A little musing today. I’ve been watching two bird feeders outside my kitchen window for the past several months, the seeds devoured by California Towhees and occasionally a Spotted one, Western Scrub Jays, Eurasian Collared Doves, and lots of White-crowned and Golden-crowned Sparrows.
In my backyard, near an intermittent creek, I built a bench next to my vegetable garden so that I can give my knees a break after picking snow peas, asparagus, and weeds. Of course, I keep a pair of binoculars within reach in the event I see anything interesting while I’m out there.
I thought the White-crowned and Golden-crowned were hanging around a bit late as it was late April, actually April 30, the time they should be nesting somewhere up north. Should I quit feeding them? Well, the next day, on the first day of May there were no more sparrows, none – they had left. Then I went out to the garden, fussed with the drip system, and planted some tomatoes and peppers. Then I sat on the bench. In the next 30 minutes I spotted an Ash-throated Flycatcher, Western Tanager, Black-headed Grosbeak, Western Kingbird, Bullock’s Oriole, Nashville Warbler, and Yellow Warbler, all migrants. I had not seen any of these before May first.
In one day, the crowned sparrows disappeared and the migrants showed up. I’m sure that’s not true, but it sure seemed like that to me.
Birds initiate their migration in response to the length of the day, whose change is very predictable as the earth tilting on its axis. Birds are slowed down or sped up by weather conditions, but it’s the photoperiod that tells them it’s time to go. Overall that’s the best strategy for them. Typically, you’ll see a bunch of birds and a variety of species on their migratory journey appear just after a storm-front (low-pressure center) rolls through as the birds follow it as they move with a high-pressure center. I remember being in couple of places, namely southern Alabama and central Illinois, at the perfect time for those conditions. In Illinois, I saw hundreds of warblers and maybe 15-20 species of them pass through a small patch of woods as I just stood there for a short time. On Dauphin Island, Alabama, one of the first pieces of land birds find after crossing the Gulf of Mexico, I saw a tree full of male Rose-breasted Grosbeaks – maybe a hundred of them. These are rare sights only made possible by being in the right place and with the right weather conditions. (Read an interesting discussion on segregation during migration.)
Why did I see all those different migrants in the same place in my yard in such a short period of time? They all winter in Central America, some down to northern South America, but they all are instigated to travel as the days got longer and they all travel about the same route. The weather has been mild, so it probably didn’t affect their travel. Cold or rainy weather would have made insects scarce and slowed down the insectivores, but the oriole and grosbeak could probably have kept traveling. Typically, that’s what I’d see – a kingbird one day, a flycatcher the next, then a grosbeak and warbler a few days later. But good conditions down south let all these species show up in my backyard, and gave the impetus to the sparrows that it was time to move on.
But in one day? On May Day? Interesting.
I found it interesting that birds follow a low pressure system in a high pressure system. I didn’t know that. I studied climatology and weather systems and never thought about birds and how they maneuver around them. Would that be because the wet weather makes more insects for them to eat, or just that flying in dry weather is the best condition? Also, since I raise Monarch butterflies, would that also be true of their migrations?
They fly in a high pressure system either behind or ahead of a low pressure system. They do it partly because the weather is better and of course food is easier to find, but mostly because the high pressure makes it easier to fly. Physics tells us that there are more air molecules in high pressure than low pressure. That’s why birds perch before a storm – the low pressure makes it more energetically costly to fly. I presume the same logic applies to butterflies (and to airplanes).