The Demography of Blackbirds Turdus merula in Rural Habitats: Is Farmland a Sub-Optimal Habitat?

Abstract

1. A population of 54-76 pairs of European blackbirds Turdus merula L. encompassing woodland, farmland and edge (wood/farm boundary) habitats was studied to investigate whether farmland is a sub-optimal habitat for a bird species whose ancestral habitat was woodland. 2. Woodland was occupied at a higher density than farmland, but there was also variation within habitats: 'hot-spot' areas of woodland were occupied at high density, 'sparse' areas at low density, while other areas were completely unoccupied. 3. There was no effect of habitat or territory type on survival, although movements between years were more likely to be from farmland to woodland, and from sparse to hot-spot territories than vice versa. 4. There was a higher proportion of young males in farmland than in woodland, and in sparse territories compared to hot-spot territories; but there was no difference in the age structure of females across habitats. 5. Timing of breeding and clutch size did not differ between habitats or territory type, but farmland females laid smaller eggs than other females. 6. Reproductive success was low in all habitats (only 19% of attempts produced any fledglings), but there were significant differences between habitats in various measures of reproductive success. Success in farmland and woodland was similar, but hot-spot territories tended to be more successful than sparse territories. Predation was the major cause of failure. 7. Our results indicate that farmland blackbirds exhibit certain characteristics of populations in sub-optimal habitat, but the effect of habitat on reproductive success probably operated at a finer scale than this relatively crude habitat classification. 8. The distribution of breeding blackbirds is discussed in relation to ideal free and source-sink models.