Food sharing and mutual aid among the homeless Since the time of Peter Kropotkin, it's been observed and theorized that cooperation and mutual aid are more common in austere environments. A classic biological example would be slime mold spore-making (Starssman &amp; Queller, 2011), and anecdotal example would be people coming together to face a natural disaster. I am interested in more systematic studies of this in modern human society under more extended time-frames than reactions to disaster.
In monkeys, apes, and primitive human societies, a typical measure of cooperation and mutual aid is food sharing (Jaeggi &amp; Gurven, 2013). I am interested in seeing studies like this that use naturalistic measures like this, instead of artificial games like dictator or ultimatum (even though these do provide interesting economic and anthropological results at times; Henrich et al., 2001).
One of the few truly austere environments remaining in Western society is the plight of the homeless. For example, the U.S. Interagency Council on Homelessness (1999) has found that 28% of homeless people nationwide sometimes or often do not get enough to eat (compared with 12% for the poor), 20% eat one meal a day or less, and 40% reported that they went one or more days in the last 30 days without anything to eat (compared to 3% for the poor).
Yet, consistent with the Kropotkin's views, anecdotal evidence (i.e. what I hear from friends that have been run aways, and from the occasional homeless that I have a chance to talk to) suggests that food sharing is common among the homeless. As a highlight, here is an anecdote I encountered recently of reddit (empasis added by me):

This is why I, as a homeless person, gets offered more food from other homeless people than from wealthier people I come in contact with. This holds even though there is no pity parties among the homeless, and many food sharers will stab your back in a heartbeat if they perceive longer term gains. Or, in some cases, when more primal needs are at stake.

This brings me to my questions:

How common is food sharing among the homeless?
Is it primarily driven by empathy/egalitarianism, reciprocity, tolerated theft, or other mechanisms?
How does it compare to rates of food sharing in modern hunter-gatherer societies?

I am more interested in anthropological studies done in western populations and developed countries, especially big urban centers, but comparisons to homeless populations in developing countries is of interest as well. I also prefer academic studies instead of governmental/activist ones, since the latter tend to have a policy bias; but I'll take what I can get!
References
Jaeggi, A. V., &amp; Gurven, M. (2013). Reciprocity explains food sharing in humans and other primates independent of kin selection and tolerated scrounging: a phylogenetic meta-analysis. Proceedings of the Royal Society B: Biological Sciences, 280(1768).
Henrich, J., Boyd, R., Bowles, S., Camerer, C., Fehr, E., Gintis, H., &amp; McElreath, R. (2001). In Search of Homo Economicus: Behavioral Experiments in 15 Small-Scale Societies. American Economic Review, 91 (2), 73-78
Strassmann, J., &amp; Queller, D. (2011). Evolution of cooperation and control of cheating in a social microbe. Proceedings of the National Academy of Sciences, 108(2).
U.S. Interagency Council on Homelessness (1999). Homelessness: Programs and the People They Serve – Findings of the National Survey of Homeless Assistance Providers and Clients.
I spent over 5 weeks hanging out with homeless people in Kalamazoo, MI by a nearby homeless shelter. I engaged with them nearly every minute of free time I had after work and on weekends. What I observed regarding food was actually an abundance. When someone releases a study indicating that someone doesn't get "enough" to eat, that is extremely subjective. I used to eat once a day myself out of pure choice and habit. The homeless may eat only once or twice a day, but they likely have many options throughout the day if they are in the right location such as in Kalamazoo. No one went hungry near the shelter, yet many chose not to eat. They also chose where to eat since there were often many locations offering food. In fact, some of them declined when I offered to take them out to eat. 

They do not hoard food for lack of anywhere to put it. Thus, they are often willing to share this food but usually do not even need to share since they can grab their own. From my experiences, I do not recall anyone giving out of their own loss given their full knowledge that more food would come tomorrow. I've seen more sharing with cigarettes and dimes, but this is clearly not a need and is, in general, a means of befriending people. Theft was very common as I was regularly told stories of how their clothes or shoes were stolen. Thus, befriending people was a rather good idea and made it less likely to have your things stolen or to get into fights. Perhaps other homeless communities are not as well-off as the one in Kalamazoo, but my own experience leads me to believe that it's not the same as watching people band together in a mutual experience of hardship. Most of the people I saw seemed deficient in the mental arena. They seemed to be stuck in high school relationship mentality with elementary school cognitive prowess. Only a handful of people seemed more capable and simply mooched the system. Perhaps you need someone's input who has been with competent yet unfortunate homeless people, but I thought I'd share my own experience.