Female mass in most altricial birds reaches its maximum during breeding
at egg laying, which coincides temporally with the fertile phase when
extra-pair paternity (EPP) is determined. Higher mass at laying may have
two different effects on EPP intensity. On the one hand, it would lead
to increased wing loading (body mass/wing area), which may impair flight
efficiency and thereby reduce female’s capacity to resist unwanted
extra-pair male approaches (sexual conflict hypothesis). On the other
hand, it would enhance female condition, favouring her capacity to evade
mate guarding and to search for extra-pair mates (female choice
hypothesis). In both cases, higher female mass at laying may lead to
enhanced EPP. To test this prediction, we reduced nest building effort
by adding a completely constructed nest in an experimental group of
female pied flycatchers (Ficedula hypoleuca).
Our treatment caused an increase in mass and thereby wing loading and
this was translated into a significantly higher EPP in the manipulated
group compared with the control group as expected. There was also a
significant negative relationship between EPP and laying date and the
extent of the white wing patch, an index of female dominance. More body
reserves at laying mean not only a higher potential fecundity but a
higher level of EPP as well. This interaction had not previously
received due attention but should be considered in future studies of
avian breeding strategies.