Nasal markers (saddles and discs) are used commonly to identify indivi
duals in studies of waterfowl ecology. The potential effects that thes
e markers have on the study animals has been poorly tested for free-ra
nging birds. We examined the effects of nasal discs on several indices
of nesting effort for wild Mallards in the prairie pothole region of
south-central Canada. Nasal discs did nor significantly influence the
proportion of birds that remained on the study area, the proportion of
birds that nested, the number of nests initiated per bird, the total
number of days a given bird devoted to laying or incubating eggs, or t
he proportion of birds that successfully hatched a nest. Compared to u
nmarked birds, however, nasal-marked birds did significantly delay the
ir first nest by 2-6 days. Investigators should carefully examine stud
y objectives before deciding to nasal-mark individuals.