Green Darners: Bird Food that Migrates

Mated Pair of Green Darner Dragonflies (Anax junius). Dragonfly reproductive biology is rather brutal. The male dragonfly (right) grasps the the female (left) by the head while she lays eggs. This ensures that the eggs he fertilized have a chance at development–rather than being forcibly removed by another male. This “guarding” behavior can lead to damage to the female’s exoskeleton. Photo taken on 9/28/13 at Fennessey Ranch, South Texas. Canon EOS 7D/600mm f/4L IS (+1.4x TC). Natural light.

I have noticed a large up-tick in the number of Green Darners (Anax junius) around the Texas Gulf Coast. This no surprise as Green Darners migrate from as far north as Alaska to as far south as Panama during the fall. The details of Green Darner migration across North America are sketchy, but many millions fly south during fall with their avian predators. Why some Green Darners migrate and others stay put is a mystery, but the north-south migration is intergenerational as the reproductive adult typically only lives for an estimated 4-7 weeks. On an individual basis, telling a migrant from a resident is generally not possible.

For a bird photographer, the waxing and waning of abundance of adults of different dragonfly species means that I get to shoot birds preying on different species of dragonflies throughout the spring, summer, and fall. For dragonflies, it seems likely that emergence (molting into the flying adult from the aquatic larval form) and mating are two times of special vulnerability to avian predation. In the case of the former, the dragonfly must sit motionless on vegetation for hours while the wings extend and harden. In the latter case, the male and female insects are attached, thus presenting a larger and slower target for predatory birds.

In any case, such temporal variation in prey abundance adds a fascinating dimension to nature photography . . . .

Little Blue Heron with Male Green Darner Dragonfly (Anax junius) at Brazos Bend State Park, Texas. This bird grabbed dragonfly after dragonfly from vegetation. Whether or not these insects had just emerged or were migrants must remain a matter of speculation–but one day there was suddenly a huge number of Green Darners in the park. Canon EOS 600mm f/4L IS (+1.4x TC). Photo taken in late September. High-speed synchronized fill-flash.

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