Reduced breeding success

Willow Warbler, photograph by Chris Knights

There is increasing evidence to suggest that Willow Warbler population declines have been driven,
at least in part, by a reduction in breeding success.

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.

Twelve species exhibit reduced FPBA over the past 48 years, indicating that their productivity has decreased over time: two red-listed species (Tree Pipit and Linnet), four amber-listed species (Nightjar, Willow WarblerMeadow Pipit and Reed Bunting) and six green-listed species (Moorhen, Great TitGarden Warbler, Treecreeper, Chaffinch and Greenfinch). While productivity of Moorhen, Great Tit, Willow WarblerGarden Warbler, Linnet and Reed Bunting has been falling consistently, trends for the other six species are curvilinear. For five species, FPBA increased between the mid 1960s and mid 1980s or mid 1990s and decreased thereafter; whereas in the case of Nightjar, productivity decreased from the mid 1960s until the mid 2000s but has increased slightly over the last ten years.

Productivity declines in migratory species: Nightjar, Willow WarblerGarden Warbler and Tree Pipit, may be driven in part by birds returning in poorer condition as a result of changes in habitat or climate on their African wintering grounds. For Willow Warbler and Garden Warbler there is evidence that conditions on the breeding grounds and, in the case of the latter, grazing pressure from deer, may also be important.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. Additionally, climatic warming may have resulted in a developing asynchrony between laying dates and the availability of insect prey on the breeding grounds. Although this report shows that many species are advancing laying dates (see early breeding), for some species these advances may not be sufficient to match the advances in peak food availability. 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 Great TitChaffinch and, to a lesser extent, Treecreeper; however, numbers of Great Tit and Chaffinch have increased over period covered by this report and we cannot exclude the possibility that the observed reduction in breeding success is due to density-dependent processes. 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 trichomonosis, although research would be needed to establish this link. The driver for increased Moorhen nest failure is at present unclear, but increases in aquatic mammalian predators and Coot populations have been proposed as potential causes.

CES ringing data integrate productivity across the whole season, including juvenile survival in the first few weeks after fledging, the key breeding success parameter being the ratio of juveniles to adults captured. According to this measure, productivity has fallen significantly for 10 of the 23 species monitored. Blue Tit, Willow Tit, Sedge Warbler and Reed Bunting have exhibited declines of more than 50% over the last 31 years, while reductions of between 25% and 49% have been observed for Great TitWillow Warbler, Blackcap, Garden WarblerBlackbird and Song Thrush. For species such as Blue Tit, Great Tit and Blackcap, where a concurrent population increase has occurred, reductions in productivity may be at least partly driven by density-dependent processes, whereby increased competition for resources in an expanding population will mean that some pairs occupy poorer quality habitat and reduces the mean breeding success per pair. Alternatively, climate induced mismatch with invertebrate food supplies may be impacting negatively on productivity and/or post-fledging survival, particularly in the case of the caterpillar-dependent tit species. 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, however, increasing evidence that a reduction in the number of offspring produced may be an important driver of Willow Warbler declines (and possibly other migratory species) and may also be preventing recovery of the UK Reed Bunting population.