Two distinct waves of synchronised breeding occurred at a colony of Great Cormorants Phalacrocorax carbo in west Scotland in 2007. The second wave led to young fledging in September, exceptionally late for this species locally. Evidence from elsewhere suggests that this second wave was not double-brooding (raising of two broods in one year by the same pair) but breeding by newly-arrived birds that had failed at a nearby colony earlier in the year.

At a small colony of Great Cormorants Phalacrocorax carbo (hereafter ‘Cormorants’) in Scotland in 2007, there were two distinct waves of successful breeding separated by 2.5 to 3 months. In June, the normal fledging season, 47 young fledged from 18 nests. Then, in September, 28 young fledged from 14 nests in the same small area (details in Appendix 1). A well-synchronised pulse of many young fledging together so late in the year is unusual, at least in this part of Scotland where such an occurrence seems not to have been reported before. The first purpose of this short note is to place this event on record.

Our second objective is to consider whether these two waves were caused by the same pairs breeding for a second time (successful ‘double-brooding’), or by incoming birds nesting in the same small colony area, or possibly a mixture of these. There were no ringing data to resolve this question directly. We therefore approach it indirectly by examining other records of late breeding in this and a closely related species. If it was genuine double-brooding, 14 of the original 18 pairs raised successful second broods. This percentage (78%) will be compared with similar measures from elsewhere.

We are grateful to Tom Callan for first alerting JCAC to the Cormorants breeding at Glas Eilean, for keeping him informed of their progress, and for assistance with ringing. We thank Jens Gregersen for his dedicated effort in observing breeding Cormorants at the Vorsø colony. We thank Robin Sellers, Martin Heubeck, Sarah Wanless and Mike Harris for considerable help and advice during writing.

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