The organizers of the bird count -- the Audubon Naturalist Society, Bird Studies Canada and the Cornell Lab of Ornithology -- are hoping to break the participation record set last year, when more than 97,000 checklists were sent in from across North America, reporting 602 species and 11.2 million individual bird sightings.

Participants in 2010 saw more than 1.8 million American robins as well as the first red-billed tropicbird in the history of the 14-year count. The rare bird was spotted near San Diego, in southern California.

The avian census has also charted the dramatic spread of the Eurasian collared-dove, which was reported in eight states in 1999 and in 39 last year.

But some of the statistics the bird counters send in are alarming, such as the sharp drop in the number of glaucous-winged gulls on the US Pacific Coast.

In California, 83 percent fewer of the gulls were spotted during last year's count, even though the number of checklists handed in from the western state was up by a quarter.

Previous counts have also shown a drop in the number of American crows since 2003, the first year of widespread outbreaks of West Nile virus in the United States.

The four-day bird count is not scientific but does give scientists a glimpse of trends in the bird world, said Miyoko Chu, director of communications at the Cornell Lab of Ornithology.

"Anyone can go out at their whim and report or not report. So the coverage can be spotty," she said.

"But when we see that American crows used to be consistently in the top four or five of birds reported before 2003, and then when West Nile hit, they dropped consistently to ninth or 10th, that's a signal that scientists look at and say, hmmm, we need to look at that more carefully," Chu said.

To take part in the Great Backyard Bird Count, participants observe and count birds in the spot of their choosing for at least 15 minutes on any or every day from Friday to Monday.

They then fill in a form on which they identify the birds and indicate how many they saw.

Because birds of a feather tend to look a lot like each other, and because they have a tendency to move around a lot, avian census takers are asked to only list the maximum number of a species that they see together.

So if they see four northern cardinals, then three fly away but five more join the lone remaining cardinal, they would note down six on their checklist, which would be sent in to the organizers of the count.

This year, novice bird-counters can download an app onto their handheld device to help them identify the avian species they are looking at.

Checklists for the birdwatch can be filled in and submitted online at www.birdcount.org -- but only by people in North America.

For everyone else, Cornell and Audubon also run a year-round global bird count, called eBird.

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