Files in this Data Supplement:

• Movie 1 -

  Movie 1: Guinea fowl running across level runway. Body center of mass (COM) energy is shown below, synchronized to video. Total kinetic energy (E_K, green) and gravitational potential energy (E_GP, blue), are summed to obtain the total mechanical energy of the body (E_tot, yellow).

• Movie 2 -

  Movie 2: Guinea fowl encountering an unexpected drop in substrate (U treatment), exhibiting the ‘E_Kh mode’ response. This movie illustrates the most common response to the perturbation (E_Kh mode, 9 of 19 trials). Body COM energy is shown as in Movie 1.

• Movie 3 -

  Movie 3: Guinea fowl encountering an unexpected drop in substrate (U treatment), exhibiting the ‘E_com mode’ response.  This movie illustrates the second most common response to the perturbation (E_com mode, 7 of 19 trials). Body COM energy is shown as in Movie 1. Note that there is a much smaller increase in total kinetic energy (E_K, green), than in the ‘E_Kh mode’ response shown in Movie 2.

• Movie 4 -

  Movie 4: Guinea fowl encountering an unexpected drop in substrate (U treatment), exhibiting the ‘E_Kv mode’ response. This response was only observed in three trials by the smallest bird. Body C)M energy is shown as in Movie 1.

• Movie 5 -

  Movie 5: Guinea fowl negotiating a visible substrate drop (V treatment). Body COM energy is shown as in Movie 1. The behavior during V trials was more variable during U trials. This video shows a common case, in which the bird slowed much more than U trials and stumbled slightly with a re-stepping response during the step back up to the original height.