Incubation delays territory defence by male blue-headed
vireos, Vireo solitarius
            

Ioana Chiver^{*, ,}
,
Eugene S. Morton^† and Bridget J.M. Stutchbury<sup>*
            

            †</sup>Smithsonian Institution Conservation and Research
Center, Front Royal, VA, U.S.A.

            ^*Department of Biology, York University, Toronto,
Canada

Received 18 November 2005;  revised 11 January 2006;  accepted 1 June
2006.  MS. number: A10304R.  Available online 2 November 2006.

Animals often face trade-offs during reproduction between
activities such as parental care and territory defence. In species that are
socially monogamous, males and females are faced with the additional problem of
coordinating their respective contributions to such activities. Here, we
examined male incubation behaviour in a passerine that shows genetic monogamy
and in which males incubate extensively. Male incubation is rare in North
American passerines and little is known about how males defend their territories
during the incubation stage and whether females compensate for lower male
incubation during territorial intrusions. Blue-headed vireo males contribute
50% of incubation time during the day. We performed playbacks to simulate
intrusions to males while they were on the nest and off the nest during the
incubation period. On average, males took 18 min to arrive at the playback
location while on the nest incubating, but took less than 2 min when not
incubating. In addition, only 44% of the males sang while on the nest, whereas
all males sang in response to the playback when off the nest. This result
suggests that males probably delay territory defence until females return to
the nest to avoid exposing the nest to predators. Females returned to the nest
to relieve their mates from incubation duties sooner during experimental
intrusions than during control periods, which also allowed males to pursue
intruders sooner. Genetic monogamy may underlie this apparent cooperation and
the sex-role convergence exemplified in this species.

We first evaluated two general hypotheses based on studies
of different bird species reported in the literature. The hypotheses are not
mutually exclusive, but they represent the two most commonly proposed functions
of T in male birds, at least during the breeding period. In both cases, we
modified the predictions of the hypotheses to address nonbreeding social
behaviour in our own study species, P. pubescens. One hypothesis is the ‘resource defence’
hypothesis, which states that males with elevated T during winter can occupy
and defend a territory to a greater degree than other males. Furthermore,
expansion of the home range could lead to an increased opportunity for males to
interact with nonmate females, who may be higher in quality than the current
mate (cf. Raouf
et al. 1997). We predicted that under the resource defence hypothesis,
T-males would expand their home range, occupy space more exclusively of other
males, associate with more females over time and interact more frequently with
other males than would controls. T-implants in the spring have these effects in
other species (Silverin
1980; Hegner
& Wingfield 1987; Wingfield
& Farner 1993; Ketterson
et al. 1996). As a consequence of these T-influenced effects, males also
neglect their mate and offspring (Ketterson
et al. 1996; Hunt
et al. 1999; Schwagmeyer
et al. 2005). Therefore, a secondary prediction we made under the resource
defence hypothesis is that T-males and T-females will associate with each other
less often than control pairs do. Mate association frequencies among
experimental pairs may also be low if T-males become more aggressive towards
their mate. Little is known about the effects of T on male–female aggression in
birds, but male woodpeckers are known to displace females from high-quality
foraging sites when resources are scarce (Kilham
1970, 1974).
Thus, T-males could show higher mate-displacement rates than controls, and the
foraging rates of T-females could be reduced because of their efforts to avoid
displacements by their mates.

A second hypothesis is the ‘pair bond’
hypothesis. Testosterone might improve a male’s potential for future
reproductive success by facilitating investment in a current or future pair
bond rather than, or in addition to, an expanded home range and resource base
(e.g. Enstrom
et al. 1997; Alonso-Alvarez
2001; Peters
2002). The hormonal state of one partner affects that of the other (Wingfield
& Farner 1993; Wingfield
& Monk 1994), and hormonal synchrony early in the breeding season
improves reproductive success (Hirschenhauser
et al. 1999). Thus, in species in which mates have the opportunity to
interact in winter, higher winter T levels could hasten the start (or increase
the frequency) of male–female interactions. Under the pair bond hypothesis, we
predicted that T-pairs would show a higher mate association frequency and
greater home range overlap between mates than would controls. We further
predicted that T-females would have greater foraging efficiency than C-females
as a consequence of more frequent mate association. Studies on avian pair
behaviour during winter have documented male protection of females against
predators and conspecific food competitors (P. pubescens: Sullivan
1984, 1985;
Kellam
2003; other examples: Lens
& Dhondt 1993; Hogstad
1995; Fusani
et al. 1997). As males become more vigilant when near the mate, female
foraging efficiency improves. This helps prevent mate loss by lowering the
risks of female starvation and depredation.

Both the resource defence and pair bond
hypotheses are based on the findings of studies conducted on a variety of bird
species, usually during the breeding period. However, the results that we
report here for P.
pubescens in winter do not tend to support either a priori
hypothesis, even though our treatment groups showed several significant
behavioural differences. We therefore conclude our paper posing a third
hypothesis for the function of T in males during nonbreeding periods: the
mate-guarding hypothesis. We use our results to suggest that T may help males
in the nonbreeding period prevent mate loss stemming from female-initiated mate
switches or desertion before mating. This hypothesis contrasts with the pair
bond hypothesis, which predicts that males and females would benefit similarly
from a nonbreeding social relationship.

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