Friday, February 26, 2016

Out Over the Perilous Deep

We received a lot of “First of the Year” Swallow-tailed Kite sighting reports on 2/24/2016, and it’s no wonder after a look at the following wind maps. The arrows show the wind directions around two high-pressure systems that were moving from west to east (left to right) on 24 and 25 February. The air in a high-pressure system always circulates clockwise and outward, whereas the air in a low flows counter-clockwise and inward. The more “feathers” on the wind arrows, the faster the winds.

Map 1.  11:00 Eastern 2/24/16

On 2/24 (Map 1), a high that had just passed into the eastern Gulf of Mexico was producing southerly winds (i.e., blowing from the south) over the Yucatan Peninsula. This would have encouraged kites staging on the northern Yucatan to begin flying northward. By late morning on the 24th, as this high was leaving the Gulf to the east, it was being replaced by another high approaching from the west. The clockwise-and-outward flow around this second, approaching high produced strong westerly winds across the northern Gulf of Mexico, which blew any northbound Swallow-tailed Kites in that area directly toward the Florida Peninsula (Map 1).  This explains the sudden up-turn in sightings in Florida on the 24th. For the previous 15 days, northerly winds had dominated the region. Most of the birds reported in peninsular Florida on the 24th probably had just reached land.

Map 2. 07:00 Eastern 2/25/16

But wind directions and velocities are continuously changing, and they change particularly fast over the Gulf of Mexico. Just 20 hours later, at 7:00 a.m. on 2/25/2016 (Map 2), the same high had shifted farther to the northeast and was now creating steady northerly winds from the southeastern U.S. southward across the Gulf and deep into the Yucatan Peninsula. This would have discouraged any kites still on the Yucatan from migrating northward. They were safe as long as they stayed on land.

This could not be said for the Swallow-tailed Kites - and all the other northbound migrants of many species - that had already struck out over the Gulf. These birds were now facing solid headwinds. Some that were already close to Florida may have gotten blown far enough south or southeast to encounter Cuba and survive.  But we know from our satellite-tracking studies that many northbound Swallow-tailed Kites come to a virtual stand-still out over the Gulf, turning in circles awaiting a change in the winds that will help them reach shore. We have learned that they can remain aloft over the water for three to four days. Most northbound kites will experience favorable winds in time and reach land. The rest will perish.