Steroid hormone profiles in cowbirds and redwings

Avian brood parasites display enhanced annual fecundity compared to other
passerine birds. Adult female Brown-headed Cowbirds (Molothrus ater), a
brood parasite in the Icterid family, are estimated to lay 40- 50
eggs/year. Some studies suggest annual fecundity in various parasitic
species can exceed even this upper estimate. However, boosting
reproductive output to this extent may come at the cost of reduced or lost
parental care. Steroids, such as testosterone, may act as a pivot point
that balances the fitness consequences between reproduction and parental
behavior. Moreover, steroids are targets of selection that shape life
history strategies and can therefore potentially contribute to the
evolution of novel behavioral phenotypes. Here, we examine how
reproductive steroids may mediate a possible tradeoff between increased
annual fecundity and parental care in female cowbirds. We compare seasonal
fluctuations in steroid profiles and follicular development in cowbirds
and Red-winged Blackbirds (Agelaius phoeniceus), a related non-parasitic
Icterid species. We aim to identify differences in seasonal steroid
patterns that correspond to the striking divergence in life history
strategies between these species.  We also use GnRH
administration in these two species to determine whether species variation
in the response to GnRH maps onto the differences in behavioral
phenotypes. We identified several mechanisms that would enhance annual
fecundity in the cowbird and one mechanism that would do this at the cost
of parental care: elevated testosterone. We show that female cowbirds are
reproductively ready before female Red-winged Blackbirds, a preferred and
abundant host of the cowbird at our study site. We find no significant
association between estrogen and follicular size in cowbirds, whereas this
association is apparent in Red-winged Blackbirds. However, follicular
development is associated with testosterone, and female cowbirds exhibit a
pattern of elevated testosterone throughout the breeding season compared
to the Red-winged Blackbird. These steroid profiles indicate divergence in
the physiological mechanisms that regulate oocyte development in these
related species. Moreover, cowbirds produce testosterone significantly
quicker and more robustly in response to GnRH administration compared to
female Red-winged Blackbirds. Divergence in the steroid seasonal profile
and responsivity to GnRH in cowbirds, particularly with respect to
testosterone, indicates the HPG axis exhibits consequential modifications
in cowbirds that can enhance reproduction while simultaneously inhibiting
caregiving behaviors.