Spring Migration Timing

Currently re-reading Scott Weidensaul’s Living on the Wind, a personal account of bird migration in the Americas, while studying how and why birds migrate, and what the future of migratory bird conversation looks like. In high school, it was one of my favorite books, as it combined both my scientific fascination of birds along with my more personal connection to birds and their environments. There’s a lot of really scientifically interesting stuff on the mechanisms of bird migration, but there’s also some really emotional stuff about the death of thousands of Swainson’s Hawks due to insecticides in Argentina, for instance.

Re-reading it now, it’s not quite as good as I remembered. The book doesn’t have a clear narrative arc, which I’m fine with, but it also doesn’t do the ‘sprawling New Yorker style’ that McPhee does so well, and that I’ve been reading so much of lately. Instead, there really doesn’t seem to be much of a structure at all, and feels like facts and stories haphazardly thrown together. There are probably better ways to structure this book.

Anyways, one small factoid caught my attention as I was reading the second chapter. It was mentioned that birds know when to migrate based primarily on two factors: genetic predisposition, and photoperiod (length of daylight).

You would think, then, that birds’ arrival and departure times would be really consistent. Photoperiod doesn’t vary much from year-to-year as far as I know, and genetic predisposition is obviously pretty immutable. The only thing delaying departures and arrivals that I can think of would be things like weather encountered en-route.

Sometimes, birds do arrive really consistently. I’m terrible about keeping records about this kind of thing, but I’ve heard of Dark-eyed Juncos showing up in people’s yards on the exact same day every single year, plus or minus one day, as just one example of consistency.

But on the other hand, spring migration for the past few years has not been consistent at all. Again, I don’t have set dates for this at all, but most other birders seem to agree, from what I can tell. Freshman year, I was getting most of the exciting neotropical migrants a week or so before Finals period started. It was fun and frustrating at the same time, I could use birding as a break from my studies, yet that only made me want to spend even more time in the field. By the end of the semester, Alison and I had even found Baltimore Oriole nests, presumably with young already inside, being tended to by adults. That must have meant that the birds had arrived many weeks before.

Yet Sophomore year, the major spring birds didn’t arrive until the last week of Finals. I didn’t even see my first Baltimore Orioles until the day before I left, not to even speak of nest-building and young-feeding. That’s a two or three week delay, compared with their arrival time my freshman year! I was abroad in Australia my junior year, but from what I heard reported on the email listservs, migration was also unusually late last year. Everyone’s been blaming it on Global Warming, but is that really the case?

So if migration is driven by genetic predisposition and photoperiod, neither of which seems variable, how could migration timing be so off? Global Warming doesn’t seem to factor into that. I thought about it some, and the best explanation I can come up with is that natural selection actually acted on genetic predisposition. Freshman year, the birds I’d seen arrive before or at the peak of migration may not have had very good breeding success, maybe because temperature-driven fruits and insects hadn’t fully emerged yet. Therefore, the late arrivals, had the highest breeding success rates, and so the next year, they and their offspring arrived late, as that is their genetic predisposition.

I obviously have no data whatsoever to back this up. If fruits and insects emerged late, I guess that means it was a cold spring, so I could go back and look at temperature data, if I wanted. What that might also mean is that migration arrivals are fluctuating, but self-regulating. Over the next few years, small genetic mutations may create early arrivers, who will do well, and gradually migration timing will become earlier again, but then another cold snap could occur, and we’d see late migrations the next year. I’m sure the data on migration timings vs. temperature exists, I just need to find the time to seek it out.

How’s my theory sound?

By the way, sorry for taking so long to write another post. It’s gonna be a busy, busy semester.


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