Spring Migration: take your seats for one of the greatest shows on earth

It was painful to watch. It was like seeing a gang beat up an innocent passer-by – except that it was happening 200 feet up in the air. The victim was an eagle. The assailants were seagulls – yellow-legged gulls, the Mediterranean version of our familiar herring gull – and they were merciless.

Dozens of them converged on the hapless predator, harrying it with swoops from all sides, easily outmanoeuvring it, since their victim, a short-toed eagle, was a snake-hunter with wings designed for hanging in the air and patiently searching the ground for reptiles, rather than for rapid flight. The late afternoon sun picked out its pale underside as it struggled to get away, on leaden wingbeats.

"They get hammered by the gulls, more than any other species," said Ian Thompson, as we gazed through binoculars at the drama unfolding above us.

The gulls renewed their attack as the eagle lurched and tottered through the air out of our sight. I wondered what would happen to it. "They get exhausted and downed, and sometimes they die," said Ian.

Here was the Rock. We were on the heights of Gibraltar, looking out over the strait to Morocco and Africa 15 miles away, observing, live, one of nature's most remarkable spectacles – the annual spring migration of millions of birds from their African winter quarters to their European breeding grounds.

More than 120 species, from tiny warblers to vultures with door-sized wingspans, fly north every year because the breeding opportunities are better – they can escape the competition in Africa, and take advantage of the much longer summer days in the high latitudes, which offer more time for finding food for hungry chicks.

Yet there is widespread concern among ornithologists that the system of bird migration between Africa and Europe is running into trouble with numbers of bird returning to their breeding grounds falling and the spectre of several species heading towards disappearance. These epic journeys are immense and full of hazard: 2,000 miles to northern Europe, crossing the Sahara, the world's biggest desert, the Mediterranean, the Pyrenees, and then the Channel, to get to Britain, say, facing bad weather, food shortages, natural predators and human hunters.

I had come as a wildlife tourist because the Rock is one of the best of all places to observe these vast movements, both in spring on the outward passage, and in autumn on the return. You might say it's Gibraltar's second, almost secret, identity, much less well known than the familiar one of Little Bit of Britain Stuck on the End of Spain. Long the UK's most important foreign naval base, guarding the entrance to the Mediterranean (a function now much reduced), Gib has been a British territory since 1704, and crowds of tourists flock ashore every day from cruise ships to look with curious eyes at the bobbies in their blue helmets, the double-decker buses, the English road signs, the English pubs and the English shops. Some also take a taxi to see the famous "apes" – not true apes, but large, tailless macaques that live semi-wild on the Upper Rock, protected by the Gibraltarian authorities.

But not that many people go to Gibraltar, as I had done, looking for wildlife. If you do, at the right time, you may be pleasantly surprised. The Rock is, in effect, another Mediterranean island.

The town at its base is noisy, crowded and packed with cars, its appearance not improved by the hectic commercial and residential development of the past 20 years, much of it on land reclaimed from the harbour. But the whole of the Upper Rock, the higher reaches around and below the 1,400ft summit, form a nature reserve of maquis, or Mediterranean scrub, mainly composed of wild olive and lentisk.

In mid-April, when I visited, this is bursting with exuberantly coloured wild flowers, from the giant Tangier fennel and the Gibraltar candytuft to the wild gladiolus, which in Britain is a great rarity found only in the New Forest. The butterflies, too, are eye-catching: almost at once I spotted a Cleopatra and a Spanish festoon, which are two of Europe's great showpiece insects.

But the birds are the real interest. Gibraltar attracts them magnetically because its strait is the Mediterranean's shortest crossing point, the narrowest hop over the water between Africa and Europe. For one group in particular, this is absolutely vital: the soaring birds. Many raptors such as eagles, kites and vultures, and other birds such as storks, are not strongly powered flyers; they have evolved wings which they use to soar on thermals, the currents of air which rise upwards from the land as it warms in the sun. They travel by soaring up on one thermal, and gliding slowly down until they find another.

