the monarchs have come here in the Fall

[Ofriendragon]

Monarchs (butterfly) from the western United States travel to a winter home on the California coast. Whole trees in Pacific Grove, California, disappear under a coating of the beautiful sleeping insects. For as long as people can remember, the monarchs have come here in the Fall.

In the Fall obviously refers to a past time, why have come---> a present time tense is used?

And why not
For as long as people can remember, the monarchs come here in the Fall
or
For as long as people can remember, the monarchs came here in the Fall

Many many thanks in advance.

[louhevly]

Monarchs (butterfly) from the western United States travel to a winter home on the California coast. Whole trees in Pacific Grove, California, disappear under a coating of the beautiful sleeping insects. For as long as people can remember, the monarchs have come here in the Fall.

In the Fall obviously refers to a past time, why have come---> a present time tense is used?

And why not
For as long as people can remember, the monarchs come here in the Fall
or
For as long as people can remember, the monarchs came here in the Fall

Many many thanks in advance.

English uses present perfect to describe actions that begin in the past and continue into the present when some point in the past is explicitly referred to. "In the fall" doesn't, in your context, refer to past time, it means "every fall". The clause "for as long as people can remember" is the explicit reference to a past time since which butterflies "have come" here in the fall.

Here are some simpler examples:
We are here.
We are always here. (No explicit reference to past time)
We have been here for an hour.
We have been here since ten.
We have been here since John called.

Exception:
This is the first time I have been here.
Though there is no explicit past time mentioned, the implicit reference is "since I was born" and so we use present perfect. Go figure.

Lou