is there an active verb for what a "thing being consumed" does?

if we consider the sentence birds consume spiders, and spiders consume flies

the "opposite" of that sentence is something like is flies <> spiders and spiders <> birds but, more active. As if the spider jumps down the birds throat.

I'm looking for that word. Something similar to "feeds into", but just one word ideally.

https://www.merriam-webster.com/thesaurus/consume was not much help, I'm considering it in the second or third context, but from the point of view of the thing being consumed

Solution 1:

The word nourish is occasionally used this way, though usually for inanimate (or at least no-longer-animate) substances. From Merriam-Webster

3. a. : to furnish or sustain with nutriment : _^FEEDPlants are nourished by rain and soil.

A couple of examples of this usage:

This hypoallergenic kibble nourishes your cat
—"James Wellbeloved Adult Cat Housecat - Duck" cat food ad

Research has shown that donated breast milk can nourish babies just as well as mother's own milk can.
—Heidi Murkoff and Sharon Mazel, What to Expect the First Year, 2014.

In your case, you could say

Flies nourish spiders and spiders nourish birds.

The primary problem with this term (and also feed, as suggested in the comments and in M-W's definition for nourish) is that with animate subjects it is usually used to mean that the subject provides (some other) nourishment to the object: I nourish my children with healthy meals, nourish their imaginations with plenty of playtime, and nourish my skin with rich creams and lotions (that last one is probably debatable, science-wise). So readers may get a mental image of spiders bringing birdseed to birds, rather than crawling into the birds' mouths.

Even with this caveat, I think this is probably the best you can do, since offering oneself up to be eaten is a rare enough activity that there's not much call for an everyday verb to describe the action.