Re: Why do birds congregate on lampposts and power lines at highway exit ramps

Although I have no references for this answer, here are my proposed 
reasons for this phenomenon:

1. People are more likely to drop trash out their window when the car is 
slow, and urban birds are likely to eat the food debris.

2. At highway onramps and offramps, there is usually an overpass of some 
kind.  The underside of these overpasses is a perfect spot for urban 
birds to nest.  Birds are more likely to perch near their nests.

-Marcy LaViollette