Svein Dale

Predation favours cryptic coloration in breeding male pied flycatchers

Male pied flycatchers, Ficedula hypoleuca, are highly variable in breeding plumage coloration, ranging from bright (black-and-white) to dull (brown and female-like). Male disappearance was studied during the breeding season in a woodland area in Norway. The disappearance peaked during the nestling period, probably because of predation by the European sparrowhawk, Accipiter nisus. Significantly more bright (11%) than dull (0%) males disappeared during breeding. Variation in male body size (wing length, body mass), age, previous breeding experience, and aspects of behaviour (effort and success in mate attraction and in feeding of the young) did not seem to explain the differences in male disappearance. Hence, the data suggest a cost associated with conspicuous coloration. Brightly coloured males may be more readily detected than dull males by a predator and detected earlier, resulting in a greater chance of a successful attack. Female disappearance (10%) was similar to that of brightly coloured males but significantly higher than that of dull males. Females may be initially less easily detected but once detected less able to escape a predator. The results support the view that predation may help explain sexual dimorphism in bird coloration, delayed plumage maturation, and why males in species with less male parental care are more colourful than males in species with more male parental care.

DAWN SINGING IN THE GREAT TIT (PARUS MAJOR): MATE ATTRACTION, MATE GUARDING, OR TERRITORIAL DEFENCE?

In a number of passerine bird species, mated males sing at dawn and this song activity peaks in the fertile period of the mate. We present the hypothesis that an important function of such dawn singing is to maintain the territory. We suggest that mate guarding and territorial defence are demanding and often mutually exclusive activities. Losing paternity is so costly that males give priority to mate guarding. Males therefore use the early morning period, before their mate emerges from the roost, to claim territory ownership. We report some preliminary tests of this hypothesis from a study of great tits (Parus major). Simulating male intrusion by a playback experiment showed that the resident male was more often absent from central parts of the territory, following the mate, during the periods of nest building and egg laying than during incubation. This supports the assumption of conflicting demands between mate following and territorial defence. From the hypothesis we expected males to spend effort in defending their territory as soon as they were free to do so. Consistent with this prediction, we found that male song activity was high before the mate left the nest at dawn, when she temporarily visited the nest during the day, and when she entered the nest to roost at night. A female removal experiment showed that unmated males, having no mate to guard, sang as much at dawn as mated males. Only one of the eight widowed males succeeded to replace their mate. We discuss some alternative functions of dawn singing in the great tit, such as attraction of own mate, a replacement mate, and extra-pair mates. We conclude that the hypotheses are not mutually exclusive, and song may serve multiple purposes.

Competition for a mate restricts mate search of female pied flycatchers

SummaryWe studied the mate sampling behaviour of female pied flycatchers, Ficedula hypoleuca, in a 40-ha area containing 10–12 unmated males whose nestboxes were monitored with videocameras. The main results were: (1) The females undertook a restricted mate search. The females that mated in the area during three monitoring periods (n = 20, 12 females released by us and 8 females that arrived naturally) sampled 1–10 males (median 4.5). This was about 40% of the available mating options. (2) Search costs in terms of time and energy were low. The search period was short (median 5.1 h) and only a small proportion of the search period was spent at the nestboxes of males (median 4%). The females visited up to seven different males in 1 h, and the time elapsing between visits to different males was short (median 13 min). The minimum distance travelled during the search was also short (median 1.4 km). (3) There was competition between the females. We recorded seven cases of two females visiting the same male at the same time, including at least one case involving physical fighting. (4) Females that experienced a high level of competition had a more restricted mate search than females that experienced a low level of competition. (5) The search pattern of most of the females did not conform to the best-of-n-males rule nor to the threshold criterion rule, because they made repeated visits to many of the males sampled.

Risk taking during parental care : a test of three hypotheses applied to the pied flycatcher

Abstract According to life-history theory, there will often be a conflict between investment in current versus future reproduction. If a predator appears during breeding, parents must make a compromise between ensuring the growth and survival of offspring (nest defence, feeding and brooding of young), and reducing the risk of predation to ensure their own survival. We model three hypotheses for the outcome of this conflict which are particularly relevant for altricial birds. They are not mutually exclusive, but focus on different costs and benefits. (1) Parental investment is determined by the parents’ own risk of predation. This hypothesis predicts that a lone parent should take smaller risks than a parent that has a mate. (2) Parental investment is related to the reproductive value of the offspring: Parents are predicted to take greater risks for larger broods, larger-sized or older offspring. (3) Finally, we present the new hypothesis that parental investment is related to the harm that offspring would suffer during a period of no parental care (incubation, brooding, feeding). This hypothesis predicts that parents should take greater risks for younger offspring, or for offspring in poorer condition, because the marginal benefit of parental care is largest in such cases. Hence, one may also expect that lone parents should take greater risks than two parents because their offspring are more in need of care. We tested these hypotheses on the pied flycatcher (Ficedula hypoleuca) by presenting a stuffed predator of the parents (a sparrowhawk, Accipiter nisus) close to the nest when parents were feeding the young. Risk taking was measured as the time that elapsed until the first visit to the nest. Most support was found for the ‘‘harm to offspring’’ hypothesis. Previous studies have usually measured the intensity of nest defence against typical nest predators, and have found evidence for the ‘‘reproductive value of offspring’’ hypothesis. However, our model predicts that the importance of the reproductive value of the offspring should decrease relative to the harm that offspring would suffer if they were not cared for when the predator type changes from a nest predator to a predator of adults, and when conditions for breeding turn from good to bad.

