Predicting big flights

Broad-winged Hawks soaring over Chichicaxtle (including one dark morph)

The raptor migration season here in Veracruz is now in its middle, high-volume part, with flights typically of 30,000 raptors or more each day (see hawkcount.org). The average day count over the last week was just over 76,000. The majority of the flight in this part of the season is of course broadwings.

the direct line measures 966 km, but that is not how the broadwing flies (image courtesy Google Earth)

I mentioned the Corpus Christi hawk watch in my previous entry, and we often look at their numbers to get a sense of what's on our doorstep. That site is located in Hazel Bazemore Park, just outside Corpus Christi, TX, about 1,000 km north of us - as the broadwing flies (their numbers are also posted to hawkcount.org).

Yesterday, the counters in Texas had a phenomenal day with more than 100,000 broadwings counted as migrants. Broad-winged Hawks migrate "up to 400 km / day in Central America" (Smith 1985, cited in Goodrich et al. 1996), or "500 km in 6 hours of ridge flight with favorable winds" (Kerlinger 1989, cited in Goodrich et al. 1996). A thousand kilometers then would probably take them 3 days or so.

September 2011 day totals for Broad-winged Hawk from Corpus Christi and Veracruz (source: hawkcount.org)

Wondering if such a traveling speed could be detected in a direct comparison between the daily counts of both sites, I decided to graph that out. I took daily totals of broadwings for both sites for September 2011. Note that the VRR (Veracruz River of Raptors) totals are really from two count sites combined: Cardel and nearby Chichicaxtle.

Obviously, weather events between southern Texas and central Veracruz will influence the broadwing flight between the two locations.

The Corpus Christi count reported their first larger broadwing flight on the 19th of September. Three days later, we had our first wave. From 21 through 24 September, CC had good sustained flights of broadwings. We (VRR) had two good days (23 & 24 September) and after that flights around the 40,000 mark.

The day after tomorrow, will we register the peak flight of broadwings that they had yesterday?

Cited literature:
Goodrich, L. J., S. C. Crocoll and S. E. Senner. 1996. Broad-winged Hawk (Buteo platypterus), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/218