Mini Migrant Wave and Fog
September 26, 2010
Here on Southeast Farallon Island during the fall songbird monitoring season, weather determines much of what we do because of the way in which it affects songbird migration. Based on work done by Pyle et al. (Condor 1993), migrating birds arriving at Southeast Farallon Island are strongly influenced by local weather conditions. During much of the fall, high pressure over the northern Pacific Ocean combined with low pressure over the interior of California creates a strong northwest wind with alternating clear skies and low fog. This weather syndrome keeps migrating birds near the mainland coast, both because the following wind allows the birds to keep migrating, and clear skies allow migrants to see and follow the coastline. Regardless of wind strength or direction, fog inhibits birds from finding the island, simply because they can’t see it. Therefore, these conditions, which are what we experienced for much of the latter half of August and the first two weeks of September, lead to very few birds arriving on the island. Very few birds on the island means that we get to do a lot of data entry and proofing.
However, this all changed on the night of the 11th of September, and when we awoke on the morning of the 12th we found a high overcast sky and light south to southeast winds. These conditions are created when a weak low pressure system moves in from the Pacific, and are typically present for just a few days before the northwest winds dominate once again. Migrating birds flying over this cloud layer at night are unable to see the land below, so when they drop below the cloud layer in the morning to find food, they sometimes find themselves over open ocean: a truly inhospitable landscape for a landbird. With this high overcast and decreased visibility, landbirds can’t see the mainland, and so are more likely to end up on the Farallon Islands. The following two days continued with similar weather, and culminated in a spectacular wave of migrants on the 14th, with small flocks of warblers and other migrants arriving almost continuously over the course of the day. Here are photos of a few of the expected western migrant species that showed up during those three days of good migrant weather.
Birds of the year on their first migration are inexperienced and are much more likely to end up migrating over open ocean. These birds comprise most of what we see out here on the island. In addition to the expected western species, we get small numbers of ‘vagrant’ eastern songbirds. It is these species that we get particularly excited about finding. A small proportion of hatch-year songbirds are genetically misoriented during migration and follow a predictable incorrect migratory direction. For example, a warbler from the northeastern United States that is supposed to migrate in a southeasterly direction will instead orient in a “mirror-image” direction, i.e. southwest, and thus eventually find itself flying over California. During the three days of light south winds that we experienced, we had relatively good numbers of these eastern warblers show up on the island, the best of which was a Connecticut Warbler found by Matt Brady up at the lighthouse. Also located were a Mourning Warbler, 3 Tennessee Warblers, 4 Chestnut-sided Warblers, 2 Magnolia Warblers, 2 Blackburnian Warblers, 7 Blackpoll Warblers, and 3 American Redstarts. Other rare eastern birds that showed up included 2 Bobolinks, a Chimney Swift, an Orchard Oriole, and a Philadelphia Vireo.