Temporal variation in the population density and structure of the Eurasian Bullfinch Pyrrhula pyrrhula in the Iberian Peninsula

The population ecology of the Eurasian Bullfinch Pyrrhula pyrrhula is almost unknown in Iberia, where the subspecies iberiae lives. The present study provides a first approach to the population attributes of this subspecies in an area located in northern Spain, characterised by a landscape dominated by hedgerows and meadows. In particular, I analysed the population density, age distribution and sex ratio during a six-year period (2001–2006). By exploring the entire area, I estimated the density in each month, and distinguished males, females and juveniles. In winter, samplings by line transect were also used to obtain abundance indices to compare different days, months and years (1999−2005 period). Density values during the breeding season were similar between years, but winter abundances changed considerably at different temporal scales. A density peak was found in July–August, with the highest percentages of juvenile individuals occurring in August– September. Individuals clearly performing post-juvenile moult were seen during August–November. Sex ratio was markedly biased towards males throughout the year. Several biological and ecological characteristics of the Bullfinch, together with a favourable habitat and small changes of environmental conditions from year to year, seemingly promoted the relatively high stable breeding population densities estimated during the study period. The high variation in winter abundances was likely due to short-medium range movements. The high population density in late summer was a consequence of the addition of juveniles each year. The greater parental effort of females compared to males – since the former are responsible for most of the reproductive tasks and directly suffer considerable predation during incubation – was probably a root cause of the skewed sex ratio.