Gibraltar, and the narrowest gap between two continents

Large soaring birds, eagles, vultures and storks among them, rely on rising columns of warm air, thermals, to gain height cheaply as they migrate; thermals form reliably over land and barely at all over open sea, which leaves birds that depend on them reluctant to fly far out over water if they can possibly avoid it. Between Europe and Africa, most of the Mediterranean offers no way around that problem. The Strait of Gibraltar does: at its narrowest point, a little over eight miles separate the two continents, close enough that a migrating bird can cross on the wing without ever losing sight of land on either side.

The result is one of the more reliable wildlife spectacles anywhere on a British cruise itinerary, and one that requires nothing more than looking up. Migration bottlenecks of this kind are rare enough globally that ornithologists tend to know them by name, the Bosphorus and the isthmus of Panama among the handful of comparable sites; the Strait of Gibraltar belongs in that company, on a considerably smaller and more accessible scale for anyone happening to be passing through by ship.

A bottleneck for hundreds of thousands of birds

Twice a year, in spring and again in autumn, huge numbers of raptors and storks funnel through this narrow gap on their way between African wintering grounds and European breeding territory, or back again. Honey buzzards, short-toed eagles, griffon vultures and white storks are among the regular passage species, sometimes moving through in flocks large enough to be visible from a considerable distance, riding the same thermals that rise from the Rock and the Spanish and Moroccan coastlines on either side of the water. Europa Point, at Gibraltar's southern tip, is one of the best-known watch points in Europe for the spectacle, and ships passing through the Strait at the right time of year get a version of the same view from the water instead.

Dolphins in the bay, most days of the year

The Strait's narrowness affects more than birds. Strong currents and a relatively shallow sill where the Atlantic meets the Mediterranean push nutrient-rich water up towards the surface, supporting a food chain that keeps common and striped dolphins present in the Bay of Gibraltar for most of the year, reliably enough that dolphin-watching trips run from the harbour on a near-daily basis. Larger cetaceans, including fin whales and occasionally sperm whales, pass through the Strait on their own migrations between the Atlantic and the Mediterranean, though sightings of these are considerably less predictable than the dolphins, which turn up often enough that regular boat trips from Gibraltar's marina treat a sighting as the expectation rather than a stroke of luck.

Watching from the rail

None of this requires a dedicated wildlife excursion. Several cruises calling at Gibraltar or simply passing through the Strait on their way between the Atlantic and the Mediterranean give passengers a reasonable chance of dolphins from the ship's own rail, and, in season, a raptor or two riding the same air currents as the ship itself threads the gap. Fred. Olsen's Discovering Italy with the Amalfi Coast transits the Strait early in its itinerary, before the ship turns east towards the rest of the Mediterranean, while Ambassador's Mediterranean Mosaic, a longer round trip from Tilbury, calls at Gibraltar itself on its way home from Barcelona.

It is easy to treat the Strait as simply the gateway to somewhere else, the narrow bit of water a ship has to get through before the real itinerary begins. For the animals using the same gap for rather more urgent reasons, it has always been the point. Time a passage through it for the right week in spring or autumn, keep half an eye on the sky rather than only on the water, and the crossing itself becomes as much a part of the trip as whatever port comes next.