Wednesday, June 22, 2011

Day 29: Sharp terns

The marsh fish fell under heavy fire today, as various seabirds took advantage of the low water to snap up easy meals. A lone skimmer swept the pond twice before snatching a killifish and gliding off to the ocean. But the major assault came from the least terns. At first, I was a little slow on the draw.

But the robin-sized seabirds gave me hundreds of opportunities to study their strategy. It begins with a jittery yet controlled hover around 30 feet off the water, where it can find a target without being detected. Its countershaded white underside makes it invisible against the sky, especially on an overcast day like this.

Once locked into position, it releases the air from its wings and begins its descent.

Dropping quickly now, the legs press against the chest to reduce drag, and the wings make minute adjustments as the target nears.

A split second before impact, the wings fold inward and backward, and tern becomes torpedo.

Immediately upon entry the tern rights itself. Wings always emerge first.

Hit and miss. The majority of the attempts I witnessed failed, which speaks to the killifishes' successful evasive strategy. But catch or no catch, the terns lift off to try again. Their dramatic emergence doesn't look like much, until it's stopped at 1/3200th of a second.

One more and I'll move on.

Sarah and I began our day with a stranded horseshoe roundup. Rammed headlong into the sand this way, they almost look like crash-landed extra terrestrials. Or cruel experiments involving cannons and schoolboys. But this is a normal predicament. When they can't escape the marsh on the outgoing tide, they dig in to stay moist until the water returns.

Since the water won't reach them for a week, we assure safe passage to the bay.

The massive tides of late pushed a lot of debris against the bank, where it made for easy removal.

While raking, a flapping commotion was heard in the heart of the marsh. This is one of the few patches of spartina that isn't regularly flooded, and it is heavily guarded by the willets. I clambered up the wood pile to investigate, and witnessed this, taken from the Cornell Lab of Ornithology:

Male approaches female from behind; when <1.5 m from female, male calls “click-click,” depresses tail, increases rate of calling, raises wings over back and waves wings with increasing frequency; display lasts 20–30 s (Hansen 1979). Receptive female lowers head, squats, and raises tail and utters “click”; male flutters to mount and grabs head or neck of female (Hansen 1979). Female regulates duration of copulation, often interrupting coitus (Vogt 1938, Douglas 1996). 

Although it's late in the season, this bodes well for one or two Rocky Point willet chicks. Meanwhile, hundreds of crab holes pockmark the sand and mud surfaces of the marsh. Despite this, one rarely sees an actual fiddler crab. The instant a threat is detected they vanish into their burrows. They eventually emerge, but only after a good long wait.  So today I put the timelapse to fiddler surveillance. This was a test... I hope to shoot a full fiddler bonanza next time the conditions are right. [One shot taken every three seconds for 40 minutes, animated at 30 fps.]

High of 80, max humidity 78%, average wind ( ) @ 9 MPH, 4.9 high tide @ 12:59 PM. Moon 74% visible.
Water level recorded at 8 inch mark.
Birds seen in marsh: red winged blackbird, willet, common tern, least tern, skimmer
Birds seen in bay: laughing gull

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