Here’s the radar from 6:00pm on 10/10 to 12:30pm on 10/11.

Frames are every 1/2 hour. Click on the thumbnail to view the full-sized animation.

This is a “make-up” post, since I was down in Cape May doing the Big Sit event and couldn’t post the radar at the time. Instead of the normal duration, I have extended these animations out to 12:30pm on 10/11. I did so because it’s interesting to see if and when birds change direction after sunrise, and whether that change is visible on the radar. First of all, the entire region experienced high migration traffic, as can be seen on the regional composite. Since the winds were out of the NW for most of the night, most traffic across the region was headed NW->SE. This pattern is a recipe for good birding along the east coast, since many birds were moving over NY state and Pennsylvania, all of which were headed to the southeast.

If you check out the loop for the Dover radar (3rd and 4th from the left) you can see that after sunrise, the direction of traffic across the radar quickly shifts from N->S, to S->N, indicating that whatever is being picked up by the radar has changed directions. This can happen for two reasons. Sometimes the prevailing wind is different from the direction that birds are migrating, and therefore once the birds disappear from the radar, whatever is left will become more apparent. Since the winds on 10/11 were still light and northerly after sunrise, this explanation doesn’t work for that day. More importantly, though, is to look at the areas which appear to experience this phenomenon. The signal is most clearly present between Lewes Delaware and Cape May, as well as at the north end of the Delaware Bay, where the river feeds into the bay. These are the narrowest points between Delaware and New Jersey, and therefore are the easiest places for birds to make the crossing. So the question, then, becomes: “why do birds want to head back north after migrating south?”. There are several theories as to why (and if you were on the hawkwatch platform on 10/11, you might have even gotten into the discussion; these include maneuverability in a headwind, seeking out optimal foraging habitat, and reorientation after a night of drift ). I’ll be devoting a special post to this topic in the near future, but in the meantime, feel free to talk amongst yourselves, here, in the comments section.

For the recap and results of our Big Sit- see Tom Reed’s and Don Freiday’s posts on the Birdcapemay.org blog here: http://www.birdcapemay.org/blog/

Good Birding

David