Changing of the Birds

There was a noticeable shift in seabird species composition this week. If you were to study a chart of the tracklines we surveyed, we wouldn’t have to tell you the reason why Cory’s and Audubon’s Shearwaters were the most abundant seabirds, easily surpassing Wilson’s Storm-Petrel for top spot. Wilson’s Storm-Petrel, formerly our commonest seabird, had to settle for third place this week. We recorded 917 birds in our 300 metre strip transect: 60% were Cory’s and Audubon’s Shearwaters.

Any guesses as to why this was so and – bonus question: where did we spend the first few days of this past week? Here’s a few more hints: we saw our first Bridled Terns of AMAPPS 2013; several Band-rumped Storm-Petrels; only our second Black-capped Petrel; and four White-tailed Tropicbirds, including a stunning adult, the first of this age class we’ve seen on this cruise.

Give up?

All of these birds at this latitude are traditionally associated with warm Gulf Stream water. Although we didn’t fully enter this famous warm water current, we certainly got close enough to see its influence on seabird distribution and abundance. Because survey effort is essentially equal from week to week, we can use the daily species total and total species seen per week as a rough measure of diversity. With that in mind, we can see little change in total species from the previous week. We found a total of 14 species, down one species from last week. However, our daily average was seven species, exceeding our previous high by one, thanks to two consecutive 10 species days on line 25.

Species composition and abundance, however, did exhibit changes with the previously mentioned warm water species being more widespread. Furthermore, several large flocks of shearwaters, primarily Cory’s and Audubon’s, with a scattering of Great, feeding over schools of tuna, contributed to the increased overall abundance recorded on the strip transect. The most unexpected bird seen this week was not a seabird at all but rather a long distance migrant wader. A Greater Yellowlegs flew past the ship about 145 nautical miles southeast of Nantucket while we were making acoustic passes of a group of Ziphius.

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