Found it - it is 'The Storm'
First there were two of us, then there were three of us,
Then there was one bird more,
Four of us--wild white sea-birds,
Treading the ocean floor;
And the wind rose, and the sea rose,
To the angry billows' roar--
With one of ustwo of usthree of us--four of us
Sea-birds on the shore.
Soon there were five of us, soon there were nine of us,
And lo! in a trice sixteen!
And the yeasty surf curdled over the sands,
The gaunt grey rocks between;
And the tempest raved, and the lightning's fire
Struck blue on the spindrift hoar--
And on four of us--ay, and on four times four of us
Sea-birds on the shore.
And our sixteen waxed to thirty-two,
And they to past three score--
A wild, white welter of winnowing wings,
And ever more and more;
And the winds lulled, and the sea went down,
And the sun streamed out on high,
Gilding the pools and the spume and the spars
'Neath the vast blue deeps of the sky;
And the isles and the bright green headlands shone,
As they'd never shone before,
Mountains and valleys of silver cloud,
Wherein to swing, sweep, soar--
A host of screeching, scolding, scrabbling
Sea-birds on the shore--
A snowy, silent, sun-washed drift
Of sea-birds on the shore.