Museum egg collections as stores of long-term phenological data

Museum collections hold large amounts of data on collecting dates and local ities of eggs collected over the past 150 years. Egg collections hold the l ongest available time series for a wide range of bird species on a large sp atial scale. Using data for two British species I investigate whether egg c ollection data can be used in phenological research. A method is presented allowing laying dates to be estimated from collecting dates. Problems and b iases in the data are highlighted. Both the dipper and song thrush have sta rted laying earlier over the past 150 years. The advance in laying is signi ficantly correlated with mean March temperature.