Most birds are protective of their nests, but some much more aggressively than others. I've never had any experience with a Tree Swallow, or any other Swallow. Mockingbirds, Jays, and Thrashers are VERY aggressive when it comes to protecting their nest and young. I know from experience that these will attack anything that gets too close.... including
me!! 
Other species are more passive, such as a Killdeer or Bobwhite Quail, which will fake an injury to try to lure a predator away from its nest. Some others will simply give up and abandon their nest if threatened and re-nest elsewhere.
We did not inherit Mother Earth from our ancestors...... we have borrowed Her from our descendants.
Chief Seattle (paraphrased)
