COUNTING BIRDS YOU CAN’T SEE?
November 26, 2009
Effective conservation of a species requires that you have some idea as to how many individuals there are in a population, whether the population is stable, growing, or declining, and what factors may influence those trends. The Farallon Islands are home to the largest seabird colony in the contiguous United States and currently have more than 300,000 birds breeding there each summer. That is a lot of birds, but this is really only a rough estimate because counting birds is a lot harder than it might seem.
For some species that nest out in the open, like the gulls and cormorants, counting is relatively straightforward. We simply count the number of birds or nests that we can find on the island. However, for those species that breed underground, like the Cassin’s Auklet, Rhinoceros Auklet and Ashy Storm-petrel, it is not quite so easy.
So how do you count birds that you can’t see? Well, one way to do this is to determine the amount of suitable nesting habitat that is available. Once we know that, we can then make corrections for how much of that habitat is actually used based on data from a small sample of followed breeding sites. For auklets, determining available habitat means going around the entire island and counting all burrows and crevices that are available as potential nest sites.
So this September, biologists and volunteers from both PRBO and the USFWS spent a week on the island repeating the 1989 census. It took 5-8 people five full days to complete, and it required scrambling up rocky slopes, crawling around in caves, and scaling the islets to be sure that we counted all the sites. This was exciting for us since we got a rare opportunity to go to some places on the island where we would normally never go.
But, it is also a lot of work, so it is not done very often. In fact, the last time the entire island was counted was in 1989. We assume that much of the habitat (particularly rock crevices) does not change very much from one year to the next but over the course of 20 years, small changes can add up. Old burrows collapse and new ones are excavated. Rockslides and erosion close off crevices and subtle changes to the habitat may make some areas more or less suitable for auklets.
The total number of sites counted was 21,044 including 10,525 medium sites (suitable for Cassin’s Auklets) and 4,681 large sites (suitable for Rhinoceros Auklets). There were also 501 collapsed burrows counted. These sites were likely former breeding sites that were unoccupied this season or had degraded by the time the census was conducted.
Now that we have these numbers, we can generate a new population estimate for Cassin’s and Rhinoceros Auklets and develop a better understanding of how these species are faring at the Farallones.