Timing of initiation of egg-laying of first clutches by Corncrakes Crex crex

Female Corncrakes are difficult to study because they are elusive and spend most of their time hidden in tall vegetation. Unlike males, they only vocalise quietly and infrequently. The timing of Corncrake breeding is therefore difficult to study, except by fitting females with tracking tags. We estimated the average date on which egg-laying of first clutches began using records of the appearance of the cloaca and abdomen of untagged adult females captured within about one month of their arrival on the Isle of Coll (Scotland) after their spring migration. The mean date of initiation of egg-laying was 11 May (95% confidence interval, 1 - 20 May), which is consistent with results on the timing of breeding derived from previous studies using radio-tracking, trail camera images and captures of chicks. The information contributes to Corncrake conservation by identifying the period in spring when management of nature reserves and through agri-environment schemes should provide sufficient habitat suitable for the laying of first clutches.