Population biologists often are much more intrigued by death than by birth, a fact brought home to me personally over the past few weeks. We follow about 60 family groups of these nonmigratory jays, and through color-banding, we get to know each jay as an individual. Over the past 10 months, nearly 50 percent of the jays in our study area have died, including 40 percent of the breeders and 85 percent of the juveniles. We strongly suspect the killer to be an insect-borne disease, perhaps encephalitis, but we are still hunting for the cause. Rapidly spreading lethal disease has been reckoned with by natural populations for millions of years.
Birds have a remarkable immune system, and it is constantly being tested by new microbes.
By watching bird populations carefully and recognizing that death is just as natural as birth, biologists continue to make progress in understanding how evolutionary forces shape the way birds live. Well-studied cases of disease in the wild are extremely rare, as they tend to appear unpredictably. I salute all who have taken part in this study already, and I urge others to join if you have House Finches at your feeders. You are helping us understand how sickness and death affect the character of the populations that remain alive.