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
If I Could Only Fly aging and the accompanying doubts, we can learn how to face our own mortality with greater equanimity and fewer regrets." Ben Ratliff of "Rolling Stone" gave the album 4 stars, enthusing, "...what a lyricist he still can be. All the songs emanate from a single persona, an aging, cloistered singer (Haggard is in his sixties) whose routine—avoiding drugs, taking comfort from cushioned bus seats, being honest with his kids — is all he has". Stephen Thomas Erlewine of AllMusic wrote: ""If I Could Only Fly" is the first album in years that deserves to be compared to Haggard's classic work."