Where wood warblers wander

November 14, 2018

Fall on the Farallones signifies both arrival and departure. Soon after the Common Murres finish breeding and leave the island, the summer interns depart as well and make room for the fall interns to set foot on their new granite home, greeted by warblers, flycatchers, and buntings. The whir of whistling Pigeon Guillemots and Tufted Puffins flying overhead is deafened amidst a cacophony of sea lions and Western Gulls. The songbirds, seemingly confused and frantic, move about the few cypress branches on the island, catching moths and cormorant flies as they prepare for their next migratory flight.

Commonly acknowledged as the seasonal movements between the breeding (summer) grounds and non-breeding (winter) grounds, migration is perhaps one of the most mysterious and fascinating aspects of bird ecology. Some birds do not migrate at all, some only slightly (e.g., elevational), and others cross much longer distances ranging from countries to continents. These long-distance migrants make incredibly arduous journeys covering thousands of miles over varied terrain, where they may be exposed to abrupt changes in weather, food availability, and predators. The physical and mental exertion required to complete the trip can take its toll.

A tremendous amount of research has been dedicated to studying how birds navigate across vast swaths of water and landscape. We currently know that migrants use a combination of learned and innate navigational means including sight and recognition of landmarks, celestial cues (the moon and stars), and by sensing Earth’s magnetic field. The latter is possible due to an impressive directional sense that is genetically inherited from the parents, which allows first-year birds to fly in a particular direction for a certain amount of time. Hatch-year birds that successfully arrive at their wintering grounds are then able to geographically pinpoint the exact location of that area. This enables them to navigate more freely the following year, even taking different routes in the spring and fall in response to seasonal shifts in weather and food availability.

In species where first-year birds migrate independently of their parents, this innate migratory ability is vital. Genetic mutations in this system are not uncommon and these defects often result in migrants mis-orienting themselves in a mirror image of their species-specific migration route. So a Magnolia Warbler from the deciduous forests of Minnesota can end up on the coast of California, rather than its typical wintering grounds in Central America and the Caribbean. These young and inexperienced birds continue flying in the wrong direction, not knowing that they are hundreds or even thousands of miles from their desired destination. Combined with wind drift, they can end up so completely displaced from their normal wintering grounds that they are doomed to vagrancy.

Eastern vagrants traveling in a southwesterly direction often fly past the Pacific shoreline at night, end up over the ocean at dawn, and are forced to stopover on the nearest land. Thus, the Farallon Islands has become a world-renowned stopover site for the number of rare vagrant birds that are found there. When the weather allows for it, the fall crew may find several vagrant birds a day, many of which are wood-warblers originating from the Eastern United States. These rarities have sparked many questions, conversations, and overall excitement in birding communities and researchers.

Some of the questions currently being researched on the island explore where these eastern warblers originated and where their departure flight path continues when they leave. In 2016, Point Blue began working with Benjamin Van Doren at Cornell University and the University of Oxford, examining stable isotopes and DNA obtained from blood and tail feather samples to determine their genetic origin. The following year, Point Blue began collaborating with Katherine Snell and Kasper Thorup at the University of Copenhagen to start answering the second question – in what directions are these vagrants departing from the island, and what decisions do they make during their stopover? Do they continue along the same trajectory used to arrive here and head out to sea, or are they able to change their course and navigate towards the mainland? And what about western warbler species that aren’t as far off course – are they able to correct for displacement too?

To tackle these questions, we captured birds in mist nets and, after obtaining morphological information and feather and blood samples, attached small radio transmitters to their backs. Using an array of antennas set up at the lighthouse with automated and handheld receivers, we were able to track their movements as they departed the island.

To gather this information, most evenings were spent at the lighthouse, receiver and antenna in hand, listening for changes in the audible pulses of each tagged bird beeping in metronome rhythm. These signals were nothing like their chirps or songs in spring, but they may tell us something just as meaningful.

If these vagrant wood-warblers continue in the southwesterly trajectory that put them on the island, it is almost certain that they will not survive and will not breed the following year. Alternatively, if they are able to orient back to the mainland and successfully overwinter, this mirror-image migration could then be passed on to the next generation, potentially leading to range expansion of that species and the development of wintering populations along the California coast. Correction would indicate that this island may serve as an important stopover site for these migrating birds to rest and feed, allowing a greater chance at survival.

Vagrants foreshadow where bird species might be found in the future. Monitoring the movement patterns and flight decisions of vagrant wood warblers is important not only because it sheds light on the mystery of vagrancy, but because it contributes to our understanding of navigational mechanisms in migratory birds, and the conservation implications of the directions they choose. Documenting the fluctuation of vagrant species occurrence, stopover duration, and departure flight decisions over time will give insight into population changes as well as how a species’ range may shift due to climate change.

At this point, there are certainly more questions than answers. And as interns on the Farallon Islands, we are fully aware of how fortunate we are to help answer them. Yet even with the puzzles we solve, migration will remain one of the most incredible events in the natural world – connecting distant landscapes, and reminding us of the importance in protecting stopover sites and wilderness areas, like the Farallon Islands.

— Text by Sarah Hecocks, Point Blue