By Keith Bildstein, Ph.D.
March, 22, 2011 (Tarifa, Spain)
We are now at the end of two days of field work in southern-most Spain, and our count of migrants crossing the Strait of Gibraltar from Africa to Europe has stalled at 11. That is about 189 observations short of the 100 flapping-rate sightings we were hoping to collect each field day.
Such is the life of raptor biologists trying the study the flight energetic of birds of prey crossing one of the world’s great raptor-migration bottlenecks. The subjects here include both Egyptian and Griffon Vultures, and the long-term goal is to compare their flight behavior with that of Turkey and Black Vultures crossing water in the Americas.
But back to the issue at hand… the lack of sufficient observations. To quote Bob Dylan “the answer my friend is blown’ in the wind, the answer is blown’ in the wind.” In our case, the answer seems to be blowing in the region’s “cross winds.”
Cross winds, or winds that blow perpendicular to a bird of prey’s desired route of travel, make it energetically more expensive for a bird to stay on track while crossing the Strait of Gibraltar… and staying on track while crossing the Strait is important for migrating raptors.
Simply put, the stronger the cross winds, the more costly the flight. And for the last two days, cross winds have been averaging 25 miles per hour, with gusts of 40. Expensive times for migrating raptors, indeed.
And then there is the problem of wind direction. Easterly winds, known locally at “Levante,” can push tired birds to the west, and, eventually, out to sea over the Atlantic Ocean. “Out-to-sea” is not a good thing for migrating raptors. And for the past two days the winds have been Levante. Energetically expensive cross winds are one thing, energetic and dangerous cross winds are altogether something else.
Ingrained in each migrating raptor’s head seems to be a rule that states “don’t fly over water in strong cross winds, and, whatever you do, don’t fly across the Strait of Gibraltar in winds that can push you west and out to sea on your return migration.” Most migrants here seem to have gotten this message, and have decided to stay put in northern Morocco until the winds die down, or at least until they change direction.
Tomorrow the forecast calls for a less forceful Levante. I hope the forecast is right, as I would like to see a flight and begin recording large numbers of flapping-flight observations.
This is fourth Spring that I have traveled to southern Spain to collect flapping-rate data. Initially, the study was to last three seasons. Levante winds have intervened each year. The results of the work should shed considerable light on the costs involved in crossing the Strait, where effective soaring flight seems to be difficult, particularly for large vultures. But I am still working on the results.
That said, I hope to collect sufficient information this year to complete the field work, analyze the data, and publish the results. There are many other projects on my to-do list. We are here for another seven days, and I remain hopeful. My enthusiastic coworker, former Hawk Mountain Sanctuary Intern Marah Ketter remains enthusiastic as well. Tomorrow the forecast calls for less wind, and the plan is to start playing catch-up.
But in the end, the answer is still “blown’ in the wind.” More in a few days.