Increased breeding success

Nuthatch. Photograph by John Harding

Nuthatch has exhibited the greatest increase in productivity of any species over
the past 49 years, due to a combination of falling failure rates
and increasing brood sizes

Our best overall measure of breeding success is Fledglings Per Breeding Attempt (FPBA), calculated from brood sizes and nest failure rates, which indicates the mean number of young leaving each nest in a given year.

FPBA has changed significantly and is currently higher than in the late 1960s for 27 species, across a wide range of taxonomic groups. This total includes 11 species for which the change has been linear, i.e. consistent increases in productivity across the last 49 years, and 16 species which show curvilinear trends (i.e. early decreases in FPBA were followed by increases, or vice-versa). For some species in the latter group, FPBA is currently only slightly higher than in the late 1960s.

Population trends are also positive for 17 of the 27 species, including raptors (Sparrowhawk, Buzzard, Barn Owl, Merlin, Peregrine), pigeons (Stock Dove, Woodpigeon, Collared Dove), corvids (Magpie, Jackdaw, Carrion Crow, Raven), and some small passerines (Nuthatch, Wren, Robin, Redstart and Pied Wagtail). It is therefore possible that increasing productivity has contributed to the population growth exhibited by these species over recent decades.

Conversely, 10 species (Little OwlTawny Owl, Kestrel, Starling, Dipper, Wheatear, House Sparrow, Tree Sparrow, Grey Wagtail and Yellowhammer) have declined in number as FPBA has increased, suggesting that a density-dependent reduction in intraspecific competition, or a retreat into better quality habitat, may have enabled breeding success to rise.

CES ringing data integrate productivity across the whole season, including juvenile survival in the first few weeks or months after fledging. According to this measure, productivity has risen significantly for just one of the 23 species monitored. (Chaffinch). The discrepancy between the positive Chaffinch CES trend and the decline in breeding success identified by the NRS warrants further study, but increased survival rates in post-fledging period could contribute to this, although data are sparse for this vital period.