Robert W. Butler
824 Ladner Street, New Westminster, B.C. V3L 4W4;
e-mail: contact@robbutler.ca

Abstract: Whereas many birds walk or hop in search of earthworm prey, the American Robin (Turdus migratorius) hunts by making short runs or hops followed by a brief period when they stand motionless. Previous studies have shown that robins use sight and sound to locate earthworm prey but they did not explain the purpose of the motionless behaviour. I test predictions for several hypotheses using published accounts and my personal observations to conclude that American Robins most likely halt to look for prey. It was less clear but plausible that earthworm movements could be heard or felt through their legs.
Key Words: American Robin, Turdus migratorius, earthworm, foraging, sensory

Introduction

An enduring feature of suburbia in North America is an American Robin (Turdus migratorius) in a tug-of-war with an earthworm (Lubricus terrestris). Robins are one of many birds that eat earthworms. Starlings, New World blackbirds and crows are examples of birds that hunt by walking across the ground while looking from side to side for an earthworm on the surface or in its burrow. Their heads bob forward and back with each step and frequently turn from side to side. Once a worm has been discovered, the bill is thrust into the soil to catch and extract the worm. Kiwis (Apteryx spp.) and some shorebirds are examples of birds that repeatedly probe soft sediments for worms. These birds have long bills with sensory cells in their tips used to locate buried worms (Cunningham et al. 2009). Some gulls and lapwings repeatedly tap the earth surface with one or both feet often in muddy locations, to bring worms to the surface where they are consumed (Pienkowski 1983). American Robins and other thrushes make alternate short runs or hops and brief pauses when they stand erect and motionless while searching for earthworms (Vanderhoff et al. 2020). Once an earthworm is detected, the robin approaches, crouches, cocks its head to look or listen, and then grabs and dispatches the worm using the bill.

Heppner (1965) used experiments to conclude that American Robins used mostly visual clues. He thought that robins were unlikely to use vibrotactile cues through their feet since the birds cocked their heads towards prey in his experiments, although he did not rule tactile detection out. Montgomerie and Weatherhead (1997) also used experiments from which they concluded that robins could find worms using auditory cues alone. They showed that robins located prey on trays without using vibrotactile cues and said that in tall grass, robins couldn’t see worms. However, neither study explained why robins ran or hopped instead of walked, nor the purpose of standing erect and motionless for brief periods while foraging.
In this paper I describe hypotheses for this erect and immobile behaviour and test predictions of foraging methods against published accounts and observed behaviour of American Robins.

Methods

Robins hunt earthworms by alternating between standing motionless and erect for brief periods and making short runs or hops. Once an earthworm is detected, the robin moves closer and crouches, cocks its head presumably to look and/or listen, and then thrusts the bill into the soil to catch the worm. I refer to these hunting elements as ‘searching’ when a robin ran or hopped to a new location, ‘standing still’ when the bird stood upright and motionless, ‘approaching’ when the robin moved closer to a prey detection, ‘crouching’ when the bird lowered the body into a horizontal position and cocked the head, and ‘attack’ when it attempted to grab the earthworm.

I used published information and my observations to test predictions from four hypotheses. I watched American Robins forage across a wide variety of habitats including urban lawns and in parks, along beaches, on small islands, along forests edges and in the alpine. My observations of American Robins over 40 years of birdwatching in British Columbia include all of these habitats.
I also closely watched through binoculars, naked eye, or video-taped 162 sequences by 7 robins while they foraged in a forested park, 2 along a busy road, 5 on playing fields, 3 on urban lawns, and 2 on sandy shorelines in Richmond, Burnaby and New Westminster in May and June 2020. I purposefully selected a partially enclosed forest where I expected the robin would be very vigilant, and beside heavy traffic noise where a robin would have difficulty hearing earthworm movements. In each occasion, I watched to see if robins turned the head to search for worms or raised the head to scan the sky while standing erect, if they adjusted their foot position, and if they foraged alone or with other robins.

Hypotheses

Vigilance
Ground foraging birds become vulnerable to aerial predators when foraging in the open away from shelter and searching for prey. The vigilance hypothesis posits that the erect response of robins is a temporary halt in foraging to judge if foraging is safe. A prediction of this hypothesis is that robins while standing still turn the head to scan around and above them for predators, and that they would forage in flocks when in dangerous locations.

