UK Butterfly Monitoring Scheme (UKBMS) data has been used in a study led by British Trust for Ornithology (BTO) researcher Dr Samantha Franks. The study, published in the journal Global Change Biology, investigates the potential impact of climate change on songbird populations in the UK.1

The time at which birds choose to breed has a strong impact on how many offspring are successfully produced, which in turn is related to food availability. One of the strongest impacts of climate change so far has been the advancement of spring events such as flowering times and the emergence of insects, due to warmer temperatures.  A number of songbirds are declining in abundance, causing concern amongst conservationists that birds have an inability to match the timing of breeding to the availability of their insect prey.

The BTO, collaborated with Butterfly Conservation, the Centre for Ecology & Hydrology (CEH), and researchers from Rothamsted Research and the Woodland Trust to test this. Comprehensive long-term datasets on the spring phenology of 280 plant and insect species from across the UK, as well as information on egg-laying for 21 songbirds were gathered together.  Much of the data was collected by volunteers.  Specific yearly datasets used for the period 1983–2010 included:

  • first flowering and/or leaf dates from the UK Phenology Network
  • aphid first flight dates from Rothamsted Research suction traps
  • moth data from the Rothamsted light traps network
  • butterfly first appearances on transects from the UK Butterfly Monitoring Scheme (UKBMS). For butterflies, data was available for 54 of the 59 regularly occurring UK species.

The annual phenology measures were used to create a single, averaged annual index of spring phenology which was modelled in relation to the timing of egg-laying, tracked by the BTO/JNCC Nest Record Scheme.

In warmer springs, birds tend to breed later than normal, relative to the timing of spring. The species most sensitive to changes in timing, including Willow Warbler and Linnet were shown to be suffering the greatest population declines. However, by looking at the number of fledglings produced by each species per year, the study found no evidence that these mismatched species were also suffering a long-term decline in breeding success.  This suggests that the population declines observed in many British songbirds are not, as many people had thought, directly caused by the effect of temporal mismatch on nesting success.

The study concluded that breeding later than normal in warmer springs may be causing other problems such as poorer survival in either adults or fledglings. Alternatively, the species which breed late in warm springs are constrained by other environmental pressures which are driving their population decline. Further research is needed to understand the ongoing causes of the decline in songbirds and the food sources on which they depend.

The research has demonstrated the immense value of biological data collected by volunteers in helping us to understand the impacts of climate change on biodiversity.

Professor Tom Brereton
Associate Director Monitoring and Research

References

1. Franks, S E, Pearce-Higgins, J W, Atkinson, S, Bell, J R, Botham, M S, Brereton, T M, Harrington, R, and Leech, D I. 2017. The sensitivity of breeding songbirds to changes in seasonal timing is linked to population change but cannot be directly attributed to the effects of trophic asynchrony on productivity. Global Change Biology 24(3):957-971. https://doi.org/10.1111/gcb.13960