"Screening" by Welcome Swallows

[Colin Scouler <>]

Hello all,

Referring to the foraging techniques of Welcome Swallows, HANZAB says:
"Forage aerially, taking prey by sallying or screening..."

I can't find the term "screening" in the glossary, and none of the  
dictionary definitions seem to fit.
Can someone explain what "screening" means in this context?

Thanks,
Colin Scouler
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