Visual Searching
The visual searching hypothesis posits that robins when standing still are searching for telltale movements of worms. This hypothesis predicts that robins will turn and cock their heads to scan around them to locate a worm.

Audible Searching
The audible searching hypothesis posits that by standing erect and immobile, a robin can hear earthworms moving nearby. A prediction of the prey listening hypothesis is that robins would cock their heads when they hear worms.

Foot sensitivity
The foot sensitivity hypothesis posits that robins stand erect and motionless to feel for underground vibrations by worms. Predictions from this hypothesis are that robins will adjust the position of their feet to better feel the surface of the earth.

Results and Discussion

There was weak support for the vigilance hypothesis. In all sequences of ‘standing still’ in open areas, I could not detect any robin obviously scanning the sky around as if looking for predators. However, robins turned their heads to look at me when I was nearby and a robin foraging beneath a tree where the visibility was obstructed scanned down a trail presumably to look for approaching walkers. However, observed from behind, robins often seemed focused on hunting and not watching around them. Robins foraged alone or with one other robin on all occasions, and never in flocks.
There was good support for the visual searching hypothesis while ‘standing still’. On most sequences, the robins remained motionless and frequently turned their heads as if looking at the ground, especially before approaching a worm. Birdwatchers will be familiar with this method to locate a small bird in a forest canopy by waiting for it to move.
There was no clear evidence that robins were listening for prey while in the ‘approach’ or ‘standing still’ phase. Robins did not scan with the head as if trying to pick up audible cues. Moreover, two robins repeatedly caught worms along a busy road where hearing was likely impaired by traffic. Montgomerie and Weatherhead (1997) showed that robins used audible cues and cocked their heads to locate nearby worms, clearly indicating robins can use audible cues when ‘crouching’.
I did not see any robin reposition its feet or give any behavioural response that indicated it felt earthworms underfoot. Many birds have Herbst corpuscles in feet and legs that are sensitive to vibrations (Shen 1983) and earthworms give off vibrations as they penetrate the earth so it is plausible that robins can feel nearby earthworms, but it was not apparent they did so from my observations.
The run and hop ‘approach’ used by robins might be an important part of a hunting strategy to get earthworms to reveal their presence. Earthworms respond to vibrations near their burrows by rapidly retracting into the soil for safety (Mitra et al. 2009). Hops and runs preceding each ‘standing still’ event might serve to get the worms to move, which robins detect by standing still. This combination might have universal application while hunting other elusive prey in diverse places such as along seashores, in the alpine and on rocky islands where earthworms were likely absent, and urban landscapes with plenty of earthworms.
I conclude that robins most likely used visual clues and that the purpose of the brief ‘standing still’ pose was to search for subtle movements by worms before attacking them. Although there was no clear evidence to support the other hypotheses, I can’t rule out that robins combine watching, listening and feeling for movements in their hunt for earthworms.

Literature Cited

Heppner, R., 1965. Sensory mechanisms and environmental clues used by American Robins in locating earthworms. Condor 67:247-256.
Mitra, O., M.A. Callaham, Jr, M.L. Smith, and  J.E. Yack. 2009. Grunting for worms: seismic vibrations cause Diplocardia earthworms to emerge from the soil. Biological Letters 5: 16–19. Published online 2008 Oct 14. <doi:10.1098/rsbl.2008.0456>.
Montgomerie R. and P. Weatherhead. 1997. How robins find worms. Animal Behaviour 43:885-894.
Pienkowski, M.W. 1983. Changes in the foraging pattern of plovers in relation to environmental factors. Animal Behaviour 31:244-264.
Shen, J.-A. 1983. Behavioral study of vibrational sensitivity in the pigeon (Columba livia). Journal of Comparative Physiology 152: 251–255.
Vanderhoff, N., P. Pyle, M.A. Patten, R. Sallabanks, and F.C. James. 2020. American Robin (Turdus migratorius), version 1.0. Birds of the World (P.G. Rodewald, ed.). Cornell Lab of Ornithology, Ithaca, N.Y. <https://doi.org/10.2173/bow.amerob.01>.