There maybe a much simpler explanation for seeming scope "insensitivity" in saturation. The differences in the lead example with the birds seems random variation to me. Probably most participants had some maximum amount they would ever consider parting with for non personal life threatening scenarios, and it doesn't take very many birds in the scenarios to reach their maximums. Seems like modeling artificially contrived scenarios like this is overthinking the issue without giving adequate thought to alternative more likely explanations.