Thursday, April 15, 2010

How to understand a bell curve

The following is an example of a Bell Curve.

What is a Bell Curve you probably ask? Well this is from wikipedia: "In probability theory and statistics, the normal distribution or Gaussian distribution is a continuous probability distribution that often gives a good description of data that cluster around the mean. The graph of the associated probability density function is bell-shaped, with a peak at the mean, and is known as the Gaussian function or bell curve"

In other words - most things are average, a few are better, a few are worse. So why am I talking about this instead of talking about birds? Well I AM talking about birds. More specifically I'm talking about spring arrivals.

Everyone is up in arms this year because it "seems so early". So the other day I did a little research and broke down the species into 3 categories using Pam Hunt's spring arrival dates list. If a bird was reported before the 1st quartile it was considered early, if after, it was considered on time, if the bird should be here (later than median) by now but has not been reported it was considered late. I ignored nocturnal birds (ie - Virginia Rail, American Bittern) and birds that also overwinter regularly (ie - Cooper's Hawk, Red-tailed Hawk, etc.) as it would be impossible to determine if the bird was an overwintering bird or an actual arrival.

7 species were considered "early", 30 species were considered "on time", and 5 species were "late". Basically most are average (on time), a few are better (early), a few are worse (late). It is interesting to note that about half of the "on time species" were clustered close to the 1st quartile date and the other half lagged slightly behind. I found this chart using google image search which sums up the data of the 7, 14, 16, 5 split very well:

In other words there have been:
7 Early Adopters - aka early bird gets the worm
14 Pragmatists - lets face it, it's a long journey to the breeding ground, what if you get lost? And building a nest and setting up territories is a lot of work!
16 Conservatives - they're enjoying their journey up and seeing some sights on their way; trip to Cape May, Cape Cod
5 Laggards - maybe they should listen to the pragmatists
... and that's still a completely normal distribution

So the next time you say its an early year because you heard a Pine Warbler on an earlier date than you ever had before... remember that it's not an early year. After all, you still haven't seen a Barn Swallow.

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