In the tree swallow, some components of the immune system deteriorate with age. Acquired T cell-mediated immunity, for example, declines with age in the female tree swallow. But, the age of a female does not affect both the acquired and innate humoral immunity; the lack of deterioration in the former contrasts with studies on barn swallows and female collared flycatchers. Because of this immunosenescence (a decrease in immune function with age), older females infected with a disease generally visit their nest less, resulting in their nestlings growing slower. They are also likely to lose weight because of an infection.