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 the latter of which included "The Walrus and the Carpenter" (a filmed recreation of the Lewis Carrol poem which survives from the largely missing sixth episode of series two) instead of "The Ravens". Neither DVD includes this sequence. "Initials", or "Old J.J.", an old piece of Cook's recorded for the NOBA pilot in November 1964 and screened in January 1965 in the first episode of the series proper, was included in a mid-1990s VHS compilation of 1960s BBC comedy extracts. A Region 1 DVD of "The Best of... What's Left of... Not Only... But Also..." was released by BBC Worldwide