Effects of Grass Length and Mowing on Foraging Behavior of the American Robin (Turdus migratorius)

Abstract

In the first of three studies, American Robins (Turdus migratorius) spent progressively less time foraging on a suburban lawn as the grass grew longer.The rate of attacks at apparent prey, however, was actually greater in long than in short grass.When, in a second study, the robins were offered a direct choice between short-and long-grassed areas, the birds showed an overwhelming preference for short grass.A third study revealed that the act of mowing itself (i.e.independent of grass length) induced a short-term enhancement in the time the robins spent foraging, implying that the birds were exploiting temporarily vulnerable prey.THE propensity of the American Robin (Turdus migratorius) for foraging on the short-grassed lawns that characterize much of North America is well known to even the most casual of birdwatchers.Indeed, the proliferation of the suburban lawn has contributed greatly to the expanded range and population that robins have achieved since humans cleared away so much of the continent's pre-Columbian forests (Eiserer 1976: 13-25).Despite the clear affinity of robins for lawns, however, little systematic research has been directed at uncovering the details of this particular species-habitat relationship, although related work has been done on other thrushes (Smith 1974a, b, Roth 1979).The present experiments investigated the effects of grass length and lawn mowing on the foraging behavior of robins.Tyler (in Bent 1949) observed that foraging robins run over short grass but are forced to hop over long grass.Heppner (1965) reported, without quantifying the fact, that robins have greater success in hunting worms where grass is sparse rather than thick.If long grass means an altered locomotor pattern and perhaps decreased hunting success, robins might be expected to spend less time foraging on a given lawn as the grass lengthens. EXPERIMENT 1Methods.--The observation area was a suburban backyard in Lancaster, Pennsylvania.The yard (15 x 18 m) contained dandelions (Taraxacum vulgare), sourweed (Oxalis corniculata), white clover (Trifolium repens), crabgrass (Digitaria sanguinalis), daisy fleabane (Erigeron annuus), and various other forbs and grasses.Data were collected daily (except when it rained) during periods that averaged 90 min and that occurred at different times of day ranging from 0700 to 1800; care was taken that the observations did not occur more frequently at some times of day relative to others.The lawn was mowed to a height of approximately 6.5 cm three separate times during June-July 1979; at least 15 days occurred between mowings.Data were not collected on any subadult robins who may have visited the lawn, because in many cases these birds were noticeably less adept at hunting than the typical adult.During each observation period, two categories of data were collected: (1)foraging time, the number of seconds per hour that a robin was seen foraging on the lawn, and (2) attack rate, the number of times per foraging minute that a bird either bill-