Are common terns really indeterminate layers? Responses to experimental egg removal

We trapped and identified known-aged female Common Terns (Sterna hirundo) within 24 hours of the onset of laying. First-laid eggs were removed from 36 experimental nests within 24 hours of trapping, and returned approximately two days after the last egg was laid. Controls were matched for laying date. In contrast to previous studies of terns and gulls, only five of the 36 females continued to lay in the same nest-site: 16 were located at new sites after egg removal, 14 were not found and one died. Eleven of the 31 deserted nest sites were taken over by new pairs within nine days. Mean clutch size was greater for experimental birds than for controls, but only by 0.23 eggs, suggesting that fewer than half of the experimental birds laid an additional egg in response to egg removal. Fresh masses of second- and third-laid eggs were more similar in experimental birds than in controls matched for clutch size (P < 0.05). Three females that were marked without trapping also deserted their nests after removal of their first eggs, indicating that desertion was a response to egg removal rather than trapping. This study suggests that while Common Terns may be indeterminate layers, responses to egg removal are more complex than previously acknowledged.