A few weeks ago I wrote about a new citizen science initiative to monitor migrating dragonflies. In getting some background on this phenomenon, I read that dragonflies tend to follow the same migratory routes as birds, following the same topographic landmarks and avoiding the same water crossings. Cape May, New Jersey, famous for its migrant raptors and other birds, also sees swarms of southbound dragonflies. I wondered if anything similar had been observed by the hawkwatchers at Hill 129 in the Marin Headlands. Here’s a response from Allen Fish, director of the Golden Gate Raptor Observatory:
“We have a seen a range of meadowhawks and darners [dragonflies], and I know I saw Libellulas [a dragonfly genus] up there, but not so much en masse or migration except for Variegated Meadowhawks, which had a big push during Oct 2010, and seem to show lots of numbers on a few days every autumn. I recall doing a 15-minute a count in 2010 of visible meadowhawks in one direction but I’d have to dig out the figure. Also, we do get GREAT Merlin flights generally in association with (and hunting) those meadowhawks at least a few days per year.”
Merlins are small, swift, highly maneuverable falcons that breed up in the northern forests and winter in our area. I had always thought of them as primarily bird-hunters. But an association with dragonflies might account for the exceptional (in my experience, anyway) merlin numbers at Cape May. The Birds of North America species account notes that recently fledged juveniles take dragonflies in late summer (starter prey?), as do all ages in migration. I don’t know whether dragonfly-hunting has been observed in the Eurasian portion of the falcon’s range.
Further back was a series of columns about animal species that mimic, either temporarily or permanently, females of the species to gain a reproductive advantage. I mentioned the Australian giant cuttlefish (Sepia apama) whose males used specialized skin cells called photophores to imitate the patterns of females. They were also somehow able to alter their body shapes to resemble females. Small male cuttlefish used this gambit to get past larger males that were guarding females prior to, or after, mating. If the big guy was distracted and the female receptive, they switched back to a male pattern.
I just learned about a refinement in this remarkable technique. Sneaker males sometimes display a male pattern the side of the body facing the female and a female pattern on the side facing the guarding male. God help them if they get confused.
Finally, years ago, there was a piece about the life spans of birds, based on a study by Daniel Wasser and Paul Sherman at Cornell. They compiled longevity data, both in the wild and in captivity, for what they described as a representative set of bird species, and looked for correlations with taxonomy and life-history traits. Among other findings, they reported that passerines (songbirds) were relatively short-lived, while parrots, flamingos, and albatrosses were avian Methuselahs. They also claimed that longevity was associated with larger size, sociality, a plant-based diet, and nesting on islands, but not with migratory behavior.
That drew, recently, an email from Julien Peter Benney, who may be in Australia. Benney points out methodological flaws in the Wasser-Sherman paper, including the omission of the “old endemic” songbirds of Australia, which are relatively long-lived (he says the green catbird, a species of bowerbird, has reached 14 years in the wild), the nightjars, and the kingfishers. He also says the authors fail to differentiate between nomads (birds that move around and nest opportunistically based on environmental conditions, like the Australian banded stilt and our own white-faced ibis) and true migrants with a fixed route between consistent breeding and wintering areas.
Benney also enclosed a pdf of the paper, which I had not seen before (it was behind a paywall, so I relied on the press release.) Lo and behold, their species list did have a heavy Northern Hemisphere bias. They not only excluded bowerbirds, birds of paradise, and all the other old Australian families, but the antbirds and ovenbirds of South America as well. That may just reflect the availability of data, although you would think a few such species might have credible longevity records. They managed to include a few Australian parrots, along with the American and African species. And yes, no kingfishers or nightjars (the family that includes nighthawks and poorwills.)
Point taken, Mr. Benney. After a few borderline abusive comments I’ve had lately, it was kind of refreshing to get a civil response.