Hermit Thrushes (Catharus guttatus) are one of my favorite birds. Quite a bit of my research focuses on their migratory ecology, and it’s always nice to have a genuine fondness for your study subject.
The other day at home I looked out my office window and saw a Hermit Thrush at my pond, getting a drink and a quick bath. I have a lot of fruiting shrubs in the yard, and Hermit Thrushes are frequent visitors in autumn. Hydrated and clean, the thrush hopped out onto the grass.
It flicked aside some leaves, looking for something to eat. Not spying anything immediately, it moved along a few feet further. Then it did something I had only read about: it rapidly vibrated one foot. This was no casual shake, but a near-violent tremor, restricted to just the foot and lower leg, sustained for five or more seconds. The shiver complete, the thrush peered into the grass, then hopped forward and foot-quivered again. This time it picked at the grass, perhaps obtaining a small insect. This was repeated four or five more times before a reckless Blue Jay careened into the yard and startled the thrush into the shrubs.
Foot shaking is not unknown in other birds. Gulls, herons, and some shorebirds use a similar maneuver to stir up prey when standing in shallow water, although it is not as rapid and usually referred to as “foot-paddling.” Among songbirds, published reports describe high-intensity quivering in various thrushes, and also Swainson’s Warbler. I suspect it may be more widespread than that, but probably rare since many foraging behaviors in birds are fairly stereotypical.
One of the earliest reports of foot-quivering in Catharus thrushes was by William Dilger, who in 1956 discussed it in a paper on thrushes. He considered foot-quivering an antagonistic behavior used in mild conflict situations by birds on their breeding grounds. “The sound of the foot against the dry leaves of the forest floor…sometimes resembles a rattlesnake’s rattle,” Digler reported, surmising that the action was the result of simultaneous opposing impulses to move forward (attack) and to retreat (escape). He never saw it employed by a foraging bird in the wild.
A few years later, a note in the journal Auk countered this, describing observations of eight Hermit Thrushes foot-quivering when foraging on lawns. Still later, ornithologist Lawrence Kilham also observed a Hermit Thrush performing high-intensity foot-quivers, which he attributed to foraging and not to being disturbed. “Neither in the round, limpid eye nor elsewhere in the bird could I detect any motion or pose indicative of excitement of any cause,” he wrote.
Migration ecologists Wang Yong and Frank Moore recorded foot-quivering in the other three common North American species of Catharus thrushes — Swainson’s, Gray-cheeked, and Veery — on migratory stopover. They state, “Although we do not question previous interpretations that regard ‘foot-quivering’ as a hostile (intraspecific) display, the context in which ‘foot-quivering’ occurred during our study and the frequent attempts to capture prey that followed the movements indicate that the behavior functioned to flush prey.”
Alas, this great photo is not the thrush I watched, but one from the yard of the digiscoping maestro Mike McDowell (who has a nicer lawn than I do!). Surf over to Mike’s Birding and Digiscoping Blog and his Bird Digiscoping site for more of his excellent work.
After the jump, some of the literature on foot-quivering, including links to some PDFs.
- Berggren, A. 2006. Topography affects foot trembling side preference in the North Island robin (Petroica longipes). New Zealand Jrl. Zool. 33:197–201.
- Brackbill, H. 1960. Foot-quivering by foraging Hermit Thrushes. Auk 77:477-478. (PDF)
- Dilger, W. C. 1956. The hostile behavior and reproductive isolating
mechanisms in the avian genera Catharus and Hylocichla. Auk 73: 313–353. (PDF)
- Kilham, L. 1977. Foot-quivering in a foraging Hermit Thrush. Jrl. Field Ornithology 48:168-169. (PDF)
- Yong, W. and F.R. Moore. 1990. “Foot-quivering” as a foraging maneuver
among migrating Catharus thrushes. Wilson Bull. 102:545-547. (PDF)