Does Swift Quiver end early if you run out of arrows after being cast?

Question

The 5th level ranger spell swift quiver (PHB, pp. 279-280) says (bold italics emphasis mine):

Components: V, S, M (a quiver containing at least one piece of ammunition)

You transmute your quiver so it produces an endless supply of nonmagical ammunition...

On each of your turns until the spell ends, you can use a bonus action to make two attacks with a weapon that uses ammunition from the quiver. Each time you make such a ranged attack, your quiver magically replaces the piece of ammunition you used with a similar piece of non magical ammunition.

The way I read this, the ammunition is replaced only when you use it to make these special bonus action attacks, and will not replace ammunition that you use when you make regular attacks with your action. However, this interpretation is not necessarily clear (see this related question, which at time of writing does not have an accepted answer), so that impacts the below scenario:

So, let's say we have a ranger who has only one arrow left. They cast swift quiver (the quiver containing only one piece of ammunition satisfies the material component requirement) and now that spell is active.

If that ranger fires their last arrow with their action (i.e. a "regular" attack, not one of these arrow-replacing bonus action attacks), or (depending on the outcome of the related question) if the quiver was turned upside-down and all the arrows fell out without an attack being involved, the quiver would now be empty. Either way, we can assume that the quiver is now empty in such a way that the spell doesn't replenish the arrows.

Would the spell end because the quiver no longer contains any ammunition? Or is it that, because the material component was satisfied at time of casting, the material component doesn't need to remain valid for the duration of the spell after that point?

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Just as a reminder to those wanting to answer this question, this question is about whether or not the spell ends, not whether the arrows of a regular attack are replaced as per the spell. Anyone that wants to address the latter point should post an answer on the related question instead.

Answer 1

The spell is already cast, so whether or not the quiver is empty is no longer relevant

The rules for spellcasting state (emphasis mine):

A spell's components are the physical requirements you must meet in order to cast it. Each spell's description indicates whether it requires verbal (V), somatic (S), or material (M) components. If you can't provide one or more of a spell's components, you are unable to cast the spell.

The emphasized part of this quote indicates that you only need components when you cast the spell, not over the entire duration. The one-arrowed quiver is only required for casting it, so taking the arrow out will not end the spell.

If this was not true, you could also argue that for spells that consume valuable components, you need two of that component or the spell would not work.

Answer 2

No, the spell does not end. There is nothing in the spell description that says the spell ends in this situation, and so it follows the normal spell rules, which say the spell lasts it's stated duration.

As to the rest of the spell and how it works:

You transmute your quiver so it produces an endless supply of nonmagical ammunition...

This states that the quiver produces an endless supply of nonmagical ammunition. This is the core rule of the spell. It then goes on to say that you get can use a bonus action to have extra attacks:

On each of your turns until the spell ends, you can use a bonus action to make two attacks with a weapon that uses ammunition from the quiver.

How the ammunition is supplied is stated in the next sentence:

Each time you make such a ranged attack, your quiver magically replaces the piece of ammunition you used with a similar piece of non magical ammunition.

The "such a ranged attack" phrase should be interpreted to mean "attacks like the ones made in the bonus action", not "only those attacks made in the bonus action", or the core rule of the spell from the first line would not make any sense.

It could have been worded better.

So in the instance of having one arrow in the quiver when you cast the spell, every time you use a piece of ammunition it is magically replaced including any ranged attacks you make that are not the bonus action attacks provided by the spell. Therefore when the spell ends you still only have one arrow in your quiver, however many arrows you shoot.