Bird migration evidence. Seabirds fly low over water but gain altitude when crossing land, and the reverse pattern is seen in landbirds. However most bird migration is in the range of . Bird strike aviation records from the United States show most collisions occur below and almost none above . Bird migration is not limited to birds that can fly. Most species of penguin (Spheniscidae) migrate by swimming. These routes can cover over . Dusky grouse "Dendragapus obscurus" perform altitudinal migration mostly by walking. Emus "Dromaius novaehollandiae" in Australia have been observed to undertake long-distance movements on foot during droughts. While participating
Not Only... But Also Cook's elderly mother) to piece together six half-hour compilation shows, screened on BBC2 from 4 November to 9 December 1990 as "The Best of What's Left of Not Only...But Also" and released in 100-minute compilation form under the same title on VHS. In 2003 a 98-minute Region 2 DVD compilation of surviving sketches was released as "The Best of Peter Cook & Dudley Moore"; this is the same as the previous video tape but missing the third series' opening sequence, "Tower Bridge". At least one sketch substitution appears to have occurred between domestic and international versions of the compilation shows,