Special Analysis, Tribute in Light: 11-12 September 2013

With forecast conditions looking rather gloomy for observers watching at the Tribute in Light but rosy for avoiding any potential for hazardous conditions for migrating birds, Team BirdCast predicted local, light movements and perhaps tens of birds in the beams of light during the memorial during the night of 11-12 September 2013. The first migrating bird appeared in the beam at 8:11pm EDT, with a scattering of birds thereafter for the first two hours after sunset. Then something changed, and what seemed like a reasonably good prediction playing itself out started to look as if it missed the mark slightly before missing the board completely! Between 10:30-11 pm EDT numbers of birds in the beam began to grow, and suddenly the forecast for tens of birds in the beams was off by two orders of magnitude!

Team BirdCast likes to be the first to point out its forecasting missteps, so here is some interpretation of where things went wrong in the forecast! A substantial line of thunderstorms from central southern Pennsylvania to central and eastern New York and western New England slowly was drifting North and East across the region, winds were primarily southerly and southwesterly, and the warm and moist air mass presiding over the region felt more like summer than fall. What we did not consider was the presence of a gust front, visible in the movie above in the first few frames; watch for a thin gray line stretching from eastern Pennsylvania North and East into eastern Massachusetts and slowly moving Southeast toward the coast. As this gust front moves Southeast, a noticeable change in targets aloft occurs, particularly along the Connecticut coast. These targets are birds taking flight, reaching moderate levels along the immediate coast, and quickly eclipsing the forecast densities! It seems that as the gust front passed, conditions changed sufficiently to cue birds to fly, albeit a couple of hours after typical post-sunset exodus. Gust fronts are also known as outflow boundaries, separating cool air flowing out of thunderstorms from the surrounding air mass. In this case the gust front had some effects similar to those of a passing cold front, with a slight wind shift and presumably a slight dip in temperature and increase in pressure. Numbers of birds, appearing as light to moderate movements along a corridor from New Jersey North and East to eastern Massachusetts in the movie, clearly interpreted these changes as signifying the time to take flight. While numbers of birds increased in these areas in the hours after the passage of the gust front, most of the remainder of the region, particularly to the West of the line of storms, saw whatever light and scattered movements diminish.

After the initial scattering of birds in the early hours of the evening, the arrival of hundreds and then thousands of migrants in the beams that had taken off with the passage of the gust front was a surprise to say the least. Here is a short clip of the nocturnal migrant activity in the beam at approximately 2 AM EDT 12 September 2013. Almost all of the white “snowflake”-like objects in this video are warblers, flying around the beams from 100-1,500 m above the ground.

Special thanks to the Municipal Arts Society for allowing NYC Audubon and the Cornell Lab of Ornithology to monitor at the Tribute in Light. Moreover, Team BirdCast would also like to acknowledge that, as in past years when numerous birds were attracted to the beams, the organizers of the Tribute turned off the beams on four separate occasions during the night to avoid potentially hazardous situations for birds attracted to the lights!

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