Territory quality or bird quality? Factors determining breeding success in the Magpie Pica pica

The effects of male, female and territory quality on eight measures of breeding performance in the Magpie Pica pica were examined using repeatability. Comparisons were made for (i) the same male or female on the same territory (ii) the same male or female on different territories during their lifetime and (iii) the same territory occupied by a different male or female. Female quality explained over 60% of the variance in clutch‐size and size of eggs laid. Male quality accounted for over 70% of a pair's breeding success within each breeding season. The differential ability of males to provision their female adequately during incubation and their chicks in the nestling period is the most likely explanation for this result. The timing of breeding appeared to be controlled primarily by territory quality, but the effect was relatively weak (19%). Although no effect of female quality on the timing of breeding was detected, this cannot be totally discounted. In addition, it is likely that environmental constraints (i.e. temperature) also exerted some influence on the date of laying. These results contrast with those of Högstedt (1980), who found that 86% of the variance in clutch‐size was due to the effects of territory quality, with only 14% of the variance attributable to female effects. Reasons for the differences are examined.