But thermals do not form over the sea, so to get over the Mediterranean, migrating soaring birds have to use the narrowest crossing, where one enormous rise on a thermal in Morocco will let them glide across to Spain, or vice versa. As a result, they congregate along the straits, twice a year, sometimes in quite enormous numbers. "One day in September, back in the 1970s, from here on the Rock I counted 11,000 honey buzzards, heading south from all directions. I'll never forget it," said John Cortes, secretary of the very active Gibraltar Ornithological and Natural History Society, or Gonhs (pronounced "Gonze").

Gonhs has a field station and bird observatory at Jews' Gate, the entrance to the Upper Rock, and it was there that I met Ian Thompson, a retired BT manager from Hatfield in Hertfordshire, who for the past six springs has come out to Gib to be the society's bird ringer.

Ringing means trapping the migrating songbirds heading for Europe, such as warblers and flycatchers, and fitting them with small lightweight leg rings with an address and unique number, which allow their movements to be traced if they are caught again, or found dead: it has provided a wealth of knowledge of bird movements, essential for conservation policies.

I watched Ian and his friend and fellow ringer Yvonne Benting catch more than 20 songbirds in mist nets in the maquis, and then, handling them with great skill and gentleness, measure them, weigh them, ring them and release them – all completely unharmed. Seeing at really close quarters such English summer visitors as whitethroats and blackcaps, willow warblers and chiffchaffs – usually glimpsed back home as a brief blur in a hedge or high in a tree – was a thrill for anyone interested in birds.

But the large birds of prey passing through were an even bigger attraction, and although Ian and I watched spellbound as the short-toed eagle was attacked by the gulls above us – there are 20,000 pairs of yellow-legged gulls on Gibraltar – the wind was in the wrong direction for a major passage over the Rock. It meant the birds were crossing the straits at the narrowest point of all, at Tarifa in Spain, a few miles along the coast, and there we headed.

Tarifa is a both a charming old town and a hip windsurfing centre, because it is not only continental Europe's most southerly point, but reputedly its windiest. In the hills a few miles outside the town we found a cafe at an observation point overlooking a valley leading up from the straits, and there at last was the great procession: short-toed eagles, booted eagles, black kites, griffon vultures, Egyptian vultures, soaring past us on the thermals in a magnificent uninterrupted stream, heading out into Europe for the breeding season.

In Tarifa we found another birdwatchers' treat that windsurfers no doubt pass by: the castle (of Guzman the Good) is a breeding site for lesser kestrels, one of Europe's rarest falcons, charming small birds of prey which nest colonially.

Back in Gibraltar Ian Thompson gave me another insider's tip: one of the best birding sites is the old North Front Cemetery. A short walk among the graves produced whitethroats and redstarts, and, an exotic rarity for the Rock, a squacco heron.

My final call was at the Alameda Botanic Gardens in the town, largely unsung, but to me the single most attractive feature of all of Gibraltar. The director is John Cortes of Gonhs, and in the past 15 years he and his team have restored the gardens and turned them into an oasis of stunning beauty, resplendent not only in its trees and flowers, but also in its butterflies – the two-tailed pasha, Europe's most spectacular insect, can be found there.

Its birds can be eye-catching, too. Ambling through its glades I saw blackcaps and woodchat shrikes, and also pied flycatchers, the latter perhaps heading for the oakwoods of Wales. It was a reminder that there is much more to the Bit of Britain Stuck on the End of Spain than bobbies, double-deckers, and English pubs.

HOW TO GET THERE

Michael McCarthy flew to Gibraltar with British Airways (0844 493 0787; ba.com), which offers return flights from £138. He stayed at the four-star O'Callaghan Eliott Hotel (00 350 70500; ocallaghan hotels.com) which offers b&b from £160 per room per night.

FURTHER INFORMATION

Gibraltar Tourist Board (020-7836 0777; visitgibraltar.gi).