I saw lots of Canadian Geese in a river in England today, and it got me thinking: if someone in Ireland or Portugal or wherever saw a shit-ton of birds flying into the ocean where there is no land until you hit europe again on the other side or maybe asia or some shit, and then coming back in the summer/winter, did it click that they were actually going somewhere (i.e. America) or did people just ignore it?
Quite an obscure question I know, but it's been bugging me all day
It's based on something of a false premise. Birds generally migrate north to south, so those Irish and Portuguese birds would either be flying south for the winter, to southern Europe or west/south Africa, or north. North of Ireland can include the Scandinavian mainland, Scotland, the Faroe Islands, Iceland and Greenland, as well as the Arctic ice.
In fact generally speaking although some northern Canadian islands are included in the East Atlantic migratory flyways (map of flyways approx half way down the page), it's not many at all. And though some Canadian geese do make their way transatlantically, most English Canadian geese are non-migratory and arrived on a boat as an introduced species.
You can through a recent post
By that point in time people already knew the earth was round for quite a very long time, and even had a pretty good guess of the size of the earth, although there were disagreements. What they couldn't agree on was what land mass was exactly "over there" and how far they were.
Also please look through the FAQ, the question on the incorrect belief that people in late medieval era thought the earth was flat is often asked.