Notes from the Field
Whether outwitting a predatory jay (to find a Blue-gray Gnatcatcher's nest) or praising a Mexican trogon (while teaching avian monitoring), field work is a PRBO hallmark. Presented here are just four of the many tales that far-flung PRBO biologists will tell from this year's field season.
Long-term research helps us understand the marine ecosystem of which the Farallones are a part. For example, we have learned that the murres' success from year to year (their ability to raise a chick and the over-winter survival of adults) is closely tied to the ocean's productivity. Monitoring the birds' success and examining what prey species they are bringing in to feed their chicks provides us with a good indication of how the whole system is functioning. With more than 30 years of observations on Farallon murres, we can see how the system has changed through time.
The murres in this colony are unusually noisy this morning, and as I open the window, I see many of them taking flight. Much to my surprise, and undoubtedly that of the murres, a juvenile California sea lion is galumphing through the plot! Sea lions commonly haul out around the island, often in large numbers, but it is very rare for one to be up this high where the murres breed. The startled murres take off, exposing and scattering their eggs, and Western Gulls are more than happy to have them for breakfast. The sea lion is probably also here looking for something to eat: it appears emaciated and weak but, apparently not recognizing the eggs as food, leaves them alone. Nonetheless, the damage is done. In a murre colony, where the birds lay their eggs on bare rock, many eggs and young chicks are lost without the adults there to protect them.
In good years, murres are capable of laying a replacement egg, and this is shaping up to be another good year- the fifth in a row, an extraordinary streak. Populations of most of the breeding species on Southeast Farallon have been increasing as more young survive and recruit into the colony. This is particularly noticeable for the murres. Last season we counted over 100,000 murres breeding on the island, the highest number recorded since PRBO began monitoring them and almost twice the number observed just four years ago. A few more years of favorable ocean conditions, coupled with high productivity and survival, could result in exponential growth for the murre population, perhaps approaching a return to historical numbers, when there were estimates of up to one million murres on the Farallones.
Unfortunately, natural disturbances, such as the appearance of a predator, an unusual storm event, or a misbehaving sea lion, can throw a kink in the works. This story was originally to have had a happy ending, in which good conditions persisted and all the murres that lost eggs laid again and later fledged chicks. The murres did indeed lay new eggs, and those that managed to keep their eggs are now caring for young chicks. But shortly before this article was due, the sea lion returned, and this time it not only flushed birds but also, apparently finding the chicks more palatable than the eggs, began feeding. It has returned several times since and caused the loss of more than 30 chicks and 40 eggs. There is nothing we can do but observe and record these events as they happen. The disturbance caused by the sea lion will hurt productivity this season but ultimately be but a blip in the long-term data. The murres will rebound next season, and the population will continue to grow as long as favorable conditions persist.
I already can't wait until next year.
William J. Sydeman, PhD, Director, Marine Ecology Division