[bcoates]

It's kind of surprising that a used cigarette butt (as opposed to, say, unburned tobacco) holds enough nicotine to act as an effective anti-parasite material for a nest. It's at least a non-obvious result.

The article linked to another nature article about birds selectively choosing herbs that repel parasites to line nests, and using scent to know when to refresh them.

Given that cigarette butts are effective and that birds are known to pick nest material carefully, the conclusion that birds are introducing them intentionally isn't unreasonable.

[kragen]

It might not be the nicotine, but the polycyclic aromatic hydrocarbons produced by burning, which are broad-spectrum venoms.

[gus_massa]

Nicotine was used as a insecticide: http://en.wikipedia.org/wiki/Nicotine

[kragen]

Oh, I'm well aware that nicotine is an insecticide. But it's present in very small quantities in tobacco smoke, compared to the PCAHs.

[xyzzy123]

As a smoker, I can tell you that nicotine becomes concentrated in the filter, along with tar and lots of nasty things.

The last half of a cigarette is stronger than the first half.