The paper talks about this. It's not just mammals and birds, proteins in general become less stable over 60C:
"This raises the question of whether temperature-driven structural changes in water affect biological macromolecules in aqueous solutions and in particular in proteins... It was found that the temperature stability range of the protein is confined to the reversible interval 45–65°C. ... In all cases the critical temperature of the protein denaturation is very close to the crossover temperature T* observed in all the properties of liquid water reviewed in this work."
This is probably why sous vide temperature is around 60C.
"This raises the question of whether temperature-driven structural changes in water affect biological macromolecules in aqueous solutions and in particular in proteins... It was found that the temperature stability range of the protein is confined to the reversible interval 45–65°C. ... In all cases the critical temperature of the protein denaturation is very close to the crossover temperature T* observed in all the properties of liquid water reviewed in this work."
This is probably why sous vide temperature is around 60C.