Experimental examination of nest reuse by an open-cup-nesting passerine: time/energy savings or nest site shortage?

Reuse of open cup nests is rare among passerine birds despite possible savings of time and/or energy that might yield reproductive advantages. Anecdotal observations of the open-cup-nesting eastern kingbird, Tyrannus tyrannus, at Malheur National Wildlife Refuge (MNWR), OR, U.S.A., showed that 10% of females reuse old nests of mainly American robins, Turdus migratorius. We therefore deployed artificial nests in 2010 and 2011 at MNWR to provide female kingbirds with the opportunity to reuse nests to test predictions of the time/energy saving hypothesis. We also used a flood that submerged nest sites in 2011 as a natural experiment to test the nest site shortage hypothesisʹ prediction that nest reuse by kingbirds should increase when the availability of nest sites is reduced. Female kingbirds used artificial nests heavily in both years, and, contrary to the time/energy saving hypothesis, they did so more in the year of earlier breeding. We also rejected three other predictions of the time/energy saving hypothesis, namely that, within seasons, females nesting in artificial nests (1) breed earlier, (2) produce larger clutches and eggs and (3) fledge more young. The only prediction of the time/energy saving hypothesis that we failed to reject was that time to lay replacement clutches following nest failure tended to be earlier in females that used artificial nests. Nest reuse was significantly more frequent in the flood year. Given this result, and our rejection of most predictions of the time/energy saving hypothesis, we conclude that frequent nest reuse by kingbirds at MNWR is likely the result of a shortage of quality nest sites.