Monday, October 5, 2009

Hawks per Hour and Broadwings per Hour

HPH = Hawks per Hour, BwPH = Broad-wings per Hour

The BwPH at Little Roundtop (New Hampshire) was way down this year. No, that doesn't mean there were fewer hawks. We don't assess population trends by HPH--that is what the Raptor Population Index is all about.

What HPH does tell us is how many hawks were concentrated during migration over a given observation spot. A site can have a very high HPH one year and a very low number the next year. If the numbers of observation hours at a given site are low, but the observers were lucky enough to record lots of hawks, then the site will achieve a high HPH. If, however, observers put in many hours of observation but the migration "tracks" that year didn't take big kettles over the site, the HPH, in this case, BwPH, will obviously be lower.

That's exactly what happened at a number of New England sites this September. Many hours were invested on these sites, thanks to excellent weather conditions. Days were neither too cold nor too hot, and sites did not need to shut down for rain. Translation: happy hawks. Great flying conditions allowed for really high Broad-wing "elevators" with no significant meteorological events that temporarily blocked the birds or steered and concentrated them near lookouts.

Just for fun I calculated the BwPH for three sites up here near the northeastern "Broadie" headwaters, looking at the years 2006 through this season. Because of the interplay of weather with the landscape, these three sites probably do not count the same hawks. For the three years before 2009, Pack Monadnock (New Hampshire) averaged 33.6 BwPH, Little Round Top averaged 18.8, and Putney Mtn. (Vermont) had an average of 15.2. Both Little Round Top and Pack saw a drop this season, but Putney saw a slight increase. The combined overall average BwPH for the three sites dropped 24% from 24.4 to 18.6, although the total number of Broad-winged Hawks counted was only 12% lower than the four year average.

What's in store for next year? Aha! That's the question that keeps bringing us back to our lookouts.

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