You would be forgiven for thinking that autumn migration is just a mirror image of spring migration; swallows and house martins in in May and out in October. In reality this is far from the case, the two seasons and related bird movements are very different. There are various reasons for this, some simple and some complex.
Whilst weather conditions play a major part it is far from the complete story and if you think about it for a little while you will realise that the motivation of the moving birds is different between spring and autumn. In spring they have an overwhelming need to get to their breeding territories and start the process of rearing young so they are in no mood to stop for long when reaching land in Dorset after crossing the Channel as they continue their relentless journey northwards. In autumn the situation is very different and more relaxed. They have young ones to think about on the return journey and so do the trip in easy stages stopping off in Dorset for a good meal before venturing out over the open water. Often, numbers will build during daylight hours and then they will be gone the following morning having set off at first light. Birds do not only migrate by day but also by night and new advances in sound recording technology are beginning to reveal more about this.
Even just a quick look at the reported sighting from May compared to October will show just how marked the difference is in visible observations of birds on the move. Whilst the spring list is fairly predictable the autumn list contains some surprises. Some species you do not think of as migratory are and, for example, large numbers of wood pigeons move in late October with over 100,000 being counted over Poole Harbour in just one day. Other common species like robin, blackbird and chaffinch are also on the move but their place is taken by a compensating influx from Europe and so we do not notice the difference. This year, for example, a robin was netted in Dorset that was last seen and ringed in the Netherlands.
The other noticeable feature of autumn migration is how birds of the same species often leave pretty much at the same time and in large flocks whereas in spring they may have set off from, say, central Africa at the same time but they have become strung out and more widely distributed by the time they get to our shores; many perish on the journey. In spring swallows can be seen coming in in hundreds on some days at places like Durlston but can be seen going out in their thousands in the autumn.
The October list also shows some other surprising features too. The hawfinch is a very rare species in Dorset during the breeding months but this autumn for a couple of weeks reports of hawfinches were coming from various locations along the Dorset coast. The same is true for the diminutive firecrest. The yellow-browed warbler is not even a UK breeding species being found mainly in Asia but still a number of sightings can be guaranteed in Dorset each the autumn.
The weather, in particular the wind strength and direction, will have a big influence on bird migration and is often the reason some real oddities that turn up here with almost bizarre records this year of barred greenish warbler, Radde’s warbler, barred warbler, pallid harrier (only the second record for Dorset), pectoral sandpiper, barnacle geese, rustic bunting, whooper swan and caspian gull.
Of September’s sandpipers the spotted sandpiper remained at Abbotsbury throughout October and the stilt sandpiper was seen at various locations in Poole harbour. They were joined by another north American plover, a lesser yellow-legs that spent much of October at Lodmoor in Weymouth.
With all the bird migration going on it easy to forget that insects migrate too and large numbers of red admiral arrived in October along with a number of clouded yellow butterflies. Convolvulus, silver -striped and death’s head hawkmoths were the stars of the incoming moths and a migrant vagrant emperor dragonfly is also worthy of note.