Reduced breeding success

Nightjar. Photograph by Chris Knights

Breeding productivity has been falling consistently for the amber-listed Nightjar.

Our best measure of nest-level breeding success is Fledglings Per Breeding Attempt (FPBA), calculated from brood sizes and nest failure rates recorded by participants in the Nest Record Scheme, which indicates the mean number of young fledging from each nest in a given year.

Ten species exhibit reduced FPBA over the past 46 years, indicating that their productivity has decreased over time: two red-listed species (Tree Pipit and Linnet), two amber-listed species (Nightjar and Reed Bunting) and six green-listed species (Moorhen, Garden Warbler, Sedge Warbler, Treecreeper, Chaffinch and Greenfinch). While productivity of Moorhen, Nightjar, Garden Warbler, Linnet and Reed Bunting has been falling consistently, trends for the other five species are curvilinear, increasing between the mid 1960s and mid 1980s and decreasing thereafter.

Productivity declines of the migrants Nightjar, Garden Warbler, Sedge Warbler and Tree Pipit may be driven by changes in habitat or climate on their African wintering grounds. The majority of species exhibiting productivity declines, including residents such as Reed Bunting, are reliant on invertebrates to feed their young and there is increasing evidence that climatic change and/or anthropogenic factors such as pesticides are leading to a reduction in the size of prey populations. Alternatively, climatic warming may have resulted in a developing asynchrony between laying dates and the availability of insect prey on the breeding grounds. Long-distance migrants are thought to be particularly susceptible to such disjunction but residents may also be affected, particularly those reliant on seasonal peaks in caterpillars, such as Chaffinch and, to a lesser extent, Treecreeper; however, numbers of Chaffinch have increased over the same period and we cannot exclude the possibility that reduced breeding success is due to a density-dependent increase in intraspecific competition. Lack of food for nestling and parent Linnet due to a paucity of stubbles and weeds in more intensively farmed agricultural habitats may have contributed to the reduction in the species' breeding success, while Greenfinch productivity may have been impacted by the continued spread of trichomoniasis. The driver for increased Moorhen nest failure is at present unclear, but increases in aquatic mammalian predators and Coot populations have been proposed.

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 fallen significantly for eight of the species monitored. The productivity of Blue Tit, Willow Tit and Sedge Warbler has fallen by more than 50% over the last 25 years, while Willow Warbler, Blackcap, Garden Warbler, Song Thrush and Reed Bunting exhibit reductions of between 25% and 50%. For species such as Blue Tit and Blackcap, where a concurrent population increase has occurred, reductions in productivity may be driven by density-dependent processes, whereby increased competition for resources in an expanding population reduces the mean breeding success per pair. Song Thrush and Sedge Warbler have experienced significant declines in abundance, either on CES sites or more widely (based on CBC/BBS figures), but previous analyses suggest that falling survival rates are likely to have been a more important contributor to population changes than reduced productivity. There is some evidence that a reduction in the number of offspring produced may be the driver of Willow Warbler declines, however, and may also be preventing recovery of the UK Reed Bunting population.