Recreational bird watchers can provide a lot of data on species abundance, but how can you separate the effects of skill and effort from actual trends? Just look at the length of the species list, says this paper in the journal Ecological Applications. The length of the species list is a good indication of survey effort, when a few factors are considered. The paper tests and refines the List Length Analysis technique first developed by Australian scientist Don Franklin.

Photo: bird watchers at the Malheur National Wildlife Refuge, Oregon. Courtesy US Fish and Wildlife.