Fig. 2. Wingbeat frequency (A,C) and duty factor (B,D) during budgerigar (A,B) and cockatiel (C,D) flights with and without a respirometric mask. Wearing a mask did not elicit substantial changes in wingbeat frequency or duty factor for either budgerigars or cockatiels. For the budgerigars, wingbeat frequency and duty factor were nearly constant across flight speed. In contrast, cockatiel wingbeats were roughly 25% longer and had downstroke durations that were relatively shorter at the fastest compared to slowest flight speeds.