Are Birds Arriving Early, Or Are There Just More Birders Out To See Them?

It's been unseasonably warm in the Washington, DC area this March, and lots of birders have been taking advantage of the warmth to seek out early migrants.  The birds have been cooperating, with species like Wood Thrush, Louisiana Waterthrush and Yellow-throated Warbler all making earlier-than-usual appearances.

Thinking on the way to work yesterday morning about these early migrants I was struck by a question I couldn't answer: How do birds wintering in South America know that we're having a warm March?  It would seem that early springs in the north shouldn't affect migrant arrival dates, unless the northern temperatures are mirrored by warmer-than-average temperatures in the migrants' wintering or staging areas. 

Coincidentally (I swear!), this exact question was posed on the MDOsprey listserv today, sparking an interesting discussion.  I won't rehash the whole line, but I was interested in a response by a Baltimore birder named Richard Edden: Maybe the warm weather is inspiring more birders than usual to be out in mid-March and they're seeing birds that would otherwise be missed?

I love the simplicity of this answer.  I've written before about how rare birds are more likely to be found in areas that are more heavily birded (sounds obvious when I type it out like that), and I think this is a variation of that theme: better weather = more birders out early = more birds found.

Of course, there's the business of figuring out if it's true.  Have there been more birders out in MD/DC this March than previous, colder Marches? 

Well, The Weather Channel reports that March 6 through 22nd have all been 60 degrees or above all but two days (70 or above 8 of those days), with little to no rain.  Nice.  In contrast, March 2011 in the DC areas was wetter and colder.  Only 5 days broke 60 degrees, and it rained (or snowed) on 9 days.  Of the remaining days in the month (22nd - 31st), it rained or snowed five times and never broke 50. A crappier month to go birding, undoubtedly.

Were there correspondingly less reports to the MDOsprey listserv in March 2011 than there have been in March 2012?  Yeah, there were.  In the second week of March 2011, there were 99 messages sent to the list.  In the second week of March 2012 there were 135.  Third week of March 2011 there were 110 messages, in the third week of March 2012 there were 140. 

Looking at this limited data I'm pretty hesitant to draw any conclusions about birder-effort in early spring, but I think it's certainly a factor.  There is no doubt that some birds are just back early.  Yellow-throated Warbler was reported in MD on March 16 in 2012, but not until the first days of April in the previous two years.  On the other hand, Louisiana Waterthrush was just three days earlier (March 18) than it was first seen the previous two years. 

There is no question that birds won't be reported if there's no one out to find them.  While there are a lot of factors involved with the timing of when birds arrive, short-term trends and early dates should be checked against birding effort in the areas of arrival.