Despite its name, the Grey Wagtail has an eye-catching yellow undertail that flashes as the bird moves along a water margin. BTO describes it as primarily associated with fast-flowing water, but also using slower rivers and urban lakes or ponds. This is particularly useful context for its registered location at the Douro estuary in Porto and Vila Nova de Gaia: a species associated with streams need not be restricted to a remote mountain reach. The relevant cruise opportunity is a shore visit around Afurada, Cabedelo and São Paio, not a claim for the inland Régua–Pinhão sector. Its small size and continuous movement make it a more demanding deck identification than a large heron. A local estuary guide also identifies it among birds attracted to low-tide margins. Species account: https://www.bto.org/learn/about-birds/birdfacts/grey-wagtail

Douro
Porto / Vila Nova de Gaia: Douro estuary reserve, Afurada–Cabedelo–São Paio; NOT a claim for inland Régua–Pinhão Accessible Porto shore visit with paths and hides; waders/gulls concentrated at low tide and migration/winter. Osprey and Short-eared Owl irregular/seasonal; no guaranteed sightings.

Bar-tailed godwit
Black Redstart