[Supermancho]

> How do you know they "know" them?

It's a statistical guess, as with most phenomena. When individuals, alone, consistently travel toward direction without observable prompting, it's expected there is another stimuli. This may be an unseen force (birds following magnetic fields). However, it appears there is a genetic component.

https://archive.is/vt6rU#selection-797.2-797.236

Notably: "They also inherit from their parents the directions in which they need to fly in the autumn and spring, and if the parents each have different genetically encoded directions, their offspring will end up with an intermediate direction."

[akira2501]

> It's expected there is another stimuli

Yes, and your link then identifies it:

> They have at least three different compasses at their disposal: one allows them to extract information from the position of the sun in the sky, another uses the patterns of the stars at night, and the third is based on Earth’s ever present magnetic field.

They clearly do not "know" paths anymore than water "knows" what gravity is.

[Supermancho]

> They clearly do not "know" paths anymore than water "knows" what gravity is.

The link identifies it as genetic. If there were no genetic component, there would be consistency, regardless of genetic lineage.

> They clearly do not "know" paths anymore than water "knows" what gravity is.

Knowing is a soft term, for which I provided a definition to answer a second order question. Diving into any further classification of "knowing", is a separate issue. The topic under discussion is not definitional knowing for other organisms. The topic is the genetic transfer of information, as per this article. Hence, graciously, it can be assumed that "knowing" is shorthand for this concept.

[int_19h]

Having a compass is one thing, but that only gives you the overall direction, not the specific path to follow.