I wouldn't even be mad

Hummingbirds make for bad spys as they are an hour away from starvation for large parts of their lives.

But it's a magic shapeshifter hummingbird. Do they even need to eat?
- Good question, how far do wild shape replicate biologic aspects?
Not in D&D. They just need to eat every other day, like anything else that doesn't have enough CON to go longer. I'm not sure how much they need to eat. I'm told there's a table on page 111 of the DMG, but I don't own one.
- Here you go
Loud as hell too! Sounds like god's own hornet when they buzz you.

How does conservation of mass work with wild shape? Is that hummingbird 180 pounds? If so, how does it fly?

There's no conservation of mass. Magic turned them into a bird. Any energy missing from the system went into the magic.
D&D ain't no Brandon Sanderson crunchy magic shit. It's Tolkien "this shit happened cause I magically said so" shit
- It goes into the magic and comes back when the spell is over. It's giving magic an interest free loan. Why does it cost spell slots to use this again?
- I like it!
It's magic.
- Mystery solved!
There's no conservation of mass, you can wildshape into an elephant, load up with goods to your carrying capacity, then change to a raven and fly back, then back to the elephant to unload.

In 3.5e one of the strongest builds I'd ever seen was a hummingbird wild shape air-domain caster druid with the natural spell feat (I believe that is what it was called). They'd fly around the battlefield in bird form which gave them such insane AC bonuses as to be basically unhittable, then they'd just cast Call Lightning and Ice Storm and destroy the enemies while remaining untouchable. It was honestly insanely OP

Hmm, interesting build. Did the player ever run into trouble with AoE saves? I feel like a Con save with no to-hit roll would be the Achilles heel there
- I only played a one-shot with the guy; it was just a fun-run kind of game so I didn't get a chance to see a lot of character, just the one day worth of OP-ness.

The cat leaps for me and I wild shape into a flea which means I'm going to hitch a ride to the kitchens on this mean s.o.b. and hopefully I've got time for a short rest because fuck trying to explore a castle with a ~~walking~~ jumping distance of 8 inches.

A flea is too small for wild shape.
- This is a miniature giant space flea.