Saturday, July 12, 2008

Are Hummingbirds in Decline This Year?

Airborne originally uploaded by c-eight

A couple days ago, Birdchick posted an email she received that asked about the status of ruby-throated hummingbirds. Apparently a lot of people have noticed declines in the number of ruby-throated hummingbirds coming to their feeders. I have received queries about this myself, both last year and this year, and it has been a frequent topic on discussion boards and email lists that deal with hummingbirds. The discussion that I have seen so far, though, has been mostly anecdotal; I would prefer to see it backed up with some data.

One easily-accessible source of distribution information is eBird, which allows users to break down sightings for a species by year. I downloaded the line graph data for ruby-throated hummingbirds during breeding seasons from 2004-2008 and created my own line graphs with Google Docs. The data covers the entire United States. (I would have liked to use a smaller area, but I could either pick the whole country or an individual state.)

The first graph is for frequency, the percentage of checklists from the breeding season (i.e., June-July) on which ruby-throated hummingbirds were reported. Each point on the graph represents a single week. Ignore the flatline near the end of line for 2008; those weeks have not happened yet.

The second graph is for abundance, the average number of birds reported on all checklists from the breeding season, including checklists that do not report the species. Again, ignore the dip at the very end of the line for 2008.

My read of the eBird statistics is that ruby-throated hummingbirds are being reported about as frequently but that birders have been seeing fewer individuals during 2007 and 2008. That might indicate a population decline. Conceivably it could result instead from a range shift or delayed start to the breeding season. In any case, the effect seems to be a small one, and the species does not appear to be undergoing a sudden population crash or catastrophic decline.

With a volunteer source like eBird, there may be some inconsistency in reporting at work in the results. It would be interesting to see whether the same result holds in data sources with more standarized protocols, like the Breeding Bird Survey (BBS) or banding stations. However, as one commenter on Birdchick's post noted, even some of these sources might underreport hummingbirds. BBS picks up mostly singing birds during short point counts, and not all bird banders are trained to band hummingbirds.

Any ideas for a better source on the hummingbird population?

Related posts:
Stokes Birding Blog also has a roundup of recent information on the hummingbird population.