TREE SWALLOWS CANNOT BE CLASSIFIED AS DETERMINATE OR INDETERMINATE LAYERS

We used egg removals, additions, and swaps (controls) to determine whe ther Tree Swallows (Tachycineta bicolor) are determinate or indetermin ate layers. Since Tree Swallows modify their clutch size in response t o.their environments, and since egg additions and removals occur natur ally in this species, we predicted full indeterminacy. Females laid mo re eggs in response to the removal of their third and fourth laid eggs but did not lay clutches outside the normal range. Females did not la y fewer eggs when an egg was added on each of the second and third day s of laying. From this we conclude that Tree Swallows have limited rem oval indeterminacy but are determinate with respect to egg addition. W hen daily egg removal was used to keep females at 1 or 0 eggs, which l ed to abandonment of the nesting attempt followed by relaying, the sec ond clutch was much larger, on average, than the first. This suggests that the first clutch had been terminated early. Thus Tree Swallows, a lthough addition determinate, were capable of early cessation of layin g. Although removal indeterminate, they were not capable of laying abn ormally large clutches. We therefore argue that determinate and indete rminate laying patterns cannot be used to infer a female's ability to modify clutch size in response to other stimuli, and that strict categ orization of Tree Swallows as determinate or indeterminate is not appr opriate.