According to Villa et al.,

Pigeons combat feather lice by removing them with their beaks during regular bouts of preening. Columbicola columbae, a parasite of feral pigeons, avoids preening by hiding in spaces between adjacent feather barbs; preening selects for C. columbae small enough to fit between the barbs. Preening also exerts selection on traits critical for locomotion on the host.

In light of this information, comment on the remark in Sect. 5.1.3 ... size-biased sampling need not be a serious concern for this experiment provided that it affects all birds equally.