> but you might also suspect like most animals they do not perceive risk (basically a probability distribution of possible outcomes)

I beg to differ. Animals are certainly risk averse. Many animals come out only at night to avoid being seen by predators. 

The swarming activity generally occurs during the most favorable times. This is usually May--June in our region. In other areas it depends on rainy season vs dry season. 

Migratory swarms are definitely fleeing risk and seeking better odds. This may not be a "conscious motivation", but it is certainly a behavior pattern that has been fine-tuned by evolution.

PLB

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