Do you love hawkwatching, and are you organized?  If you answer yes to both questions, then you might want to plan for the following.  It’s well known that the broad-winged hawk migration is temporally compressed relative to other hawks.  For instance, sharpie’s migrate into late October and November and their numbers plot a much gentler curve on a graph.

The bell curve for broad-winged hawks is centered the third week of September – no news there.  But how many of us have time to spend the entire third week of September on the mountain each year.  I have already talked about the value of weather watching – see the following link to an article from the Monadnock Shopper (http://beyondbirding.wordpress.com/2010/09/21/big-day-on-pack/).  I decided to see whether it was possible, based on available data, to further refine the curve.

Pack Monadnock has less than a decades worth of data, so I used numbers from the premier hawkwatch site in the US, Hawk Mountain, where data goes back to 1966 (hawkcount.org).  I tallied all days where more than 1,000 birds were counted, from both Hawk Mountain and Pack Monadnock, and then plotted the frequency of these by date.  As you can see, the bell curve is centered around September 18th for Hawk Mountain, and the sparse data available for Pack Monadnock suggests that it might be heading in that direction.  Hawk Mountain lies slightly more than 2 degrees latitude to the south of us, about  250 miles worth of gradient, so if anything, the curve for Pack Monadnock would shift slightly to the left based on the assumption that the birds might pass over Pack Monadnock slightly earlier.

Big Hawk Days by Date

So there you have it.  If you dont want to miss this sight next year, in addition to the weather, watch the calendar.  Fortune is on the side of the working man as September 17/18 2011 falls on a Saturday/Sunday.  If gambling is legal in NH by then, the odds will be short.

And remember, bell curves are reflections of past data.  They are indicators of future behavior, not guarantees.  Dont blame me if the birds pass by the mountain on September 10!

Nobody wants to miss the big day. September 18th 2011 anyone?