eurasian tree sparrow struggling in spring snowfall affected by avian pox a delicate dance with winter and disease

zoology winter wildlife welfare weakened vulnerability vulnerabilities visibly viral vibrant veterinary tree threatened struggling stress springtime spring species sparrow snowfall snow sight seemingly science reproduction prevention pox population plumage pest patchy passerine ornithology normally mortality mix loss lesion landscapes health habitat greys finds facing evident european eurasian environmental efforts ecology dull double disease delicate dance crusty control