ARE COMMON TERNS REALLY INDETERMINATE LAYERS - RESPONSES TO EXPERIMENTAL EGG REMOVAL

We trapped and identified known-aged female Common Terns (Sterna hirun do) within 24 hours of the onset of laying. First-laid eggs were remov ed from 36 experimental nests within 24 hours of trapping, and returne d approximately two days after the last egg was laid. Controls were ma tched for laying date. In contrast to previous studies of terns and gu lls, only five of the 36 females continued to lay in the same nest-sit e: 16 were located at new sites after egg removal, 14 were nut found a nd one died. Eleven of the 31 deserted nest sites were taken over by n ew pairs within nine days. Mean clutch size was greater for experiment al birds than for controls, but only by 0.23 eggs, suggesting that few er than half of the experimental birds laid an additional egg in respo nse to egg removal. Fresh masses of second-anti third-laid eggs were m ore 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 tha t desertion was a response to egg removal rather than trapping. This s tudy suggests that while Common Terns may be indeterminate layers, res ponses to egg removal are more complex than previously acknowledged.