In the Spring, Male Songbirds Sing a Lot and Have

In the spring, male songbirds sing a lot and have high testosterone (T) levels, but song output declines significantly in August and September. One hypothesis to explain this observation is that the number of T receptors declines, which would

A) shift the dose response curve to the right.
B) mean lower levels of T could accomplish the same level of song output.
C) cause reduced affinity of T for the remaining receptors.
D) cause T to bind to unrelated receptors.
E) have no effect on sensitivity to T.