Female brood desertion increases with number of available mates in the Rock Sparrow

We studied the reproductive strategy of a Rock Sparrow Petronia petronia po pulation. breeding in nest boxes in the Western Alps (Italy). Over seven ye ars of study (1991-1997) 19% of the females laid second clutches after succ essfully fledging the first one. Among these, about 50% deserted the first nest when nestlings were 14.3 d old (range = 8-19 d). 3.6 d before fledging (range = 1-8 d). In all these cases the primary male mate took over all pa rental duties and successfully reared the young. Inter-clutch lime of deser ting females was 8.1 d shorter than that of non-deserting double-brooded fe males. The breeding success of deserting females was significantly greater than that of both single-brooded females and double-brooded females that di d not desert their first brood. The fledging success of the second clutches depended on the status of the secondary male: females paired with previous ly unpaired males had a higher fledging success than those that paired with a polygynous male. The frequency of deserting females varied among years f rom 0 to 16%, and was significantly and positively correlated with the freq uency of males available as males at the time of desertion. In this study w e showed that sequential polyandry with brood desertion is a regularly occu rring strategy in the female Rock Sparrow.