© Susana Simião
In the swimming images, the uniform dark brown colour and greyish bill with dark tip are visible, as is the bird's smaller size than Cory's Shearwater.
I work as a marine biologist and nature guide for a whale-watching company, Espaço Talassa, which is based at Lajes do Pico, Azores. On one of our whale-watching tours, on 7 August 2020, late in the afternoon, one of our crew members noticed an anomalous-coloured shearwater sitting in the water, in a raft of Cory's Shearwaters. They were close to a group of Atlantic Spotted Dolphins that we were watching, but the birds were not interacting directly with the dolphins.
The dark-coloured shearwaters or petrels that we regularly see in this area are Bulwer's Petrel and Sooty Shearwater. However, the bird was larger than a Bulwer's and showed a completely different bill and leg/foot colour to Sooty.
We approached the birds to try and identify what it was. It would let us approach, while the surrounding Cory's Shearwaters flew away, and we noted how when it flew it landed shortly after taking off, not far away (this being unusual behaviour for Cory's). At first we thought it could be a melanistic Cory's Shearwater, but it was noticeably smaller in size.
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