From long-distance and short-distance migrants: A migratory season at Falsterbo

In my past blog entry I have told you about migratory bottlenecks in general, Falsterbo in particular, methods to monitor migration and the sophisticated radio-telemetry system CanMove has on the Falsterbo Peninsula. So today it is time to write about what I am actually doing there. 

A Robin hanging safely in a mist net.

In the lighthouse garden staff from the Falsterbo Bird Observatory catches and rings migratory birds in a standardised programme. This programme is running every autumn since 1980 with the same method! Every morning before sunrise 21 nets are opened for at least 6 hours. These are so called mist nets. These nets are very thin and put up in the shade of bushes they are hardly visible. When birds fly into them they get entangled and fall in so-called pockets of the net. There they hang safely.  
  

Experienced ringers can take out birds quickly and safe.

Every 30 minutes the net are controlled and experienced bird ringers take birds out of the net. Birds are brought to a little ringing hut, where every bird gets a metal ring with a unique number. They also get aged and measured before they are released again. With help of the unique number, a bird can be identified whenever and wherever it will be found back; be it in Europe, Africa or a later autumn in Falsterbo. This way we can learn a lot about birds and their lives. I am joining regularly during this catching sessions because I take blood samples from some birds. Later in winter I will analyse the blood samples in the lab to study immune function. On a few birds I attach a radio-transmitter. The automated radio-telemetry system in Falsterbo will monitor the presence of a transmitter-bird continuously. That way we can find out how long it stays and when it departs and crossed the sea towards Denmark.     

End of August and the first half of September is the migratory season for all birds that migrate to Africa. These are usually species that eat (almost) exclusively insects, like Swallows and Warblers. Birds that migrate to Africa are called long-distance migrants. They leave so early because they need to leave before insects gets scarce and arriving early in Africa is also an advantage. In the second half of September and in October migration is dominated by species that winter in western and southern Europe. They are called short-distance migrants (even though they might travel several thousands of km as well from northern Europe to the Mediterranean). In mid-September the number of long-distance migrants usually declines steeply and the number of short-distance migrants increases steadily. 

Goldcrest: this tiny bird migrates at night and winters in Europe,
a so called short-distance migrant

On a typical day in mid-September we caught Willow Warblers, Tree Pipits, Garden warblers and Common Redstarts (all going to Africa) as well as Robins, Dunnocks, Chaffinches, Song Thrushes and Goldcrests (all wintering in Europe). The later species increase in numbers and peak in early October, the former ones are declining and by end of September all of those will have left Sweden. 

  

Wood Warbler – a nocturnal long-distance migrant.

In September catching has been going slowly during most days. The reason was the weather, which wasn’t ideal for catching. We had mainly easterly winds for most of September. Winds from east or north mean tailwind for migrants in autumn. Having tailwinds, birds fly high and they are also less hesitant to cross the sea. Thus birds are just flying high and on a broader front across the Baltic with fewer birds ending up in Falsterbo. Luckily for our research, we still got a decent number of birds.

One aim of our transmitter work is to look when birds depart. Will they leave during daytime or at night? Some species migrate exclusively at night, like Robins or Warbler. Others migrate (almost) exclusively during daytime like Raptors, but also many small birds (passerines) like Yellow Wagtails and Finches. For Dunnocks it is highly debated if they are nocturnal or more diurnal migrants. We put radio-transmitter on some Dunnocks and the radio-tagged birds will give us an answer. But you need to be patient, we first need to analyse the tens of thousands of data points we collected.

When birds want to fly, they need energy. Just as you need energy when you hike, ride your bike or go for a run. During migration, when birds fly for long distances they need a lot of energy, they need fuel for the journey. Birds store this fuel in form of fat. They fatten up before migration. As more fat they store, as longer they can fly without eating. Especially when birds have to cross big distances without the possibility to eat, e.g. when crossing sea or dessert, they need to store enough fat beforehand. Without fuel they can’t fly. And birds can get very fat before migration. Some species double their body weight before migration. During the flight they burn the fat. Fat is an ideal energy store for birds, because it contains few water (reducing weight) and thus have a high energy content per g. And keeping an eye on the weight is important for a flying bird. When you catch birds for ringing, you can see how fat an individual is. Ringers usually take the so called “fat score”. On a scale between 0 and 8, the amount of fat is noted. 0 means the bird is very lean, has no fat. 8 is a very fat bird, basically a flying fat ball. Fat scores of 6-8 are most common in birds before they cross a huge ecological barrier, like the Sahara. In Falsterbo most birds have a fat score between 2 and 5; enough to cross the Baltic Sea. See attached picture for a fat and a lean bird. 

A fat bird; lots of yellowish fat on the belly.

A lean bird: no yellowish fat on the belly.

So that’s it for today. Hope you enjoyed reading and might have learned something. More soon.

Arne Hegemann