Ecology of the Basilisk lair

5e

The Monster Manual tell us that basilisks live in arid, tropical, and temperate climates, and seek sheltered lairs such as caves, often living underground.

Basilisks move slowly ("ponderous" predators) and can consume prey they have petrified with their strong jaws.

And that's about it.

"The Ecology of the Basilisk," Ed Greenwood, Dragon 81

The next best--and only other--reference I know is this article (January, 1984, predicated on 1e lore^*) which tells us:

• Basilisks subsist largely on berries, fish, small fowl, and carrion. They do not eat stone in this edition, so their petrification gaze is purely defensive.
• Basilisks may be found solitarily, in a mating pair (they mate for life), or as a parent-pair with young. They do not congregate in larger communities.

(among many other interesting tidbits)

Pulling it together.

I think of basilisks more like cacti than like lions. They've got their space and no-one except silly adventurers and crazy mages bothers them. They'll ruin your day/life if you get too close, so other animals just don't get close. Thus a basilisk lair would be typified by a sparseness of "normal" fauna.

Their petrification gaze functions both as a strong defense and as a hunting aid. Beyond this, whether they still hunt small game and feed on carrion or they ambush large game (including humanoids) and consume their petrified forms at their leisure... is up to you.

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^{* - 5e's most-closely related to 2e both by design and in my evaluation. However, the 2e Monstrous Manual does not contain an "ecology" section for the Basilisk, leaving this 1e contribution a little less-well related to your 5e uses, but still (IMO) the best canonical reference.}

^{2e MM does tell us that a basilisk lives in "any land", is a carnivore, and moves slowly. That's about it, though, for these purposes.}