Reference: Meyer, S K, Spaar, R, & Bruderer, B, To cross the sea or to follow the coast? Flight directions and behaviour of migrating raptors approaching the Mediterranean Sea in autumn, Behaviour 137: 379-399 (2000).

• Study of flight behaviour of raptors using radar beams at a location near Malaga, Spain, at a site from which the African coast, 150 km away, was out of sight.

• Raptors had to decide whether to directly cross the sea to the south or follow the coast westwards to the short crossing at Gibraltar/Tarifa.

• No Black Kites and only a few Honey Buzzards crossed the sea, the vast majority following the coastline from east-west. Soaring-gliding was the main mode of flight of these birds.

• The minority of sea-crossing Honey Buzzards (moving S/SW) occurred towards the end of the migratory season and were probably juveniles which mainly migrate singly and have no experience about the migratory flight route.

• Under good thermal conditions, birds following the coast migrated in soaring-gliding flight which consumes much less energy compared to flapping-gliding flight across the sea.

• For the average Honey Buzzard, flying 250 km along the coast in soaring-gliding flight consumed an estimated 4.1g of fat compared to 15.8g of fat in a direct 150 km sea crossing using flapping-gliding flight. However, the duration of the flights was an estimated 6h 52m along the coast and only 3h 51m across the sea. So most birds opted to conserve energy at the expense of time.

• Birds in bad shape, for example those with low fat reserves, should follow the coast in order to minimise the risk of starvation, to feed whenever possible and to use economical soaring-gliding flight.

• Large proportions of harriers, falcons and ospreys crossed the sea. Flapping-gliding was extensively used by these birds.

• The difference in energy consumption between soaring-gliding and flapping-gliding is less for these birds so they often choose to save time with a direct crossing.

• In strong opposing winds (4m/s and above) sea crossings were not usually attempted.

• Median flight altitude for Honey Buzzard was 862m, for Black Kite 636m, for harriers 910m, for falcons 949m and for Osprey 951m.

• In soaring-gliding, mean cross-country airspeed was 10.4 m/s (metres/second, 1m/s = 3.6km/hour) for Honey Buzzard, 9.9 for Black Kite, 9.3 for harriers, 9.5 for falcons and 11.3 for Osprey.

• In horizontal flapping-gliding flight, mean ground-speed was 13.8 m/s for Honey Buzzard, 10.9 for Black Kite, 11.6 for harriers, 12.2 for falcons and 14.6 for Osprey.

• Raptors crossing the sea in flapping/gliding flight increased airspeeds with side winds to reduce drift.

  • However, unexpectedly, they did not decrease airspeed with increasing tail wind support indicating that they wished to minimise the flight time over the sea.

  • In general the increase in airspeeds across the sea was in the range of 1-5m/s.

• The full paper contains full statistics to quantify many of the above statements.

• Honey Buzzards were clearly reluctant to cross the open sea, preferring the energy-minimising route of following the coast to the short crossing at Gibraltar/Tarifa.

• The few that did cross the open sea were thought to be juveniles who would not be aware of the short sea crossing available further west.

• The juveniles that did cross the open sea moved in a S/SW direction showing their strong sense of direction.

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