144 SOUTHWARD HO! 
a fourth. In a few minutes the air is alive 
with them. They, too, have come to say 
good-bye. 
A few house-martins are with them, easily 
distinguishable by their’smaller size and pure- 
white rumps; and there may be a sand-martin 
or two, smaller even and more dingy than the 
swallows. On some days you will see groups 
all swallows, or all house-martins, or all sand- 
martins; on others only one species will pay 
the estuary a last call before leaving on the 
long journey to Africa. It is all according to 
wind and weather. 
I have known the estuary so swarming with 
house-martins on a still, warm, hazy late- 
summer dawn that one could not count them, 
and wondered wherever in Britain they could 
all have come from; and yet within a few 
hours they had vanished, and were, as like as 
not, many miles across the English Channel, 
or perhaps well over France, by then. 
Listen! What was that ? 
‘Tyoo! tyoo! tyoo!’ rings out in a clear, 
piercing whistle. 
It is a greenshank wishing us farewell. He 
always calls like that, in a treble whistle; other- 
wise one would not know his cry from that of 
the far commoner redshank. 
Ah! there are some redshanks now, an- 
