This article below is from today's NY Times.  It may shed some light (pardon the pun) on why bright lights from tall buildings affect night-migrating birds.



Casey



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For Birds, Help at Night



Migratory songbirds perform some neat navigational tricks, feats that are made even neater because most songbirds migrate at night. So they take cues from the earth's magnetic field and from the stars to find their way when the sun is down.



Now researchers from the University of Oldenburg in Germany and Duke University Medical Center have found an area in the brain of night-migrating songbirds that is specialized for night vision. The area, which they call Cluster N, appears to be activated by dim light.



The researchers studied two night-migrating birds, garden warblers and European robins, and two nonmigratory songbirds, zebra finches and canaries, exposing them to periods of simulated night. The birds were then killed, and the brains were preserved, sliced and stained to show gene expression related to the firing of neurons. This enabled the researchers to spot parts of the brain that were active during the simulated night. The experiments are described in The Proceedings of the National Academy of Sciences.



Cluster N was active only in the brains of the warblers and robins, and only at night. And when some of the robins were fitted with eye caps that completely blocked their vision, the activity in Cluster N disappeared. Cluster N is near a visual pathway that transmits information from the retina to other parts of the brain, so the researchers suggest that the cluster is probably processing visual information as well, although at much lower light levels.



Stars are very dim, of course, so the cluster could be processing the light from them. But there's no light from the earth's magnetic field, right?



True, but other researchers have presented evidence that the magnetic field affects the light sensitivity of parts of the retina, so that birds sense the magnetic field as visual patterns. If so, they may process them through Cluster N as well.