Exploding the Myth of the Raptor Peak Week

Throughout the 1980s–90s, the GGRO Peak Week—that is, the seven-day stretch that contained the highest hawk count days of the year—truly happened in a week (or roughly a week). In those not-so-ancient times, we recorded highest count days as early as September 15 (569 sightings in 1987) and as late as September 23 (1,082 sightings in 1991). But as the years have passed, the precision of the single week has been lost. Peak days may be spread across weeks. Our recent highest count days have even occurred in October—for example, 834 raptor sightings on October 8.

Extreme peak days run a gamut from a meager 351 sightings on September 22 in 1986 to a muscular 2,833 sightings on September 21 in 1984. The latter is a day of mythical proportions in the history of GGRO, the day Carter Faust and Herb Brandt and others patiently tallied 1,380 Sharp-shinned and 983 Cooper’s hawks over a six-hour period. That’s about one hawk every eight seconds. (Can’t believe I missed that day.)

But, yes, it’s those Sharp-shins—our bread and butter hawks—that tend to drive the peak days up. Of any single species, the Sharp-shinned Hawk graph seems to most closely replicate the shape of the all-species graph. (See the GGRO Migration Timing Graphs (PDF) for more information.) And interestingly, a peak hawk-count day does not always mean a great banding day. GGRO Research Director Buzz Hull notes that, “On some of those big Sharp-shin days, especially if the Headlands are a little on the warm side, the flight of hawks is so high that hawkwatchers have to scan straight overhead to see the big flight, and banders haven’t got a chance.”

For different species, peak days can be all over the place. Osprey are among the earliest, hitting their highest numbers on September 10.  Sharp-shins and Cooper’s reach peaks on September 23 and24 but slide off gradually into early October. Broad-winged Hawks, those mysterious eastern Buteos, peak on September 26, and their congener, the Red-shouldered Hawk, shows a slight double peak—one on September 25 and a second higher spire on October 7. Merlins show a broad peak of about two weeks (October 22 to November 3), while our tundra-nesting Roughlegs are latecomers to the Golden Gate party, peaking around November 6.

Red-tailed Hawks are so consistent in showing an annual double peak (one around September 24 and a second higher peak at November 6) that raptor genetics expert Josh Hull analyzed the two apparent groups of Redtails. He learned that the early Redtails are more likely to be from California, while the November phase has a greater mix of Nevada and Great Basin Redtails.

If you look at 25 years of GGRO hawk count data, with all the species and years thrown together in one grand August through December graph, there is one distinct mountain peak that stands up above the 60 rph (raptors per hour) line. That’s one hawk a minute, an absolutely arbitrary line that appeals to my sense of order. It starts on September 16 and ends on October 3.

A true “Peak Week”? Not at all, but “Peak 2.5 Weeks” just has no ring to it, so Peak Week it is.