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)
I don't think I have ever noticed this behavior. I will have to keep an eye out for it now that the hermit thrushes are back in DC.
Posted by: John | 22 October 2006 at 07:54 PM
Great post! What a great opportunity to observe that behavior. I always learn something here- Thanks!
Posted by: Lynne | 23 October 2006 at 09:17 AM
Cool. Now I'm all a quiver.
Posted by: Clare | 23 October 2006 at 11:27 AM
Great post! I just have to respond, to see whether you've heard of something similar in toads-- we used to see this all the time in captive toads when they were fed (live crickets). As a prelude to striking, they focus on a cricket, then a toe quivers, as if in excitement. I think in herpetology it's interpreted as an effort to distract or attract the insect prey. Very cool that there's a similar behavior in birds!
Posted by: Sally | 23 October 2006 at 12:38 PM
Hmm, no, I haven't heard anything about that in toads or other amphibians. Interesting!
Posted by: Nuthatch | 23 October 2006 at 12:42 PM
It's post like these that make me so glad I visit your blog. This is interesting stuff.
Posted by: pablo | 24 October 2006 at 07:22 AM
I actually witnessed this today during the Sandy Hook CBC! It was amazing!
Posted by: Patrick | 16 December 2006 at 08:30 PM
How cool!
Posted by: Nuthatch | 17 December 2006 at 05:12 AM
I missed this post in October. You might like to know that Dr Dilger is my best friend and lives across the street. Despite his years of work with Agapornis, thrushes remain his favorite birds. I'll pass this article on to him.
Posted by: Mary Ann | 10 February 2007 at 10:11 AM
That's fantastic! Thanks.
Posted by: Nuthatch | 10 February 2007 at 05:48 PM