The study of monogamy, considered to be the dominant mating system among avian species, has suffered
significant changes with the application of molecular tools, showing it to be more complex than was
initially assumed. A number of hypotheses have been suggested to explain the huge variation in the rate of
extra-pair fertilization among species. According to the good genes hypothesis, if males differ in genetic
quality, females paired with low quality males are compensated when searching for extra-pair copulation
with males of higher quality, because this behaviour would improve their offspring's survival and
reproductive chances. Nevertheless, males cannot maximize the search for extra-pair copulation and mate
guarding simultaneously, as these behaviours are mutually exclusive. Therefore, extra-pair copulation is
only valuable for males with good chances of extra-pair fertilization, but may be undesirable for those who
lose paternity in their own nest. This paper presents a qualitative reasoning simulation model developed to
compare alternative hypotheses and as such to increase understanding about the role of individual features
and behavioural factors associated with the occurrence of extra-pair paternity. Qualitative models
contribute to ecological theory development with rapid assessment of assumptions, hypotheses, and other
ideas by representing the system structure, establishing causal relations and predicting the system
behaviour using incomplete knowledge. This model attempts to answer questions such as: What is the effect
of mate quality on female pursuit for promiscuous copulations? Is mate guarding an effective strategy for
decreasing paternity loss? Does male genetic quality affect male and female individual fitness? The
answers provided by the model to these questions are based on the most relevant mechanisms identified in
the literature to explain sexual behaviour in birds. The model demonstrates that population density and the
quality of the male with whom the female is mated influences female predisposition to search for extra-pair
copulation. Accordingly, high-quality males should have a higher number of within and extra-pair young
than low quality males. Since the interests of males and females are similar, both sexes may achieve an
increment in their fitness. The model is also used to demonstrate how extra-pair paternity may be explained
by the paternal care hypothesis, which provides an alternative explanation. Ongoing work includes
improving representations of both male and female behaviour. However, the results obtained so far confirm
the potential of qualitative reasoning modelling contribution to our understanding of the theoretical basis of
complex aspects of sexual selection in birds.