On a balmy day in late June, Scott Williams waits for a white-footed mouse (Peromyscus leucopus) to fall asleep. Williams, a wildlife biologist with the Connecticut Agricultural Experiment Station in New Haven, has just transferred the animal from a trap to a plastic bag containing a cotton ball doused in anaesthetic. As soon as the mouse's breathing slows to one breath per second, Williams will take it out, draw blood, weigh it, put an ear tag on it for identification and check the animal for ticks, saving any that are engorged with blood. He must work quickly. The mouse will wake up in about two minutes, and she might be grumpy.