FORAGING ECOLOGY OF SYMPATRIC PARIDS: INDIVIDUAL AND POPULATION RESPONSES TO WINTER FOOD SCARCITY (COMPETITION, NICHE, BEHAVIOR, OVERLAP)

KUMTHORN THIRAKHUPT, Purdue University

Abstract

This study investigated individual variation in foraging behavior in response to seasonal food shortage in populations of Carolina Chickadees (Parus carolinensis) and Tufted Titmice (Parus bicolor) at the Ross Biological Reserve, Tippecanoe County, Indiana. Each individual was banded, sexed, measured (wing, beak, and tarsus lengths), and observed to quantify foraging behavior in the field (foraging height, foraging distance from trunk, foraging substrate diameter, and foraging technique). The data provided descriptions of associations between intraspecific morphological variation and foraging behavior, individual and interindividual variation in foraging, changes in population total niche width and its components, and seasonal variation in breadth and overlap of the feeding niche between the two parid populations. I measured changes in the degree of generalization in foraging at both the individual and the population levels. The flexibility of individuals and populations in response to food reduction from early to late winter helped to test predictions made by foraging theory and competition theory. Correlations between morphological characters and foraging behavior were not strong and provided only partial support for the expectation that intraspecific morphological variation is related to resource utilization and is an adaptation to avoid niche overlap among individuals in the population. Individuals of both species were generalists. Individual foraging generalization increased to a degree that approached population-level generalization in late winter when food is most scarce. Population-level patterns are not always mirrored on the individual level, as is often assumed. Although most individuals showed similar changes in foraging behavior and became more generalized when food was scarce, some individuals shifted in opposite directions or generalized much more than others. Most results are consistent with the prediction based on foraging theory that food scarcity will favor behavioral generalization and broadening diet. Competition pressure for avoidance of overlap in resource use among individuals and between species seems weak in comparison to individualistic generalization. Competition between and within these populations was not strong enough to force either significant divergence among conspecifics or niche divergence of populations in spite of a declining food supply.

Degree

Ph.D.

Subject Area

Ecology

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