30. Egg-laying Dates of Birds
HQP Crick

The dates on which at least 5% of Robins or Chaffinches have started egg-laying each year shown in relation to mean temperatures in March in Central England. Day 91= 1 April

Specific Indicator
The British Trust for Ornithology has gathered more than a million records on the breeding performance of 225 species of birds using a network of about 1000 volunteers across the UK. A recent analysis of 65 species showed that 20 species have tended to lay their eggs 4-17 days earlier in the year over the past 25 years. The specific indicators chosen are the earliest egg-laying dates of the Robin (Erithacus rubecula) and Chaffinch (Fringilla coelebs), with about 80% of the records coming from England. The figures presented are the dates by which 5% of the birds have begun laying eggs (the 5th percentile of the distribution).
[Source: Nest Record Scheme, British Trust for Ornithology, The Nunnery, Thetford, Norfolk IP24 2PU.]

Relevance
Laying date is a crucial factor in determining the nesting success and productivity of birds. For many species, average clutch size decreases as the season progresses. Also, the chances of young surviving to the next year declines as the date on which they are reared becomes later. Birds have evolved to lay their eggs so that the nestlings are hatched when food is available and there is food when they leave the nest. If food supplies respond to changes in climate differently from birds, then mismatches in the timing of laying could reduce bird populations.

Sensitivity to climatic and other factors
Birds have generally been selected through evolution to start laying as soon as conditions permit. Increasing day length stimulates the development of reproductive behaviour and then the availability of sufficient food to form eggs is a necessary pre-requisite. However, fine-tuning is provided by birds= responses to weather, particularly temperature and to a lesser extent rainfall, which affect feeding conditions and the ability of birds to begin reproduction.

Annual variation in laying dates is often very strongly related to variation in spring temperatures: warmer temperatures lead to earlier egg-laying. The earliest laying dates of Robins and Chaffinches are closely related to mean temperatures in March in Central England.

Change over time
Robins lay about 20 days earlier than Chaffinches, on average, and the earliest 5% has varied each year between 11 March and 17 April for Robins, compared with between 2 April and 2 May for Chaffinch. Earlier laying occurs in years with warm March temperatures: for Chaffinch every degree of extra warmth leads to 2.4 days advancement in laying, and for Robin it leads to 3.1 days advancement.

Overall, there has been a tendency for March temperatures to become warmer in recent years and this is reflected in consistently earlier laying by both species. Years with particularly warm March temperatures occurred in 1957, 1959, 1961, 1981, 1990 and 1997-2000, often with correspondingly early laying by both species. Years with cold March temperatures, for example 1955, 1958, 1962 and 1969 were associated with later laying.