High-flying muttonbirds?

[Aidan Sudbury via Birding-Aus <>]

Recently someone told me of "one of the  best talks they ever attended". It
described how, at this time of the year, certain clouds appear which we
can't see, but the birds can. When they see them, the muttonbirds fly high
into the sky to take advantage of the upper level winds.

I must admit that I have always assumed that sea-birds never fly high. Can
someone sort the truth from the fiction here?

Aidan Sudbury
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