Tweets and Gary,

According to Birds of the World on line, via e-bird, for Eurasian Collared-Dove, "One report of white sphere, assumed to be fecal sac, removed from nest (Rose and Rose 1999)."

Rose, R. E. and M. C. Rose. (1999). Observations of nesting Eurasian Collared-Doves (Streptopelia decaocto) in Gulf Breeze, Florida. Alabama Birdlife 45:1-3.

Seems a reasonable explanation for your observation.

Alan Grenon
Seattle