Many models proposed to explain the occurrence of polygyny in birds im
plicitly assume that females actively choose males offering the highes
t quality breeding situations. However, strong evidence of active mate
choice by females in polygynous bird species is scarce. This study as
ked whether females in a polygynous population of house wrens Troglody
tes aedon, a small, cavity-nesting songbird, base their choice of a ma
te at least partly on characteristics of the nest site that he control
s. Already-mated males were randomly assigned one of two types of nest
boxes to which they could attempt to attract secondary mates. Some ma
les were given standard-type boxes with small entrance holes in the fr
ont sides of boxes, while other males were given boxes with larger ent
rance holes located in tile boxes' roofs. A preliminary experiment rev
ealed that female house wrens overwhelmingly prefer front-entrance to
roof-entrance boxes when both types of boxes are available on the same
territory. Females chose mated males with front-entrance boxes over m
ated males with roof-entrance boxes significantly more often than expe
cted by chance. This preference demonstrates that female house wrens d
o discriminate among potential mates and base their choice of a mate a
t least partly on characteristics of the nest site he has to offer. Th
is observation is compatible with the hypothesis that fern ale house w
rens choose mated males because they gain access to high-quality nest
sites.