Spring season nearing its end

juvenile Bald Eagle

The Sandy Hook Migration Watch began its last week of the spring 2011 season today, with a sum total of zero raptors. Clear skies and calm weather this late in the season usually translate into minimal raptor migration; only a weather event (i.e. a front passage) may trigger some concentrated movement at this point in time.

Looking at the weather forecast for the rest of the week, a major hawk flight seems rather unlikely, and a surprise appearance in the form of a kite is really the best that can be hoped for.

It's not too premature then to look at the Sandy Hook spring 2011 season and see how it measures up against previous years.

Some species did exceptionally well, like Bald Eagle and American Kestrel. Other species, like Northern Harrier, Red-tailed Hawk and (especially) Cooper's Hawk had lower than average numbers this year.

Bald Eagle numbers are increasing everywhere, so no surprises there. But high numbers for a regionally decreasing species (American Kestrel) and low numbers for an increasing species (Cooper's Hawk) highlight the fact that isolated count results from just one year and just one site are of limited value. Weather is always a key factor that determines how many migrants are seen at a monitoring site, and some years the migration of certain species happens to be more coastal, while in other years inland routes are chosen. Or, on light winds or tail winds, the flight can be extremely high and over a broad front, beyond the vision of the observer on the ground. All these things are into play, and produce annual fluctuations.

With longer data sets, however, population trends become measurable. See HMANA's* Raptor Population Index program for an analysis of such trends.

* HMANA = Hawk Migration Association of North America