Climate change and egg-laying dates

by Humphrey Q P Crick

Global warming is a fact: over the last 150 years, average temperatures on the earth have risen by 0.6°C. The last two decades of the millennium are the two warmest since recording began in the 1850s. One of the key questions that has arisen is whether these apparently small changes have already had an impact on wildlife.

The Nest Record Scheme of the British Trust for Ornithology (BTO) holds more than a million detailed records of individual nesting attempts for a large number of species, gathered over the whole country and collected by volunteer ornithologists from the 1930s onwards (Crick & Baillie 1995). It has provided some of the strongest evidence to date of the broad-scale impact of climate change on wildlife. The scheme is part-funded under the BTO/JNCC Partnership.

Using this dataset, Crick et al. (1997) demonstrated that 51 of 65 species tended to nest earlier during the period from 1971-1995. Twenty (31%) of the 65 species showed statistically significant trends towards earlier laying over the past 25 years. The average advancement for these 20 species was 8d, ranging from 4 to 18 d.

The species showing significant trends showed no common taxonomic or ecological groups, suggesting that a wider factor, such as climate change was likely to be causative:

The species covered a wide range of early to late season nesters which suggests that the recorded trends were not due to changes in observer behaviour. The only significant trend towards later laying was for Columba oenas which nests opportunistically throughout the year and may therefore be a special case.

This work has since been followed this up by looking at a longer run of 57 years (1939-1995) of data and seeing how the changes are related to temperature and rainfall for 36 well-recorded species. A very clear pattern has emerged: most species (19, or 53%) showing a pattern of early laying dates in the 1940s and 1950s, later laying dates in the 1960s and 1970s and then earlier laying dates in the 1980s and 1990s. This pattern matches well with changes in average spring temperatures: these have become cooler and then warmer over the last half century. An example is provided below, which shows how weather has affected the laying dates of Chaffinch:

Figure legend:

Laying date is numbered such that day 110 is 20 April, 121 is 1 May and so on, and the smoothed lines are calculated using a LOWESS (Locally Weighted Scatterplot Smoother) method.

In fact, when we analyse the data statistically, we found that for seven of the most widespread species, the trends in laying dates can be accounted for purely by the trends in temperatures and for 10 others, temperature and rainfall account for much of the changes. Overall, these results from the Nest Record Scheme constitute one of the strongest pieces of evidence, to date, for the widespread effect of climate change on wildlife (Crick & Sparks 1999).

We don't know yet what will be the implications of these changes for birds. They may be beneficial because early fledglings often survive better than later nestlings. However, if birds become out of synchrony with their food supply or if they get hit by bad weather later on the spring, then the early nesters will suffer, perhaps decreasing overall productivity. There is already some good evidence from intensive studies (Perrins 1991, Visser et al. 1998) that caterpillars speed up their development in warmer springs, while birds are unable to lay early enough or to shorten their incubation periods such that later nestlings and fledglings are faced with food shortages. In very early years, early-fledged chicks have a greater chance of being recruited into the breeding population in the following year. The general importance and significance of such findings urgently needs further research by investigation of the BTO's long-term datasets.

Government Climate Change Indicators

In June 1999, the Minister of State for the Environment, the Rt Hon. Michael Meacher, announced a new set of Government Indicators of Climate Change in the UK (Cannell et al. 1999). The aim is to help raise awareness of how our climate is changing and how it is changing both the manmade and natural worlds in which we live. The evidence for climate change is mounting and it is beginning to become obvious even to us, here in the UK. In order to track these changes as they happen across the UK, the Government has gathered together a first set of indicators that are influenced by climate.

There are three bird indicators among those that have been chosen. The first is from the BTO's Nest Records Scheme: the laying dates of Chaffinch and Robin; both of which are strongly related to spring temperatures (http://www.nbu.ac.uk/iccuk/indicators/30.htm). The second is the population change of the Wren, as measured by the BTO's Common Birds Census (http://www.nbu.ac.uk/iccuk/indicators/31.htm). Wren numbers fluctuate widely, mainly due to high mortality in severe winters. Small bodied birds are particularly prone to prolonged spells of cold, wet, snowy or frosty weather because they cannot obtain enough food to maintain body temperatures. Big population declines have occurred after severe winters such as 1962/63, 1978/79 and so on. It is expected that wren populations will not increase indefinitely if winters become progressively milder, but they will tend to stabilise at a relatively high level. The third bird indicator is the arrival date of Swallows, as measured by the daily records of birds seen at British coastal Bird Observatories (http://www.nbu.ac.uk/iccuk/indicators/29.htm). Tim Sparks and Dick Loxton have extracted and analysed information from four Observatories (Dungeness, Portland, Bardsey and Calf of Man) to show that arrival dates have become earlier with our warmer springs. It is very important that BTO data has been chosen to be used in such Government statistics. It is data from such long-running schemes that are going to be vital in monitoring the impacts of climate change as it develops over the next few decades and centuries.

References

Cannell, M.G.R., Palutikof, J.P. & Sparks, T.H. (Eds) (1999) Indicators of Climate Change in the UK. DETR & NERC, London. (http://www.nbu.ac.uk/iccuk/)

Crick, H.Q.P. & Baillie, S.R. 1996. A Review of the BTO's Nest Record Scheme. BTO Research Report no. 159. BTO, Thetford.

Crick, H.Q.P., Dudley, C., Glue, D.E. & Thomson, D.L. (1997) UK birds are laying eggs earlier. Nature 388: 526.

Crick, H.Q.P. & Sparks, T.H. (1999) Climate change related to egg-laying trends. Nature 399: 423-424.

Parker, D.E., Legg, T.P. & Folland, C.K. (1992) A new daily central England temperature series, 1772-1991. Int. J. Climatol. 12: 317-342.

Perrins, C.M. (1991) Tits and their caterpillar food supply. Ibis 133 (suppl.): 49-54.

Visser, M.E., van Noordwijk, A.J., Tinbergen, J.M. & Lessells, C.M. (1998) Warmer springs lead to mistimed reproduction in great tits (Parus major). Proc. R. Soc. Lond. B. 265: 1867-1870.

This advice page was prepared as part of work funded by a partnership of the BTO and Joint Nature Conservation Committee (on behalf of Natural England, Scottish Natural Heritage, the Countryside Council for Wales, and also on behalf of the Environment & Heritage Service in Northern Ireland).

Back to Information for Conservationists index