What is Instinct?

The case of the American bald eagle that wound up in Ireland was most likely a case of the bird getting lost and following the prevailing winds. If you think about it, the cruel work of nature has produced bald eagles that tend not to fly too far out over the ocean.

In the distant past, any eagle with a predisposition to stay close to land would have been more likely to live longer, and thereby produce more eaglets that carried this tendency. Meanwhile the percentage of birds that didn't think twice about flying too far out over the sea would be gradually shrinking, as these birds died out quicker from lack of perch or food, or ended up being caught in a storm.

This slaughter (just one of an endless variety of lethal scenarios) would continue over countless generations until the eagles we have now will all instinctively avoid this danger, having descended ancestors that would pass along this tendency more and more in time. This is the basic scientific understanding for how pretty much all behaviors in all species have come about - traits that allow for better survivability and more offspring tend to get passed along until they become uniform throughout the species, over successive generations.

I put an asterisk next to 'all' above, because there are always mutations and outliers...