One of my ducks is sick-she came out with the rest when i let them out of their pen into the garden but then she kept sitting down constantly and I noticed her back end keeps moving up and down. She also is not walking so well. She's one of my oldest ducks at 4.5yrs old and has only laid sporadically the past couple of years-we had a fox attack and she's not really laid regularly since she lost her first two duck pals when that happened. What does everyone think-egg bound or a respiratory infection? I will be taking her to the vet if she doesn't improve. All other birds seem fine at the moment.