This study provides novel information about gregariousness and intraspecific aggression in Iberian
bullfinches (Pyrrhula pyrrhula iberiae) in northwestern Spain. Small monospecific parties never
exceeding 10 individuals were seen throughout the year, larger in winter on average. Males considerably
outnumbered females within the groups. Adult flocks were frequent only in winter. In spring,
many of the adult groups were mixed-sex assemblages composed of pairs plus supernumerary
males. Sightings of juvenile groups, up to seven individuals, were common in summer–autumn.
The vigilance role in mixed-sex assemblages, including pairs, appeared to be the responsibility
of males based on sex-specific vigilance rates. The highest frequency of aggressive encounters,
mainly male against male, occurred during the breeding season, associated with mate defence.
Females attacked males, not the contrary, which supports reversed sexual dominance in bullfinches.
Gregariousness probably acted as an anti-predatory and foraging strategy.
agonistic behaviour, dominance, flocking, mobility, seasonal variation, vigilance.