David M. Larson
Herring Gull with clam. Photograph by Andrew Cannizzaro (CC BY 2.0).
Anyone who lives along our coast has seen the phenomenon—gulls find a clam, fly up, drop their prey, and swoop down to retrieve the edibles from the usually shattered bivalve. But haven’t you wondered, “How did they figure that out?” and “Did they learn that technique or is it innate?”
There is considerable literature on feeding behavior in gulls, including The Herring Gull’s World by Niko Tinbergen (1953), one of the true classics of bird behavioral studies. As one might expect, juvenile gulls are less efficient at foraging and feeding than are adults. Juveniles may be less skilled at picking feeding areas, may be kept away from high-quality feeding areas by adults, may be less able to discriminate optimal prey items, or may be less efficient at foraging due to inexperience or physical or social deficits. Large gulls are identifiable in age class until reaching maturity at four years, so age-related changes in foraging have been well documented. Recently, Cristol et al. (2017) reported on their studies into the reasons for age-related foraging differences in American Herring Gulls (Larus argentatus) feeding on Atlantic rangia wedge clams (Rangia cuneata), primarily in a shallow coastal estuary in Virginia. Follow-up observations were conducted at a boat landing on Chesapeake Bay.
Herring Gulls winter in the primary study area and are present from late November to early March. Most of the birds present were either juveniles or adults (roughly even numbers), with about 10% subadults. While other species of gulls were present, only the Herring Gulls preyed upon the abundant rangia clams. The Herring Gulls dropped the clams on a large exposed rock in the estuary or on a paved causeway nearby. The density of clams in the foraging area was measured by sampling in 34 marked quadrants, and the marked quadrants were used during observation of foraging adult and juvenile gulls. The size distribution of clams in the mudflats was assessed by sampling of substrate, and ten size classes of clams were assigned, where 1 equaled the smallest and 10 equaled the largest. In addition, the size of clams preyed upon by the gulls was assessed by measuring shells found on the causeway after feeding. By comparing the sizes available with the sizes taken, the researchers developed a preference/avoidance index ranging from -1 to 1. Positive numbers indicate preference and negative ones avoidance.
At the alternative site, clams were not normally available to the gulls. The site was at a dock in a salt marsh, without mud flats. The principal food supply for gulls was offal from fishermen. The researchers provided fresh live clams at this site in different size classes on a frequent basis. They then assessed the utilization of this new resource by adult and juvenile gulls.
Preferred sizes of clams: Based on the shells of clams broken on the causeway at the main study site, gulls preferred clams in the 6–9 size classes (medium to large rangia). Smaller and larger clams were avoided.
Did juvenile gulls forage in areas with fewer, smaller clams? There was no statistical difference in quality of prey in foraging quadrants used by adults and juveniles—both used areas with higher than average foraging quality.
Did search behavior efficiency vary with age? Adults were three times more likely than juveniles to end a foraging session with suitable prey. Juveniles did not make more probes per clam or take longer to find a droppable clam.
Could juveniles recognize suitable clams? When juveniles found a clam, they spent more than twice as long as adult birds in rejecting unsuitable clams. Juveniles also rejected a higher proportion of clams before settling on one. Of cases where known-age birds dropped clams that could be measured afterwards, the juveniles dropped a wider range of clam sizes, suggesting less size discrimination in prey selection compared to the adults. At the alternate study site, juveniles failed to discriminate between optimal and suboptimal clam sizes—essentially at chance level—while adults did better than chance.
Prey-dropping behavior: Juveniles were more likely than adults to drop clams onto the rock in the primary study area than onto the pavement. Juveniles also performed their first drop attempt at lower altitudes than adults. Juveniles and adults did not vary in their success rate for breaking clams on the first drop or in the recovery of food from broken clams (eaten rather than stolen or lost).
When did juveniles begin breaking clams? At the secondary site, where clams were not part of the normal local food for gulls, juveniles dropped only 4% of proffered clams while adults dropped 59%. After six weeks of supplying clams, juvenile dropping rate climbed to 62%.
From these studies, the authors have concluded that of all of the measured components of acquiring food from dropped clams, the most likely explanation for the lower performance of juvenile gulls is the lack of experience in recognizing the most suitable clams. Juveniles foraged in the same areas as adults, had similar success in breaking clams on the first drop, and were as successful in acquiring the meat from broken clams as were the adults. Such behavioral traits could be easily learned by observation of adults. The juveniles’ principal deficits seemed to be the time it took them to select a clam (long inspection time and many rejected items) and the utilization of suboptimal clams (small ones with little meat or large ones that were difficult to carry). Since the selection of a clam and the reward—meat from a broken clam—are temporally separated, acquiring expertise in the subtleties of optimal prey selection may not be as quick as in other learning scenarios.
While the paucity of intermediate-age gulls in the study area did not allow a full picture of improvements with aging, other reports suggest that young gulls learn quickly, as these researchers suggested based on their provisioning studies at the secondary study site.
Reference
- Cristol, D.A., J.G. Akst, M.K. Curatola, E.G. Dunlavey, K.A. Fisk and K.E. Moody. 2017. Age-Related Differences in Foraging Ability Among Clam-dropping Herring Gulls (Larus argentatus). The Wilson Journal of Ornithology 129 (2): 301-10.
- Tinbergen, N. 1953. The Herring Gull’s World. London, Collins.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon’s Joppa Flats Education Center in Newburyport, the Director of Mass Audubon’s Birder’s Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.