Southeast Farallon Island is the largest island in the Farallon National Wildlife Refuge and is an ideal location to study bird migration. Located 18 miles south of Point Reyes and 27 miles west of San Francisco, the island is a mere 120 acres of decomposing granite. The island rises steeply out of the ocean to 350 feet where the Coast Guard operates a lighthouse. On the leeward, southeast side of the island is a flat area called the Marine Terrace where there are two houses, two maintenance buildings, and four trees (three Monterey cypresses and one Monterey pine). During the fall, most of the ground cover turns brown and shrivels due to the dry Mediterranean-type summer. This severe shortage of suitable songbird habitat means that when fall migrant birds arrive on the island, the majority of them congregate around the houses or in the four trees where they are easy to study and relocate. Birds on other parts of the island are also relatively easy to spot as they flit about on the granite or hop amongst dry plantain on the terrace.
Migration is not a steady phenomenon, but occurs in pulses. Local weather is the primary factor that dictates whether birds stopover on the Farallon Islands during the fall. According to Pyle et al. (Condor, 1993), most fall nocturnal migrants that arrive on the Farallones are first blown out over the Pacific Ocean by easterly winds. The top of the coastal marine layer is relatively low, which allows these migrants to fly above the cloud deck in order to navigate by the stars. By flying above it, though, many don’t realize that they have inadvertently flown over the ocean. In the morning when they fly down through the clouds to find food, they discover that they are over a vast, inhospitable ocean. For birds that descend near the island, cloud ceiling height determines whether a big fallout occurs or just a few new birds arrive. If the bottom of the marine layer is too high, the birds will see the mainland and fly directly to it where suitable habitats can provide more food and shelter. If the cloud ceiling is too low, we experience a thick fog that prohibits the birds from seeing the island and us from seeing the birds, sharks, whales, or just about anything else. When weather conditions are just right, the island acts as a vacuum, sucking in all the birds that cannot find anywhere else to land.
The Farallon Islands are well known for the spectacular number of wayward birds that show up seemingly every year that are unusual to California and occasionally unusual to North America. However, the majority of fall migrants that arrive here are common West Coast birds that have been blown slightly off course and these are the focus of our studies. Since 1967, PRBO Conservation Science, formerly known as the Point Reyes Bird Observatory, has conducted annual surveys of fall migrants in an attempt to understand their stopover ecology, migratory behavior, and population trends. Because the island is small enough that a few people can cover the entire area, we are able to produce an approximate census of each species that we call a “daily estimated total.” Recently, Farallon survey protocols were improved to increase accuracy and precision of estimates, resulting in better comparisons of year to year variability and population trends. For instance, PRBO biologists conduct two daily landbird area searches (one in the morning and one in the afternoon) of all accessible areas on the island. This ensures that all parts of the island are visited by at least one person everyday so that few birds are missed. Based mostly on topography and habitat, we divided the island into five survey areas so that we know the location of the birds and can better estimate their abundance. The areas are: 1) the PRBO and Coast Guard houses, and the three cypress trees in the lee of these houses, 2) Heligoland Hill and its shrubby pine, the derelict water tanks, Shubrick, and Twitville, 3) the Marine Terrace, 4) Corm Blind Hill to North Landing, and 5) Lighthouse Hill. On a slow day, the entire area search takes approximately two hours, but on a busy wave day, it can take twice that long. For this reason, we split the area search into two halves (East Side and West Side) during the last two weeks of September and all of October so that one person only does half of the area search.
In order to understand seasonal abundance of species and migratory behavior, we need to know how many days birds are stopping over on the island. To help us keep track of individuals, we capture as many birds as we can in mist-nets and attach a single aluminum band with a unique number to a bird’s leg for individual identification. To aid our ability to track these individuals in the field, we band birds on the right leg during even-numbered days and the left leg during odd-numbered days. When only a few individuals are present for a given species, it is usually possible to differentiate a few left or right-banded individuals by differences in their plumage. However, when a species arrives on the island in large numbers for a few days in a row, it can be extremely difficult to remember individual plumages of up to ten right-banded and ten left-banded birds. To improve our ability to monitor individual stopover duration and our daily estimated totals, we recently began color banding the six most frequently-caught species with unique color-band combinations. These species are Yellow Warbler, Townsend’s Warbler, Yellow-rumped Warbler, White-crowned Sparrow, Golden-crowned Sparrow, and Dark-eyed Junco. This has greatly improved our ability to estimate the total numbers of individuals present on a given day. Several times we compared our daily estimated totals for color-banded species with and without color-band data and found that our totals were consistently low (by as much as 20%) when we did not incorporate the color-band data. This color-banded Yellow Warbler would be recorded in the field as GS/YO, or green over silver on the left leg AND yellow over orange on the right.
Banding birds also allows us to determine a bird’s age, sex, and energetic condition. Aging birds is important for determining species demographics, reproductive rates, and population trends. One way we determine a bird’s age is by examining whether the skull has completed ossification. A juvenile songbird’s skull is composed of a single layer of bone. Over the course of a few months, a second layer of bone is grown below the first and small bone pillars grow in between to connect the layers. By looking through the skin on the bird’s head, it is possible to determine the extent of ossification. Ossified areas appear speckled with white dots, while unossified areas appear pink. Most birds do not complete ossifying their skulls until October or November. After that, we need to rely on plumage features to differentiate adult and juvenile birds. In general, because nestlings grow all of their feathers simultaneously and as quickly as possible to avoid predators, the quality of these juvenal feathers is not as strong as that of feathers produced in later molts, which can be grown in smaller groups. Many birds in their first year only molt some of these weaker, juvenal feathers making it possible to distinguish these birds from adults by their mixed generations of feathers.
Determining the sex of migrant birds also helps us determine population demographics and trends. During the breeding season, most songbirds can be easily sexed by whether the bird has a brood patch for incubating eggs (female), or an enlarged cloacal protuberance used for storing sperm (male). In the fall, prior to migration, adult females grow new belly feathers and the male’s cloacal protuberance regresses so the bird is not encumbered by it. Thus, we must rely on morphological traits to identify a bird’s sex. In some species, this is possible by simply measuring the wing since males are typically larger. However, in most species there is so much overlap that we need to rely instead on plumage differences which is, again, easy in some species and impossible in others. This Rusty Blackbird was aged as an adult based on its uniform wing coverts and sexed as a female by plumage and wing length.
Fat is the primary fuel used by birds to provide them the energy to migrate. After a night of migrating, birds may exhaust this fuel supply and they need to replenish it by eating food during the day. After we capture a bird, we determine its energetic condition by examining its subcutaneous fat deposits. A lean bird will have a deep, concave furcular hollow, while a fat bird’s furcular hollow is filled with a yellow substance that looks like chicken fat. In addition, the weight of a bird divided by its body size (determined by wing length) provides another estimate of energetic condition. Every time we recapture birds, we reassess their fat and weight to determine the rate at which they are gaining energy for their next flight. This information can provide important clues to the migratory strategies of songbirds, which may ultimately help scientists and conservationists protect or improve critical stopover habitats. This Orchard Oriole had no fat in its furcular hollow.
Although songbird migration is the focus of our research during the fall period at the Farallon National Wildlife Refuge, we also study several other taxa such as seabirds, shorebirds, owls, sharks, fur seals, and bats. Future blogs will introduce these other taxa and provide updates on bird migration and rare bird sightings.