A spring overshoot discussion, continued, again . . .

Andrew Farnsworth The Cornell Lab Apr 09, 2013

Last week, Team BirdCast forecast that conditions looked favorable for spring overshoot migrants to occur in areas far to the north of their expected seasonal range. Here, we provide some additional discussion and speculation for what actually transpired.

After a frontal boundary swept across the Florida Peninsula, many birds were clearly grounded by unfavorable conditions. The composite satellite and synoptic conditions image below from Friday 5 April 2013 shows this frontal boundary stretching from the northern tip of Yucatan north and then northeast to the continental shelf off of North Carolina. The color scale on the left represents temperatures that correspond to cloud heights, the colder the temperature, the higher the cloud tops, and the more red the color. The coldest, highest, and in this image, reddest areas of clouds are associated with significant storm activity, such as some intense areas of precipitation over the southern tip of the Florida Peninsula through the Bahamas and over the continental shelf.

Birds encountering this nasty weather likely did one of two things: they continued “on course” and eventually landed in the closest available stopover habitat (a classic “fallout”) or in the water (classic over-water migration mortality) or they became entrained in the system and were transported for some distance with the system (leading to the classic southern overshoot events). This system clearly grounded many birds, although there is a hint of the southern overshoot event that might have been.

Perhaps the most dramatic report received in eBird associated with this weather system was the fallout reported from Key West, where large numbers of Palm and Prairie Warblers dominated a nice diversity of species. See this checklist submitted by Carl Goodrich. Less dramatic, but still associated with the passage of this system and with birds staying the course and falling out, widespread arrivals of Indigo Bunting were reported, with some concentrations and some fallouts along Florida’s Gulf Coast, as well of Summer Tanager and Blue Grosbeak.


Unfortunately, no proximate reports of southern overshoots were submitted to eBird for any New England or Canadian maritime locations; nor have we heard of any reports of mainland southern overshoots expected in this event (particularly those species listed above). Presumably, because birders were out in reasonable force this past week in many coastal areas, these migrants simply did not overshoot to more northerly coastal Atlantic sites as forecast.

However, we did receive an interesting report of a photographed Blue Grosbeak from Bermuda on 7 April 2013 and a report of an Indigo Bunting from New Brunswick on 9 April 2013. This hint of an overshoot event is scant evidence but perhaps some indication that southern overshoots associated with the sweeping cold front moving off the Atlantic coast did occur. We hope to receive additional reports of migrants from Bermuda that might lend additional support to our forecast.

One plausible and unfortunate outcome, if we are to assume that southern overshoot migration occurred but went undetected much farther to the East of the Atlantic coastline, is that some number of these early migrants entrained in the system may have met an untimely death in the Atlantic.