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Since 1939, the nest record scheme of the British Trust for Ornithology has gathered more than a million records on the breeding performance of 225 species of birds in the United Kingdom. A network of 1,000 volunteer ornithologists provided details of nest site, habitat, contents at each visit and evidence for success or failure of each nest found3. Only a proportion of this information is available on computer databases and is sufficiently detailed to provide information on egg-laying dates.

We have now analysed 74,258 records from 65 species to investigate trends in the distributions of laying dates of the first egg in each clutch over the 25-year period from 1971 to 1995. We found significant trends towards earlier laying dates for 20 species (31%) with only one species laying significantly later. The shift towards earlier laying for the 20 species averaged 8.8 days, ranging from 4 to 17 days, s.d. = 3.4 (Fig. 1). The analyses of all species also show a tendency towards earlier laying (Fig. 2).

Figure 1: Temporal changes in laying dates for early-, mid- and late-season nesters.
figure 1

a, Miliaria calandra, F1,23.5 = 5.30, P = 0.030. b,Phylloscopus collybita,F1,27.1 = 23.00, P = 0.0001. c,Pica pica, F1,18.9 = 61.89, P = 0.0001. Laying date is numbered such that day 60 is 1 March, 121 is 1 May and so on. Points show annual means ± s.e.m. We analysed between 150 and 5,700 records per species. Laying dates were estimated with an accuracy of at least ± 5 days (ref. 3). For each species we selected the most significant of three mixed linear models with Year (for example, a and b), Year2 (c), or Year and Year2 combined, fitted as continuous fixed effects; Year was also fitted as a categorical random effect to contend with the non-independence of observations within years.

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Figure 2: Frequency distribution of P values of temporal trends in laying dates for 65 species of UK birds from 1971 to 1995.
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Significant trends are shown in black, non-significant in grey. Trends towards earlier laying were found in 51 species and towards later laying in 14 species, a significant difference at P = 0.000005 (binomial test).

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Species showing significant trends were not confined to any one ecological or taxonomic type, and comprised water birds (Haematopus ostralegus, Numenius arquata, Tringa totanus), resident insectivores (Cinclus cinclus, Troglodytes troglodytes,Aegithalos caudatus, Sitta europaea, Sturnus vulgaris), migrant insectivores (Anthus trivialis, Phoenicurus phoenicurus, Sylvia communis, S. atricapilla, Phylloscopus sibilatrix, P. collybita, P. trochilus), corvids (Pica pica, Corvus corone) and seed-eaters (Fringilla coelebs, Carduelis chloris, Miliaria calandra).

The species also covered a wide range of nesting times, from early to late season (Fig. 1), which suggests that the recorded trends were not due to changes in the behaviour of observers. The only significant trend towards later laying was for the stock dove (Columba oenas), which nests opportunistically throughout the year and may therefore be a special case.

Trends towards earlier laying times are expected if ambient temperatures rise earlier in the year. There are fitness benefits to nesting early, and food availibility is often the important determinant of laying date4. Average flowering and leafing dates may be advanced by high spring temperatures5,6 and these are likely to have pronounced effects on the availability of the arthropod food supplies for birds. There is evidence that two species of waders in The Netherlands nest earlier in warmer springs7. Amphibians in Britain have also been shown to spawn earlier in recent years as spring temperatures have risen8.

The tendency to nest or reproduce earlier may be a more general phenomenon for wildlife in Britain. This could have considerable consequences for their ecology and conservation. For birds, earlier nesting could be beneficial if juvenile survival is enhanced by a prolonged period before winter. Conversely, birds may be adversely affected if they become unsynchronized with the phenology of their food supplies.