common in these waters. In fact, some ornithologists have suggested that this species is among the most abundant of any bird on the planet. It just goes to show that if you can adapt to harsh conditions, there are resources to be found. Indeed, the ocean provides for an abundance of life… hence the title of this post. What amazes me most about seeing Wilson’s Storm-Petrels off the Delaware coast is that they don’t breed anywhere near North America but rather way down in Antarctica and the Subantarctic islands. The thing is that when not breeding during the short austral summer (December – February) they roam the oceans and spend their time feeding at sea far from their breeding colonies. The birds in this photo are just a tiny fraction of a huge feeding flock.

The next two photos show dorsal views of Wilson’s Storm-Petrels and the prominent white rump and pale wing coverts, creating a diagonal bar on the wings (although this can be obscured by harsh direct sunlight)

Revisiting the feeding flock, you can easily see the foraging behaviour known as "paterring" in the following photos. Pattering is hitting the surface of the water with the feet while thrusting the head forward to feed on tiny organisms near the surface, using the wings to balance. The long legs of the Wilson's Strom-Petrel allow them to execute this type of foraging with characteristic "dancing" style.

A closer examination reveals that the webs between the toes are yellow in colour (look at the bird on the left). When I compare Wilson's with other storm-petrels it seems possible that the yellow webs might be a foraging aid, possibly attracting some prey items or assisting them to hunt in low light (many Storm-Petrels also feed at night). On the other hand, older birds develop black spots on the webbing so it could be an indication of age. Interestingly, other long-legged storm-petrels such as White-faced Storm-Petrel also have yellow webbing whereas some of the shorter legged ones such as Leach's Storm-Petrel that typically use more of a hovering style to feed and rarely "patter" do not.

The long-legged Wilson's are so good at pattering that they sometimes seem to be standing or walking on the water...

But when they hop using both legs, their legs trail backwards as the upper body is propelled forward so it looks like they’re on pogo-sticks.

Here are a few more sea dance moves

More to come from the water soon...

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