Can a Beast Master ranger choose a swarm as an animal companion?

Question

The Beast Master ranger's Ranger's Companion feature says:

At 3rd level, you gain a beast companion that accompanies you on your adventures and is trained to fight alongside you. Choose a beast that is no larger than Medium and that has a challenge rating of 1/4 or lower (appendix D presents statistics for the hawk, mastiff, and panther as examples). (PHB, p. 93)

May a Beast Master ranger choose a swarm of ravens as an animal companion?

A swarm of ravens has a challenge rating of 1/4, and is classified as a beast, as required by the Ranger's Companion feature.

Answer 1

No, you can not choose a swarm as an animal companion

The simplest argument is that the Ranger must choose a beast, singular, not a swarm of beasts, plural. Rules designer Jeremy Crawford even supports this interpretation in an unofficial tweet from January 2016 (although the tweet is responding to a question to the druid's Wild Shape feature).

In addition to Jeremy Crawford's tweet, there is now an official ruling in the Sage Advice Compendium (as of October 2020) that says a swarm is not an option for the conjure animals spell:

Can conjure animals summon a swarm?

No. Conjure animals summons individual creatures, and swarms are groups of creatures.

The spell conjure animals refers to being able to summon a specified number of beasts, and the Wild Shape feature lets a druid turn into a single beast, just as a Beast Master gains a single beast as an animal companion. As such, the similar logic would suggest that the same restrictions apply.

Natural vs. unnatural

There are also additional arguments to be made against a swarm being a valid option. For example, consider the description of the Beast type, in the introduction to the Monster Manual (p. 6)

Beasts are nonhumanoid creatures that are a natural part of the fantasy ecology.

And then look at the sidebar titled "The Nature of Swarms" in the section listing various types of swarms (Monster Manual, p. 377; emphasis mine):

The swarms presented here aren't ordinary or benign assemblies of little creatures. They form as a result of some sinister or unwholesome influence.

A creature with the beast type is a natural creature. A swarm is something that does not occur naturally.

Buyer beware

Of course, your DM may choose to allow you to choose a swarm. However, be wary of making such a choice. Each swarm, including the Swarm of Ravens, includes the following text in its Swarm trait:

The swarm can't regain hit points or gain temporary hit points.

Not being able to heal your swarm is likely going to cause you issues, especially given the relative squishiness of the animal companion in general.

Answer 2

RAW, no you could not choose a swarm

Throughout the relevant rules text, the companion is continually referred to in the singular as 'a beast or 'the beast'. Most crucially in the section that details eligible choices it says 'Choose a beast'.

The swarm of ravens is never referred to as a singular beast in the rules, it is a 'swarm of Tiny beasts' (plural) or thereafter simply 'the swarm' - therefore, RAW it's not an eligible pick.

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As of the 2020 edition of the Sage Advice Compendium, this ruling has now been explicitly confirmed (Thanks to V2Blast for noticing this):

Can conjure animals summon a swarm?
No. Conjure animals summons individual creatures, and swarms are groups of creatures.