 Birds that are heavily " wing @-@ loaded " — that is , heavy @-@ bodied birds with relatively short wings — have great difficulty flying with the loss of even a few flight feathers . A protracted moult like the one described above would leave them vulnerable to predators for a sizeable portion of the year . Instead , these birds lose all their flight feathers at once . This leaves them completely flightless for a period of three to four weeks , but means their overall period of vulnerability is significantly shorter than it would otherwise be . Eleven families of birds , including loons , grebes and most waterfowl , have this moult strategy . 
