| Summary: | 1) This study investigated the reproductive strategies employed by European starlings (Sturnus vulgaris) nesting in two separate study areas in England and another near Wellington, New Zealand. Evidence from behavioural observations and single-locus DNA profiling was used to establish the occurrence and frequency of alternative reproductive strategies within this generally socially monogamous species. 2) In total, 8% (42/526) of nestlings were sired by an extra-pair male and 28% of broods (37/134) contained at least one extra-pair young (EPY). There were no cases of extra-pair males gaining more than 50% of the paternity within a clutch. The distribution of EPY was best explained by the 'genetic diversity' hypothesis. 19% of the males breeding within the English study areas were paired to more than one female and extra- pair paternity (EPP) was significantly more common in the broods of these polygynous males. Nevertheless polygynous males sired significantly more fledglings than males paired to a single female. EPP was equally common in the broods of primary and secondary females. 3) An examination of the possible methods of paternity protection employed by male starlings failed to find evidence for strong mate guarding. Males followed a greater proportion of female-initiated departure flights during the female's fertile period (81 - 94%). However, in each study area males actively left their females unguarded during the female's fertile period. Frequent copulation is suggested to be the primary paternity guard in starlings. 4) The frequency of intraspecific brood parasitism (IBP) was determined through regular nest inspections and DNA profiling. Double laying was detected in 9% (34/394) of all clutches monitored. The hatching success of eggs thought to be parasitic was significantly lower than that of eggs laid by host femmes. DNA profiling revealed that 2% (12/576) of chicks were parasitic and 7% (11/154) of broods contained at least one parasitic chick. A single case of communal nesting was found. I suggest that IBP is the result of nest desertion principally caused by intraspecific disturbance and is a 'best-of-a- bad-job' strategy. 5) The existence of anti-parasitism strategies such as nest guarding, egg removal, nest desertion or retaliatory parasitism was investigated. Behavioural observations revealed that the intensity of nest guarding did not increase during the laying period, the time when potential hosts are most at risk from parasitic activity. The experimental presentation of a female during the laying period failed to increase the intensity of nest guarding. An egg addition experiment did not apparently induce parasitic behaviour in host birds and the host clutch size was not significantly different to the clutch size in non-experimental nests. Egg removal was no more frequent in experimental nests than neighbouring nests or nests elsewhere in the study area. The apparent absence of anti-parasitism behaviours in starlings is discussed. 6) The influence of breeding density, colony size and nearest neighbour distance on the frequency of IBP, EPP and intraspecific disturbance was investigated. There was no evidence to suggest that the composition of colonies of different sizes or nest distribution differed relative to male age. The mean frequency of EPP among nests was significantly negatively correlated with nearest neighbour distance. The frequency of intraspecific disturbance also showed a significant negative correlation with increasing inter-nest distance. No relationship was found between the frequency of EPP and the degree of breeding synchrony. IBP was significantly less frequent in the asynchronously laid intermediate clutches. The distribution of EPP among nests is discussed against a background of frequent copulation as the principal paternity guard.
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