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
Pay what you can Pay what you can Pay what you can (PWYC) is a non-profit or for-profit business model which does not depend on set prices for its goods, but instead asks customers to pay what they feel the product or service is worth to them. It is often used as a promotional tactic, but can also be the regular method of doing business. It is a variation on the gift economy and cross-subsidization, in that it depends on reciprocity and trust to succeed. "Pay what you want" is sometimes used synonymously, but "pay what you can" is often more oriented to charity