I found this article very interesting because, in truth, I never even considered the fact that birds collect pollution in their feathers! But, when you look at the two pictures, it's so very evident! But, although I do realize that collecting specimens and embalming them for future reference is good for science, I can't stand to look at the hundreds of dead birds that you usually find in the Natural History Museums... You don't actually see them in the one in NYC or the one in Washington DC but I remember going to the one in Helsinky, where they have a super complete collection of artic animals and being completely repulsed by the sight of so many corpses... Read on.
https://www.popsci.com/museum-bird-soot-air-pollution