Frugivorous Insectivores

For the past few weeks at work, I have been watching an Eastern Phoebe. The running joke is that it's a vegetarian since we see it gorging itself on Winged Sumac berries more than we see it flycatching insects.

Yesterday, I was admiring a swarm of Tree Swallows. The flock was composed of at least 5,000 birds. While not quite the intricate murmurations of starlings, this flock was forming interesting shapes and patterns, flying as a shifting mass only to descend and flatten over a shrubby section of marsh. I asked my coworker, "how often do you think an individual bird in that flock actually gets an insect?" A large flock suddenly descending on the marsh might flush up a few more insects than a single bird, but I couldn't imagine it was enough to compensate for the competition an individual swallow would have to catch those few insects.

On my lunch break, I decided to look up the diet of Tree Swallows on Birds of the World. I was curious to know what size insects the swallows prefer. Are they eating mosquitoes or primarily larger insects? What I was surprised to discover is Tree Swallows regularly eat Wax Myrtle berries in the winter. Suddenly, it all made sense. They weren't eating insects at all. Those shrubs out in the marsh were Wax Myrtles!

While I already knew that Eastern Phoebes will eat berries in the winter when insects are scarce, I never suspected it with Tree Swallows. Further reading revealed that Tree Swallows' unique ability to digest the waxy coating on the berries is what allows them to winter much farther north than other swallow species that are more reliant on insects. It was fascinating to learn something new about a species that I originally thought I knew so much about!

I had one final revelation when I googled to see what other species feed on Wax Myrtle berries. Listed were Yellow-rumped Warblers and Gray Catbirds. That came as no surprise; I regularly see both foraging in the Wax Myrtle thickets. What finally clicked for me was the eastern subspecies name for Yellow-rumped Warbler, Myrtle Warbler! I knew they ate Wax Myrtle berries sometimes, and I knew they were called Myrtle Warblers, but I had never associated the two. Yesterday certainly exemplified the adage, you learn something new every day.

My ebird checklist for the day can be found here.

Posted on Ιανουάριος 21, 2022 1237 ΜΜ by swampster