Molecular analyses

We sexed nestlings using DNA from red blood cells and a standard protocol for sexing Tree Swallows (for details, see [19]). Adults were sexed by plumage and/or presence of a brood patch (which only females possess).

Adult male Tree Swallows average heavier than females [20] and, because heavy nestlings were selected for tagging, 29 of 36 tagged fledglings (80.6%) were males (Fig 2).

Some juveniles continued to visit the natal site for days or weeks after fledging. Females are shown with black circles and males with gray triangles. Each symbol represents one day on which the individual was detected at the site; the first day on which a symbol appears for a given individual is its fledge day. Individuals are ordered from top to bottom by latest detection date. Breaks in horizontal lines represent days on which the base station was not functioning. The nest number of each fledgling is displayed on the y axis and the x axis is the number of days since a given individual’s natal nest hatched.