Full Idea
Psychologically, 'or' corresponds to a state of hesitation. A dog waits at a fork in the road, to see which way you are going. For crumbs on a windowsill, birds behave in a manner we would express by 'shall I be brave, or go hungry?'.
Gist of Idea
'Or' expresses hesitation, in a dog at a crossroads, or birds risking grabbing crumbs
Source
Bertrand Russell (An Inquiry into Meaning and Truth [1940], 5)
Book Reference
Russell,Bertrand: 'An Inquiry into Meaning and Truth' [Penguin 1967], p.79
A Reaction
I love two facts here - first, that Russell wants to link the connective to the psychology of experience, and second, that a great logician wants to connect his logic to the minds of animals.