The sensitivity of breeding songbirds to change in seasonal timing is linked to population change but cannot be directly attributed to the effects of trophic asynchrony on productivity. 

Willow Warbler

Author(s): Franks, S.E., Pearce-Higgins, J.W., Atkinson, S., Bell, J.R., Botham, M.S., Brereton, T.M., Harrington, R. & Leech, D.I.

Published: 2 November 2017

Journal: Global Change Biology

Digital Identifier No. (DOI): 10.1111/gcb.13960

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The time that birds decide to breed has a strong impact upon the likely success of raising their young to independence. Pairs of birds will generally time egg laying to maximise the availability of food for their chicks. However, one of the strongest impacts of climate change so far has been to alter the timing of spring. In response to milder temperatures, flowers and insects are all appearing earlier than they used to. Songbirds struggling to match their timing to the changing climate could be facing population declines.

To test this, BTO, in collaboration with The Woodland Trust, Rothamsted Research, Butterfly Conservation and the Centre for Ecology and Hydrology (CEH), have collated some of the most comprehensive long-term data on the timing of spring from across the UK. They have related changes in the timing of leaf and flower emergence as well as aphid, butterfly and moth emergence, to changes in the timing of egg-laying, tracked by the BTO/JNCC Nest Record Scheme. As expected, in warmer springs, birds do tend to breed late relative to the timing of spring. However, by using data from the BTO/JNCC ringing scheme to monitor changes in the number of fledglings produced by each species per year, the study’s authors 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 directly caused by the effect of mismatch on breeding success.

These results significantly increase our understanding of the impacts of climate change upon birds by showing there is not a strong link between changes in the timing of spring and long-term trends in breeding success. The heroic efforts of thousands of volunteers recording information on birds, insects and plants were vital in providing the long-term data required to understand the impacts of climate change upon bird populations, and we thank them for their hard work and dedication.

Abstract

A consequence of climate change has been an advance in the timing of seasonal events. Differences in the rate of advance between trophic levels may result in predators becoming mismatched with prey availability, reducing fitness and potentially driving population declines. Such “trophic asynchrony” is hypothesized to have contributed to recent population declines of long-distance migratory birds in particular. Using spatially extensive survey data from 1983 to 2010 to estimate variation in spring phenology from 280 plant and insect species and the egg-laying phenology of 21 British songbird species, we explored the effects of trophic asynchrony on avian population trends and potential underlying demographic mechanisms. Species which advanced their laying dates least over the last three decades, and were therefore at greatest risk of asynchrony, exhibited the most negative population trends. We expressed asynchrony as the annual variation in bird phenology relative to spring phenology, and related asynchrony to annual avian productivity. In warmer springs, birds were more asynchronous, but productivity was only marginally reduced; long-distance migrants, short-distance migrants and resident bird species all exhibited effects of similar magnitude. Long-term population, but not productivity, declines were greatest among those species whose annual productivity was most greatly reduced by asynchrony. This suggests that population change is not mechanistically driven by the negative effects of asynchrony on productivity. The apparent effects of asynchrony on population trends are therefore either more likely to be strongly expressed via other demographic pathways, or alternatively, are a surrogate for species' sensitivity to other environmental pressures which are the ultimate cause of decline.
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