Thanks for the question, Kevin.
Here's my answer, though it's a simplified one to your question that deserves more ink than this digital retorte provides.
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Given your interesting photo of the Black-Throated Gray Warbler that landed on your boat’s deck while amid the Pacific Ocean, it’s 100% certain that your unfortunate friend is lost and wayward from its normal southern migration route. More exact, no wood-warbler species on the West Coast have yet been discovered to migrate to non-breeding/wintering grounds via an oceanic route.
On the West Coast, only disoriented and/or wind blown wood-warblers show up on offshore islands, such as those often seen by bird banders/researchers stationed at the central California chain of islands called the Farallones. Here, banders have captured in their nets various species of so-called eastern wood-warblers. Others, like the Black-Throated Gray in the photo, below, are wayward sojourners desperate for a wayside to rest upon while fighting to survive in a pelagic habitat that offers no food resources.
Unfortunately, the vast majority of wood-warblers seen resting on boat decks, buoys, and rip-rap along coastal and deep water habitats typically are hatch year birds. Most will either perish while traveling over the Pacific ocean before reaching island refugia such as the Farallones or incur high mortality after being set free by banders that discover them. Some researchers suggest eastern wood-warblers found on the West Coast (including Farallone Island individuals") are inherently "dyslexic" in the sense they do not have the orientation design necessary to complete the classic migration routes that their brethren successfully negotiate each spring and autumn on their north and south peregrinations.
Beyond the West Coast, trans-oceanic migration by songbirds is rare -- and, in the wood-warblers seen in N. America north of Mexico, it is only documented to occur in a few species.
One of them, for example, the Blackpoll is known to contain populations that in autumn perform the high-octane feat of an ocean migration route that totals more than 2,150 miles (NE USA/Maritime Provinces to northern South America).
How do researchers know the Blackpoll performs such a magician's stunt annually?
It's because bird bander's in Bermuda (an island east-southeast of the southeastern USA) band birds in the autumn, and, thereby, sometimes catch Blackpoll in their nets. Evidently, Bermuda is in line with the route over which Blackpoll travel during their southbound migration and this small island serves as a stopover wayside area for Blackpoll that wish to stop and "refuel" before leaving to migrate south again at night.
High-octane is an apt description of the Blackpoll's Herculean task because many leave their "staging" grounds en masse with other Blackpolls and fly en route together as heavyweight butterballs while weighing as much as 26 grams (nearly an ounce) at the beginning of their air treks.
By the end of the Blackpolls' long-distance trip, however, they have been documented to have lost half their starting weights. Emaciated and Twiggy-Thin Blackpoll, therefore, in some cases, are known to digest their muscle to serve as a last resort energy source.
While winging south, researchers have figured Blackpolls burn .08 grams per hour during their three to four days of travel, a process that is a non-stop direct flight, if the Blackpoll does not stop at island refugia such as Bermuda. In comparison, such a weight loss program for club members and gym rats on two legs would mean a 20-pound or more evaporation of girth per day (for the typical weight of 6' tall male or 5'6" female).
Now there's a weight loss program that would attract headlines and lead to a manic panic for (I imagine) a best-selling book titled: "Migrating With a TailWind To A Fat-Free Lifestyle." :-)
Regards, Daniel Edelstein
Birding Guide
&
Avian Biologist
warblerwatch.com
