[s0ulphire]

For those interested in an actual answer, and to absolutely nobody's surprise, it is indeed earth's magnetic field.

http://ketheridge.sites.gettysburg.edu/BIO206/wp-content/upl...

[blamazon]

Researchers have simulated what they think this might look like to the birds:

http://www.ks.uiuc.edu/Research/cryptochrome/Images/magnetic...

http://www.ks.uiuc.edu/Research/cryptochrome/

[exo-pla-net]

Per the article:

> A long-held view was that birds were aided in their orientation by a cluster of iron neurons in their beaks, which interacted with the earth’s magnetic field. “But, in the past ten years, people have begun to ask if maybe it’s something else.” Gow posits that this something could well be olfaction. “If you have a colony of birds,” she says, “they can find their individual nest among thousands by smell.”