The most likely statement is: "The proportion of short-tailed squirrels may have decreased due to increased food availability."
This conclusion is based on the assumption that long-tailed squirrels are better adapted to their environment for activities like jumping and balancing in trees. If the conditions (e.g., food availability) have improved, this could lead to an increase in long-tailed squirrels while potentially causing the population of short-tailed squirrels to decline due to competition or changing environmental pressures.