Age-Related Differences in the Prey-Dropping Behavior of Herring Gulls (Larus argentatus)

Several avian species are known to drop prey with hard exteriors from a height in order to smash the prey open and gain access to the food inside. Such a complex foraging behavior involving multiple steps and use of novel foraging methods borders on tool use and is a fertile area in which to study cognitive development of foraging skills. I observed a wintering population of herring gulls utilizing prey-dropping behavior to feed on wedge clams (Rangia cuneata) at Jamestown Island, VA. Prior studies on age-related differences in foraging efficiency in birds have shown that young or inexperienced birds are less effective foragers than older conspecifics. I examined age-related differences in prey-dropping efficiency in herring gulls using plumage and soft-body part coloration to age gulls as first, second, third year or adult. I compared the variables of height of drop and size of prey dropped between these age classes to determine if there was a difference in foraging behavior. I found no detectable differences in height of drop between any age classes of herring gull. All ages of herring gulls also dropped similar sizes of clams on average. However, yearling or naive herring gulls dropped a broader size range of clams, including clams of low profitability in terms of caloric gain. I tested three hypotheses to determine why yearling gulls included these less profitable clams in their diets. Two of these hypotheses were focused on where yearlings were foraging for clams while the third looked at their ability to discriminate between different sizes of clams. Yearling and experienced gulls differed with regard to prey size dropped due to lower prey discrimination abilities in the yearling gulls rather than yearlings foraging in poorer quality patches due to inexperience or exclusion by older gulls. Individual prey assessment skill is likely gained through trial-and-error learning after repeated drops of various sized clams. Implications for the development of preydropping behavior in general as well as directions of future research are discussed. AGE-RELATED DIFFERENCES IN THE PREY-DROPPING BEHAVIOR OF HERRING GULLS (LARUS ARGENT A TVS) GENERAL INTRODUCTION Many species of crows, gulls and raptors drop prey items in order to gain access to food that would otherwise be unavailable or difficult to consume (Cristol and Switzer 1999). Although there are some instances of softer-bodied prey being dropped, such as a rat or a spiny dogfish (Harber and Johns 1947, Cavanagh 1992), in general, birds drop prey with hard exterior coverings such as mollusks or nuts. A typical avian prey drop involves a search period that ends when the prey item is obtained. The bird will then fly some distance with the prey to a hard surface such as a rocky beach or a road, ascend vertically and release the prey item from a particular height. This will result in the prey item being smashed open as a result of the impact, or remaining intact, in which case the bird will often repeat the drop until success is achieved. This typical series of steps for an avian prey drop is well described and well documented in gulls (Tinbergen 1960, Barash et al. 1975, Siegfried 1977, Beck 1980). Such foraging behavior that involves multiple steps, multiple decisions and the solving of novel problems is a fertile area in which to study the cognitive abilities of animals. Prey-dropping behavior in herring gulls (Lams argentatus), in particular, has been compared to tool use by non-human primates in terms of its cognitive complexity (Beck 1980). Age-related changes in