Friday, January 16, 2009

New York's Geese are exploding in another way

The Christmas Bird Count is one of the great success stories of Citizen Science. Every year in late December, tens of thousands of volunteers across North America brave snow and sleet and Christmas Shopping traffic to go out and see how many birds they can see. Everyone writes down how many individual birds they see of each species, and where they were looking, and all these data are collected into a central database. The Audubon Society's scientists know how many people were looking, and where, and they have large numbers of observers, so they can calculate pretty reliable winter range maps, and calculate rates of population change in each area.

Looking at their data for Canada Geese, a pretty remarkable number emerges. The number of Canada Geese in New York State in December has, for quite a while, been increasing by 22% per year! At that rate of increase, population would double about every three and a half years (1.22^3.5=2), and the population doubled at this rate starting about 1955 and leveled off about 1990, so it had time to double about 10 times. 2^10=1024. So for every goose in New York in late December before the 1950s, there is now a kilogoose in the same area at the same season. To put it another way, most CBC observers in NY in the first half of the 20th century saw not one geese. The average NY state observer these days sees 50 or 60 geese.

Here is a graph from the CBC historical query page.

There is a lot of noise in the data, but the trend is very clear.

So why are there so many more geese in NY in the winter? Partly, there are just more geese everywhere. Humans have been good to Canada Geese. Lawns, golf courses and grain stubble are all feasts for geese. We've killed off a lot of the natural predators, and we don't hunt them ourselves as much as we used to. And New York winters aren't nearly as cold and snow-buried as they used to be, especially around the city, which means fewer of the geese bother migrating any further south than New York.

In considering the causes of the bird-plane collision that caused yesterday's much publicized crash, we should keep in mind that a few decades ago, there would have been no geese to hit in January above the Bronx.

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