Tuesday, October 29, 2013

Cordell Bank Storm-Petrel Uberflocks

On the September 20th Shearwater Journeys trip out of Bodega Bay, I spotted a distant shimmering haze that I knew was a large flock of storm-petrels rising and settling on the water. We motored over there knowing there would be hella birds, but none of us had any idea how many we would actually be dealing with.

There were hordes of birds. We encountered enormous storm-petrel rafts on both the east and west sides of the northern tip of the Cordell Bank. It was flabbergasting. Unbelievable.

At first, the number of Fork-tailed Storm-Petrel in the flocks was really eye-catching. There were more than I had ever seen, thousands of them. But once we started to realize the number of birds present, several on board began paying more attention to the Ashy Storm-Petrels, clearly the most abundant birds there.

Ashy Storm-Petrels occupy a relatively small range off the west coast of North America, most of which stay in California and nest on the Farrallon Islands. Previous population estimates numbered them at about 10,000 birds. Note the Black Storm-Petrel on the left, appearing significantly larger and darker than the Ashies, which are smaller and darker than the Fork-taileds.

It was really difficult to effectively capture the sheer number of birds out there. Here is a full sized photo of a slice of one of the rafts (click above to enbiggen), it should at least give you an inkling of what we were dealing with.

Luckily, bird wizards Peter Pyle and Steve Howell were on board and did their best to count the birds and arrived at similar estimates. Their final counts for these flocks were 10,500 Ashy Storm-Petrels, 6,500 Fork-tailed Storm-Petrels, 300 Black Storm-Petrels, and 25 Wilson's Storm-Petrels, totaling 17,325 birds...insane!

Assuming the Ashy Storm-Petrel estimate was anywhere near correct (it was quite possibly low), that gives an informal new world population estimate for the species...and if one assumes that those rafts did not contain every single Ashy Storm-Petrel in the world (a safe assumption), then their actual population size becomes more of an open question.

6500 Fork-tailed Storm-Petrels also seems to be a new high count for the state (at least in recent years) according to eBird.

A pelagic trip to the same area reported similar numbers of birds two days later, but to my knowledge these huge flocks were never seen again. It's a mystery where a lot of these birds feed during much of the year...all part of the fun of seabirding, which truly is a different frontier of birding.

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