Phenotypic basis for a feeding change in an insular population of garter snakes

Abstract

Predation on birds, an unusual behavior for the common garter snake (Thamnophis sirtalis), has been observed and documented in an insular garter snake population. Feeding preference tests were performed on naive neonatal snakes and experienced adults from both the island test population and a nearby mainland control population. No significant preference for birds as a prey item was found for either neonates or adults from either population. Therefore, neither a genetic nor an ontogenetic basis for the observed behavior was demonstrable. Birds tainted with a piscine odor, however, elicited a positive feeding response from adult snakes from both geographical sources. We propose that the observed behavior in the field is a result of learning to exploit an abundant, readily available resource already in the predator's repertoire.

Evidence of Predation on Nestling Birds by Eastern Gartersnakes (<i>Thamnophis sirtalis sirtalis</i>)

“…Fish were the dominant prey item in stomachs of snakes from the fish hatchery, and amphibians and earthworms were the dominant prey items in snakes from reference sites. A similar pattern has been observed on islands with nesting colonies of birds (Fetterolf 1979;Greenwell et al 1984), although these researchers did not compare the diets of Common Gartersnakes, but rather observed Common Gartersnakes eating nestling birds at locations where they were abundant. One of these studies (Greenwell et al 1984) examined the prey preference of Common Gartersnakes in the laboratory by presenting them with various food items, including birds.…”

“…A similar pattern has been observed on islands with nesting colonies of birds (Fetterolf 1979;Greenwell et al 1984), although these researchers did not compare the diets of Common Gartersnakes, but rather observed Common Gartersnakes eating nestling birds at locations where they were abundant. One of these studies (Greenwell et al 1984) examined the prey preference of Common Gartersnakes in the laboratory by presenting them with various food items, including birds. Common Gartersnakes from the island that had been observed eating nestling terns had a similar prey preference to snakes from the mainland: both populations preferred frogs, followed by earthworms and fish; birds were rarely consumed.…”

“…Given the relatively recent introduction of earthworms to North America (Hendrix and Bohlen 2002), the preference of Common Gartersnakes for earthworms is a clear example of opportunism in diet selection. Greenwell et al (1984) conducted an interesting trial in which 12 Common Gartersnakes were presented with bird carcasses, and none consumed the birds. Yet when the researchers infused the bird carcasses with the smell of fish, 9 of the 12 Common Gartersnakes ate the birds.…”

“…In areas with a high density of ground nests and fledglings, young birds could make up a substantial part of the Common Gartersnake diet during part of the summer (Fetterolf 1979;Greenwell et al 1984). *Number of references (including review articles and books) that mention each prey item.…”

Historical Contingency and Animal Diets: The Origins of Egg Eating in Snakes

“…A natural example of a chemically mediated feeding change has been suggested for Thamnophis sirtalis (com-mon garter snakes) on certain islands in Lake Michigan (Greenwell et al 1984). On the mainland, common garter snakes rarely eat birds, but island snakes feed heavily on nestling Sterna hirundo (common terns).…”