Birds are significantly more susceptible to acute radiation injury than humans (cf. https://www.nature.com/articles/srep16594 https://www.nature.com/articles/sr

Birds are significantly more susceptible to acute radiation injury than humans (cf. https://www.nature.com/articles/srep16594 https://www.nature.com/articles/srep16594 - not directly on point, but the dose rate relationships can be trivially compared with those widely documented for humans, e.g. at https://en.wikipedia.org/wiki/Acute_radiation_syndrome#Dose_effects https://en.wikipedia.org/wiki/Acute_radiation_syndrome#Dose_...). I suspect that's where the association originates, and the use of the trope fits in especially well with Jericho's plot, which opens with a nuclear war.