It’s not only the more conspicuous birds such as waders that are on the move; smaller ones are busy moving south as well, but they’re much more difficult to see and the best way is to get out the mist nets and go ringing in a reed bed.
Early this morning I joined a local group of ringers and did just that. The weather was right, clear overhead last evening, with cloud moving in before dawn. Many small migrants such as warblers navigate by the stars and when blotted out by clouds, tend to ‘fall’ into the nearest reed bed. The results can be dramatic and in the past I’ve experienced huge falls of sedge warblers in particular.
Today’s catch was reasonable; with only 6 nets we caught in excess of 80 birds. Apart from the usual residents they were mostly sedge warblers, with a variety of other migrants such as reed warblers, whitethroats, blackcaps, willow warblers and a single lesser whitethroat.
The autumn migration strategy of sedge warbler is particularly interesting. In order to get across the Sahara they store up energy for the journey in a remarkable way. Plum-reed aphids provide the fuel and they find these on the underside of phragmites leaves in reed beds on the south coast of England and in northern France. Here they gorge on these insects and can sometimes double their weight in a matter of days. Once ready and when the weather is right, they set off and can do the trip in one hop, a remarkable feat for a small bird normally weighing only about 10 grams.
Reed Warblers have a different strategy. They too use plum reed aphids as fuel, but fatten up less, hopping from reed bed to reed bed through Europe before the final leg across the Sahara.
None of our sedge or reed warblers today weighed anything like this, but some showed signs of subcutaneous fat deposits under their wings, a sure sign that they were on the move.