I was just outside eating a hamburger on a park bench and a bunch of sparrows came in front of me, looking for something to eat. I tossed them a few tiny crumbs and they seemed happy with that.
There was one thing I noticed, though: sometimes one sparrow would grab a crumb and feed it to another (adult, as far as I can tell) sparrow. What’s the possible motivation for this? Would they be mother and son? Husband and wife? Sparrow-of-the-evening and client?
That would be mating behavior. Birds do it, there is also something about bees.
You will not see the same food sharing in the winter.
I had a hunch it wasn’t just altruism! Thanks.
There’s a Pirates of the Caribbean joke in here somewhere.
I’ve heard that called tidbitting when chickens or quail do it.
They were probably babies grown up enough to look like adults. We had a sparrow mecca in our back yard and I saw this all the time. It’s a little more obvious when there’s more than one “juvenile” mobbing mom with open mouths.
I suppose it’s possible, although the second sparrow certainly didn’t have any mobility problems hopping around on the sidewalk or flying away when people walked by. I.e., he/she seemed perfectly capable of feeding himself/herself.
That’s normal. The hatchling leaves the nest as soon as it is physically able to fly, but still hangs around and begs food from its parents for as long as they’ll put up with it.
This. I don’t know about sparrows, but doves are very nearly full size by the time they leave the nest. (I have a nest just outside my front window.) Looking at them on the ground you would not assume they were babies.
Here in San Antonio the major local bird is the great-tailed grackle. One funny behavior I’ve seen is what I assume are juveniles hopping around on the ledges of the windows at my office with their mouths open towards the window. They’re begging for food from their own reflection. 
Sort of. Around 0:30, it looks like one bird is dropping a crumb in the other bird’s mouth, as I saw. But in my case, the feeder was darting around picking up the crumb and the feed-ee was just sitting there. Then the feeder hopped over to the feed-ee, the feed-ee opened its mouth, and the feeder dropped the crumb in.