"... moulting their rectrices so cannot control a dive...",

Adding my two cents...We have frequently witnessed immature Anna's attempting dives, only to produce an anaemic churp. The best dives are from considerable altitude, and are for the clear benefit of a coy female, and may be repeated over and over, with the male taking a break when necessary. But we have also seen dives that seemed to be more related to teritoriality, on both bemused adult and juvenile male Anna's, and on what must been hallucinations since we were unable to identify any target bird in vicinity of the bottom of the dive. Successful dives culminate in a noisy roll in the underbrush.

chuck reinsch, magnolia, seattle, wa, (creinsch at humbirds.org)