In 1967 Eric Eastwood published a book called Radar Ornithology. Whilst research has moved on since then, this is a seminal book and gives really useful hard information on the migration habits of Passerines including thrushes, continental robins, warblers, starlings and woodcock.
These birds make their migratory journeys to England in the autumn from the continent with some from north Sweden and Norway often after having summered in the high arctic. In the spring they reverse the journey going back to breed. They fly in straight tracks over sea and land and do not deviate as they fly. Birds may use the sun when it is available, but then why do Passerines migrate at night? We can see that dusk triggers the migration of these Passerines. Radar seems to show that these birds do in fact travel in groups.
Birds are very seldom disorientated and only when there is a simultaneous presence of total cloud or overcast, wind and rain. In these circumstances the birds tend to fly circular course waiting for the weather to improve in order to continue on their way. Birds seem to need gaps in the clouds or find space between cloud layers to continue their journeys
Birds look as if they use temperature drops in autumn and rises in spring to initiate migration but they also pay attention to the weather and do not tend to fly in the rain. They tend to fly when rain ceases and the sky clears with following winds of low speed. Observed nocturnal flight activity off the Essex coast in the spring is closely linked to the weather conditions. The birds seem to like warm, spring nights with a light westerly wind, not exceeding 5 knots combined with clear skies and moon near full. As the migration season develops the birds get less picky about the conditions to migrate, just as you would expect.
It is important to note that not all birds make this same journey and some birds such a swallows migrate in the autumn to South Africa crossing the Sahara to get there and back.
Birds tend to fly low when they encounter head winds and fly high when the wind is favourable. This is a good strategy as the wind speed is lower at low altitudes due to friction with the sea surface.
It has been observed that the autumn migrant birds arriving over the Shetlands do a dawn ascent and gain altitude probably to reorientate, this often leads to drastic reorientation. For example during the latter stages of the night (still out at sea and thus nowhere to land) passerine migrants gradually loose altitude on (for example) a W heading towards America?. Then as dawn breaks they change their headings dramatically and gain height . They change to (say SE towards NE Scotland or NW back towards Norway). Birds from other regions seem to carry out a dawn ascent also. This perhaps is triggered by encountering the Sun at dawn.
The average height birds fly is about 2,500 ft but some fly as high as 7000ft. Passerines tends to fly at about 20-25 knots. As expected they seem to travel faster in the spring as the urge to mate drives them to go as fast as they can to get the best spots and mates.
Bird density is much higher along the migratory highway of the Southern North Sea. It is interesting to see that the birds criss-cross these “flyways “ throughout the year and there are always birds “going the wrong way”. There are less birds off the east coast of Scotland as this is far off from the bird highways.
Activity over the North Sea in the spring had the lowest activity at around 14.00 hours but highest at 22.00 hours with simple curve joining these times. The spring activity comes from mass departures from East Anglia of the winter visitors to Denmark, Holland and Belgium – a sea crossing of 100-500 miles depending on the destination with crossing times of between 2 and 10 hours. In autumn the graph is not so clear and it looks as if the birds leave at the late afternoon and peak before midnight in order to arrive off the coast of England in the morning.
It is important to realise that there is some migrations going on in the North sea all the time even if there are the spring and autumn migratory peaks. Birds often fly in suitable directions to avoid bad weather or look for food. However, reversed migratory flights are made with the same precision as the more normal track. This makes sense if they are following imprinted flyways and taking a reciprocal course is very normal.
Radar observations showed that birds normally travel on straight tracks both by day and night no matter what he wind state and that a completely overcast conditions. A proposal that birds used an inertial navigation system was proposed which is now thought to be conducted via the Pineal Gland.
Summary by Richard Nissen with the invaluable help of Ted Gerrard, this year’s winner of the Grocott Prize for the best article on animal navigation for Navigation News, the journal of the Royal Institute of Navigation.