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 lasted an hour and were edited more heavily. Unlike "Not Only ... But Also", all the tapes survive, although only in black and white. The show was originally recorded in colour (some sketches prerecorded on film still exist in colour). The BBC wiped most editions of "Not Only...But Also" in the late 1960s and early 1970s, as it did with many other programmes in this era. Cook and Moore even offered to pay for the cost of preservation and buy new videotapes so that the old tapes would not need to be reused, but this offer was rejected. Some telerecordings