Keeping Mistresses

European Pied Flycatcher (male) (Author-Estormiz) (CC0)

European Pied Flycatcher (male) (Author-Estormiz) (CC0)

The European pied flycatcher (Ficedula hypoleuca), also known as pied flycatcher, predominately practices a mixed mating system of monogamy (having only one mate at a time) and polygyny (marriage with several females). Their mating system has also been described as successive polygyny.

Within the latter system, males leave their home territory after their primary mate lays its first egg and create a second territory away from the first one, presumably in order to attract a secondary female to breed. After succeeding in their endeavor they return to the first female to exclusively provide for her and her offspring. Sometimes males care for both mates provided their nests are not too far from each other. Males may also care for both mates once the offspring of the primary female have fledged. Males usually do not exceed two mates, practicing bigamy. Only two cases of trigyny had been observed.

Gender difference in mating behavior

European Pied Flycatcher (female) (Author-Crowbared) (CC BY 2.0)

European Pied Flycatcher (female) (Author-Crowbared) (CC BY 2.0)

The male mating behavior has two key characteristics: desertion of the primary female and polyterritoriality. Males in this species travel distances varying on an average from 200 to 3,500 metres, to find the second mate. Being polyterritorial, they acquire multiple nest sites to attract a female. Upon breeding with this first female, they will procure more nesting sites, some distance from the site of the primary female, in order to attract a second wife or mistress. Interestingly, males that have better success at polygyny are typically larger, older and more experienced at arriving earlier to the mating sites.

As far as females are concerned, in polygynous relationship the first female does not suffer much in comparison to females in monogamous situations. In fact these primary females gain greater reproductive success because they secure full-time help from the male once he returns from his search for a second mate. The second female, however, often suffers from polygyny and produces 60 per cent less offspring than females that are in a monogamous relationship. In addition to this it also lays a smaller clutch which she is more likely to be able to rear on her own.

Extra-pair copulation (EPC) is also relatively frequent in these flycatchers. Thus, the male practicing EPC will have a group of offspring raised successfully without any parental investment on his part. The female may benefit from EPC if the second male is judged to have superior genes to the original male. Another benefit that EPC adds is that there is an increase in genetic variability.

It is believed that females are not typically very welcoming of EPC. A female that is being pursued for an EPC will either passively allow the male to copulate with her, or will resist it and risk injury due to the male’s aggression.

Evolution of polygyny

Since most bird species exhibit monogamous mating behaviors, the polygynous behavior of the European pied flycatcher has sparked much research. There are three main hypotheses that seek to explain why females settle polygynously when it lowers their overall fitness and reproductive success compared to a monogamous relationship.

“Sexy son” theory

The first theory is the “sexy son” hypothesis that says that although females experience an initial reproductive loss with their first generation, the reproductive success of the second generation compensates for the initial loss. The second generation of males is thought to be privileged because it will inherit attractiveness or the increased mating ability from their fathers and thus will have high success in procuring mates upon maturation. Since these “sexy sons” are projected to have heightened reproductive success, the secondary female’s reproductive success in turn improves. Some experts do not agree with this theory. According to them offspring of secondary female would suffer from poor nutrition, which would result in shorter tarsi and lower weights than the progeny of primary and monogamous females. These phenotypic traits contribute to lesser success in mate acquisition, rejecting the “sexy son” hypothesis.

Deception theory

The second theory claims that deception from the male explains a female’s choice to mate with an already-mated male despite the relative decrease in reproductive success. The deception arises from the polyterritoriality of the males, meaning that the males are able to deceive the females through the use of separate territories. This hypothesis attempts to show why males have developed polyterritorial behavior. The long distances between nest sites suggest that males acquire multiple nest sites to deceive their secondary female. A study showed that females leave their males upon discovering that he is already mated, as long as she discovers this fact before laying season.

However, another study on European pied flycatchers in Norway showed results that refute the deception theory. (i) The secondary females in the study raised larger clutches than primary ones. (ii) deception is not advantageous for males in the long run as the secondary females would sooner or later notice the frequent visits of males to the primary females and then they will decide to choose another mate. (iii) According to the deception hypothesis, already-mated males display polyterritorial behavior that increases their chances of acquiring another mate. Unmated males were shown to display mating behavior, consisting mostly of singing, at their nest site. On the other hand, already-mated males would need to disrupt their singing at their secondary territories in order to return to their primary nest. This can occur both before and after the time of their second mating. As a result, it decreases the chance that females would be deceived.

Female aggression hypothesis

The third hypothesis says that females reconcile for polygyny because it is hard to find unmated males. It assumes that there is aggression between females to find mates and asserts that polyterritoriality actually helps in easing this aggression and allows the second female a place to settle and breed peacefully. Although the deception hypothesis suggests that males are more successful at farther secondary territories because they can hide their marital status, the female-female aggression suggests that males occupy distant secondary territories to reduce hostility between the primary and secondary females. Primary females exhibit belligerence and prevent other females from settling near the initial nest to make sure that they get the male parental care.

In experiments, primary females have been observed visiting the second territory and behave aggressively towards the secondary female. The number of such visits declined with increasing distance between the nests. For the primary female it is also important to be able to detect an intruding female as soon as possible. Reason is the longer the intruder has been present in a territory; the harder it will be to evict her. Female flycatchers have the capacity to identify the songs of their own mates and check if they establish a second territory. The primary male was also shown to spend less time in the second territories during incubation periods than before they acquired a secondary mate, especially with greater distances between the two territories.

Parental Care

Studies have also been done to check the amount of contribution European pied flycatcher males make in parental care and also why some females opt to mate with already mated males. When the comparison between the young and older monogamous males was made there was no difference in feeding rate between each nest, but in the case of females it was primary and monogamous benefited significantly from the male in terms of parental care than polygynous ones. The latter group could only partially compensate for the absence of a male, leading to secondary females and widows raising fewer offspring than the monogamous pairs did. The results suggest that the males can control multiple territories and are thus able to deceive females into accepting polygyny, while the females do not have enough time to discover the marital status of the males.

It was also found that polygynously mated females received far less feeds than monogamously mated females, despite having no difference in the food delivery rates by the male. The reduction in delivery rate to the polygynously mated females led to a negative effect on their incubation efficiency, because they needed to spend more time away from the nest acquiring food. This also prolonged the incubation period when compared to monogamous females.

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