I rubbed one of my ducks chest and I can feel it vibrating. Like you would think she was scared. It would happen and then stop for a minute and start up again but all of a sudden it would quite for a longer period and then start all over again. My other duck is not doing this. Does anyone know what this is or what it could be and what to do about it? I am a little worried so PLEASE any info would be appreciated. Thanks.