Variation in the relationship between numbers of breeding pairs and woodland area for passerines in fragmented habitat

Species may differ in the relationship between the numbers of breeding pairs present and woodland area, because the proportion of a wood that forms suitable habitat will vary with woodland size. In this paper, we examine the pattern of variation in abundance with woodland area for eight breeding bird species, and also show how this pattern varied between years. During 1990-1997, we made annual censuses of 53-160 woods, of up to 10 ha in size, and fitted a power function to describe the relationships between numbers of breeding pairs and woodland area. Seven of the sight species, blackbird Turdus merula, dunnock Prunella modularis, wren Troglodytes troglodytes, great tit Purus major, chaffinch Fringilla coelebs, robin Erithacus rubecula and blue tit Purus caerleus showed a pattern of proportionally higher numbers in smaller woods. Only long-tailed tit Aegithalos caudatus occurred in proportionally higher numbers in larger woods. Blackbird and dunnock showed a trend towards lower numbers in large woods during years with low regional population levels: for these species large woods may provide sub-optimal habitat. Great tit, blue tit, chaffinch and robin showed the opposite trend, towards lower numbers in small woods during years with low regional population levels; for these species small woods may provide sub-optimal habitat. Wren and long-tailed tit, which also showed large annual population fluctuations. showed no change in distribution with regional population level. In great tit and chaffinch, the distribution of pairs in any one year may have been influenced by site fidelity producing a lag in the response associated with regional population levels.