Protandry in a partially migrant population of the Stonechat (Saxicola rubicola): a field study in the central Apennines (Italy)

Here I present the results of a 3-year field study on some aspects of spring migration in a partially sedentary population of the stonechat Saxicola rubicola in central Italy. By monitoring all suitable territories daily, including those occupied by resident pairs, I recorded the daily increase in the number of both sexes of stonechats in the study area. As first-day sightings showed themselves to be insufficient to test protandry, the study demonstrates the existence of protandry in the stonechat in the field by calculating the daily male-to-female ratio of immigrant individuals, as typically done at ringing stations, and by analyzing the average arrival dates of males and females while correcting for a slightly male-biased detection. Nevertheless, in apparent contradiction to the sex-biased arrival, about 40% of the stonechats arrived already paired in their breeding territories. For the first time, this fraction of stonechats arriving already paired on the breeding grounds, also reported by other authors, has been better analyzed and shown to be restricted to peak migration. Additionally, the analysis of early male arrival and subsequent female mate acquisition suggested that any correlation was purely coincidental, in contrast with both the rank hypothesis and the mate opportunity hypothesis, proposed to explain the maintenance of protandry. The discussion explores a hypothesis to reconcile these contradictory findings with the protandrous arrival of stonechats at the breeding grounds.