The Desert cottontails are regular daily visitors at our water dishes. The Black-tailed Jackrabbits only show up when daytime highs approach the 100-degree mark. Though the two species show many similarities, they are not so closely related, and their features are more the result of convergence driven by similar environmental challenges.     The little cottontails are much more assertive than their bigger cousins. They are not reluctant to muscle their way through crowds of doves and quail which have come for the morning feeding. If there is a cottontail already drinking from a single water dish, the jackrabbits will wait patiently until the cottontail finishes -- that can be five or ten minutes sometimes, as neither are very efficient drinkers.     Having two dishes with water considerably reduces competition, but the appearance of a pair of thirsty Gambel's Quail proved too much for the jack.