Mr and Mrs Borin are on their way back

An explosion in colors

An explosion in colors

It is beginning of May and the air is full of chirping, lovemaking sounds of House Sparrows. And the fluty tunes of newly arrived long-distance migrants! The marked seasonality is really fantastic at our latitude. A few weeks ago we had some late snow, and everyone was desperately longing for spring. And now it is here – slippery and fast as quicksilver! In a few days most trees will have unfolded their leaves, the insect life started to boom and all other animals just try to follow as fast they can.

For the birds, timing is everything, and they just have to get it right. If you are a migrant bird and arrive a bit too late, your chances of getting laid, and thereby producing offspring that survives, are slim. Either the earlier fellows already got the best territories and you are stuck with a shitty little shrub, or, you and your partner started breeding so late that your offspring were born mismatched in relation to the peak of surplus food; simply, the caterpillars and all other juicy insect stuff exploded in abundance while your chicks were still in the eggs.

There is a strict phenology to bird migration, each species adhering to its own itinerary. Some species are always first, some, like the Chiff-chaff, even start to appear when winter is still dominating, and spring is just almoooost is getting there; others are always late to arrive, when you think that spring really is summer. The late ones include some of the species that have travelled the longest, like the Marsh Warbler, the Icterine Warbler, the Swift and others. Species that spend the winter in Sub-Saharan Africa.

One of my favorites – the Garden Warbler – is among those late stragglers. It is not a stunning beauty; no red breast or fancy orbital ring, just shifts of grey, grey-brown, and brownish-grey. Somewhat non-descript, perhaps even drab, or boring (the Latin name is Sylvia borin, not boring, mind you). No, it isn’t very beautiful. But it has a fine voice and a rich, melodious song. The Garden Warbler is acquired taste; the first cigarette doesn’t taste good either, nor the first glass of wine, or the first taste of coriander. But Garden Warblers grow on you. They are really stunning creatures, in more ways than one.

One thing I find exceptionally intriguing is their plasticity in weight. There is nothing on this planet as fat as a fat Garden Warbler. The laws of gravity work hard on any animal that wants to sail the winds. The heavier, the more work is needed to stay in the air. Thus there is a balance on how much cargo you can put aboard and still fly, or where costs exceed benefits. Garden Warblers and other migratory birds use fat as fuel.

Early morning at Amurum, Jos Plateau State, Nigeria, April 2003

Early morning at Amurum, Jos Plateau State, Nigeria, April 2003

My friends and I, particularly Dr Ulf Ottosson, have long been interested in Palearctic migrants in Africa; specifically in Nigeria in West Africa, where many of our Swedish migrants either winter or pass through on their way south or north. I did my master thesis project in Nigeria, at Malamfatori in the extreme NE of the country, just up in the northern Sahel zone. Dry and hot most of the year, but exceedingly inhospitable in April at the end of the dry season. But at the same time a place where many migrants make their last stopover before passing the Sahara desert and the Mediterranean Sea. It is mind-boggling to realize that Whitethroats really choose this site actively for a last gluttony meal before the most challenging leg of the journey. All because of the Salvadora persica bush that fruits in April.

A really fat Garden warbler - the whole belly, tracheal pit and body sides are covered with a layer of fat

A really fat Garden warbler – the whole belly, tracheal pit and body sides are covered with a layer of fat

But this text wasn’t on the Whitethroat (I have to tell you more later, tales of gunshots, rebels and birds). The Garden Warbler doesn’t use the Sahel zone, it starts the spring migration further south, in the Guinea Savanna, some 300-400 km south of Mallamfatori. As they do not stop to forage until they reach the northern shores of the Mediterranean they need to put on a lot of fat. Huge amounts! A Garden Warbler that may weigh 16 g on the breeding grounds may reach 32 g in Nigeria, and it is not unusual that the leanest ones caught on Italian islands after the passage weigh as little as 13-15 g. It cost half of your weight to cross the dessert-Mediterranean Sea barrier. And then you still may need to travel some 3000 km to the breeding grounds. Better eat well and prepare for your journey!

Jonas Waldenström

Ottosson, U., Waldenström, J., Hjort, C. & McGregor, R. 2005. Garden Warbler Sylvia borin migration in sub-Saharan West Africa: phenology and body mass change. Ibis 147: 750-757.

http://onlinelibrary.wiley.com/doi/10.1111/j.1474-919X.2005.00460.x/full

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