Free birds aren't fat: Weight gain in captured wild pigeons maintained under laboratory conditions.

Iwata et al. (1990) moved wild city pigeons into lab cages. The birds could eat all they wanted, any time. The team weighed the birds every day to see what happened.

No food limits, no training, just free grain. The goal was to check if our usual 80% body-weight rule is harsher than we think.

The pigeons gained 17% body weight in a few weeks. They stayed heavier than their street weight for the whole study.

The gain shows that city birds normally eat less than lab birds with open feeders. Standard lab deprivation may not be as tough as it sounds.

Szempruch et al. (1993) also saw big weight change, but in kids. Their cystic-fibrosis patients gained calories and grew after a short group program. Both papers show body mass jumps when food rules relax.

Fabbretti et al. (1997) used the same pigeon lab setup to test choice. They found birds like two small keys more than one big key. Together with A et al., the lesson is that small setup tweaks—free food or extra keys—can swing behavior more than we expect.

Spanoudis et al. (2011) showed pigeons shift choice fast when payoff odds change. A et al. adds a body-level result: the birds also shift weight fast when the payoff is unlimited food. Same species, same lab, different yardstick.

If you run pigeon labs, the 80% weight rule may already be mild. Watch birds for signs of too much gain, not too little. For human clients, the broader point is the same: when access opens, intake can jump fast. Track body weight or calorie data along with behavior so you can adjust plans before drift becomes a problem.