We do eat some legumes raw
Indeed; peas straight from the plant are one of my favorites!
I looked up feedipedia on beans, it says
"Common beans contain several antinutritional compounds, notably enzyme inhibitors (trypsin, chymotrypsin, α-amylase), phytic acid, flatulence factors, saponins and lectins (
Lyimo et al., 1992). The nature and activity of those compounds vary greatly among cultivars: for instance, raw navy beans were shown to contain twice the trypsin inhibitor activity than red kidney beans but half the lectin activity (
Dhurandhar et al., 1990). Those antinutritional factors may hamper the performance of monogastric animals or even be toxic: for example lectins are known to have caused food poisining in humans who have eaten undercooked or raw beans. It is then highly recommended to process raw common beans before feeding them to pigs, poultry and other monogastric livestock. Treatments such as heating, autoclaving, cooking or extruding have been shown to remove heat-sensitive antinutritional factors (
Bollini et al., 1999;
Akande et al., 2010a;
Carvalho et al., 1997;
Dhurandhar et al., 1990;
Kakade et al., 1965). Biological treatments such as germination, ensiling, treatment with pancreatin or with chemicals can also be effective in removing antinutritional factors in common beans (
Schulze et al., 1997;
Savelkoul et al., 1994). Lectin content may be reduced by cooking above 100°C, but cooking at a lower temperature (80°C) increased lectin content and toxicity (
FDA, 2012)."
That last bit is a surprise.
Currently I'm sourcing peas as part of one mix or another designed for pigeons, which I then mix with grains, so singling out peas to cook is not an option for me. And some of the flock select the peas to eat first and really like them, so I don't think toxicity per se is an issue, nor fermenting as a method of reducing the anti-nutritional factors. Maybe it just comes down to individual birds' microbiomes and taste preferences. As long as there's variety and alternatives - the more the merrier - so no-one is forced to eat them against their own better judgement, I don't think it's a problem. The leftovers get flung onto the lawn, and something comes along to eat them sooner or later, so they're feeding the web of life, and the chickens benefit from that.