Thursday, June 9, 2011

Bonaparte's Gull

I saw a Bonaparte's Gull on May 30 at Jackson Bottom Wetlands Preserve. It was floating on Pintail Pond, zigzagging back and forth looking for things to eat. Quite often it would thrust its head into the water in hopes of catching something. According to Cornell's All About Birds, it eats small fish and large invertebrates, including insects, so its actions were in line with its typical feeding habits.
I wasn't able to get very good pictures of it as it stayed a ways off shore. I watched it quite awhile, using the tall grass that has grown along the service road that surrounds the pond as a blind, hoping it would come in closer, but it was satisfied with its position. The top image above shows the gull in one of the pictures I took uncropped at its closest using my 300mm lens (actually 450mm in 35mm film terms for those who understand that kind of thing). The image below it is a fairly tight crop. I typically don't like to crop that much as the resulting picture loses a fair amount of detail. None-the-less, there is usually enough detail for identifying and studying the bird.
Last November, I saw a couple of Bonaparte's Gulls along the South Jetty at Fort Stephens State Park. One was an adult and the other a first winter gull as seen in the image below:The adult's tail is completely white compared to the black tips on the feathers of the first winter. The tops of the wings of the adult are relatively clean with black tips on the primaries whereas the first winter gull's upper wings have mottled brown on the lesser coverts and on the leading edge of the primaries, plus black tips on the secondaries. Also, the wintering adult's black ear spot is fairly round as apposed to the oval ear spot of the first winter's. Of course, in the summer, during the breeding season, the head of the Bonaparte's Gull is completely black.
Here's a better view of the first winter's upper wings and tail:
That brings me back to the gull I saw a week ago. What can I tell about it from it's plumage? Well, from the image below, I'd say it is a first winter and it has obvious black tail feather tips. But this picture was taken on May 30th. According to Sibley's Field Guide, an adult should have a black head in April.
Looking closely at the image, you will notice a few things. First, the mottling on the lesser coverts and lead edge of the primaries is nearly gone and the ear spot is more round than oval. Sibley states that it take a Bonaparte's Gull a year to acquire adult plumage. Therefore, I deduce that a first winter does not have a black head in late May like this one, but it is clearly transitioning towards adult coloring. And an even closer look at the back of its head reveals some darker feathers starting to fill in.

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