Disappearance of Female Pied Flycatchers in Relation to Breeding Stage and Experimentally Induced Molt

According to life history theory, adult mortality during the breeding season may have an important influence on the evolution of several aspects of breeding ecology in birds, yet few studies have tried to quantify such mortality. We studied disappearance of Pied Flycatchers (Ficedula hypoleuca) during four breeding seasons in a woodland area in Norway provided with nest boxes. The main cause of disappearance was probably predation by the European Sparrowhawk (Accipiter nisus). Disappearance was nonsignif- icantly higher in females (10% per season, n = 305) than in males (7% per season, n = 269). Female disappearance peaked during egg-laying (0.53% per day), but was also high during the nest-building (0.42% per day) and nestling (0.36% per day) stages. It was low during incubation (0.05% per day), probably because less time was spent outside the nest. Low risk of predation during incubation may help to explain why female body mass remains high during this stage of breeding but drops soon after hatching. Females with selected flight feathers experimentally removed to simulate molt suffered a much higher disap- pearance per season (24%, n = 109) than did control females (10%, n = 305). This may help to explain why breeding and molt usually are temporally segregated activities in birds. Variation in female body mass and size (wing length, tarsus length), age, previous breeding experience, mating date, laying date, clutch size, and mating status could not account for the variation found in female disappearance. Disappearance was lower in males than in females during the nest-building period, despite the more conspicuous plumage color of males. This may be explained by the fact that only the female builds the nest. We suggest that risk of predation is an important constraint on sexual selection of male plumage color in species in which males take part in nest building.

Costs and benefits of hatching asynchrony in blue tits Parus caeruleus

We studied the significance of hatching asynchrony in blue tits Parus caeruleus during a 4-year period at Oslo, Norway, by comparing breeding success of broods manipulated to hatch over a shorter or longer period than average. Nestling mortality was high and mainly caused by starvation. Mean body mass of fledglings on day 14 was significantly higher for asynchronous than for synchronous broods. No significant difference was found in the mean number of young fledged between the treatment groups, nor in the number of offspring recruited into the local breeding population, even in a year with heavy nestling mortality. However, the number of recruits was few. The results are consistent with the view that asynchronous hatching ensures high quality of some offspring (the offspring quality assurance hypothesis). This benefit is achieved even when no brood reduction occurs. We suggest a potential cost to hatching asynchrony (the diminishing return hypothesis): in asynchronous broods, early hatched offspring may require so much food that their own subsequent survival is reduced or at least no further improved. Resources may therefore be wasted on first hatched offspring that might have improved the growth of younger siblings. Female parents had lower post-breeding survival with asynchronous than with synchronous hatching; the opposite result was found for male parents. We propose the exploitation of mate hypothesis to explain this result. The hypothesis presumes there is sexual conflict over the amount of parental investment in current versus future reproduction, and over the investment in particular offspring within the brood. We suggest that with asynchronous hatching, females have to invest more to keep less competitive, late-hatching nestlings alive.

Mate sampling behaviour of female pied flycatchers: evidence for active mate choice

SummaryThis paper presents major new evidence for active mate choice of female pied flycatchers, Ficedula hypoleuca. Fifteen color-ringed females were released into a study area containing 23 unmated males defending one nestbox each. Through intensive surveillance, the behavior of the females was observed during 2 consecutive days. Twenty-two of the males received a total of 131 female visits. Six of the females settled in the study area, and their premating period lasted 1.3–2.5 days. The females were seen searching for mates for at least 6–32 h and were seen visiting at least 1–9 different males. Hence, some of the females rejected males before mating. Nevertheless, the females settled close to the site of release (range: 16–243 m), suggesting that they mated with one of the first males encountered. Females visited males most frequently in the morning, and the diurnal distribution of visits was significantly correlated with male song activity.

Female-female aggression explains polyterritoriality in male pied flycatchers

Abstract Many male pied flycatchers, Ficedula hypoleuca , try to attract a second mate in a distant territory around the time their initial mate lays her clutch. In this study their success at becoming polygynous increased with the distance between the two territories. Two hypotheses may explain this result: either females are prevented from settling close to the males initial nest because of aggression from the initial female (the female aggression hypothesis), or males can more easily hide their mating status and deceive females into polygyny the further apart the nests are (the deception hypothesis). These hypotheses were tested by presenting a female in a cage near a males secondary nestbox during 3-h trials. (1) Females (and males) were able to locate and identify their mates a long distance from their own nests. (2) The males initial mate often visited the males secondary territory and (3) she was aggressive towards the caged female. (4) The proportion of initial mates visiting the males secondary territory decreased with the distance from their own nests. (5) The initial mates spent less time in the males secondary territory during their incubation periods than they did earlier on. (6) The already-mated males were more often absent from their secondary territories the further they were from their initial nests; this occurred despite the presence of the caged female. These results are most, consistent with the female aggression hypothesis.

POPULATION DIVERGENCE IN SEXUAL ORNAMENTS : THE WHITE FOREHEAD PATCH OF NORWEGIAN PIED FLYCATCHERS IS SMALL AND UNSEXY

Models of sexual selection suggest that populations may easily diverge in male secondary sexual characters. Studies of a Spanish population of the pied flycatcher, Ficedula hypoleuca, and a Swedish population of the closely related collared flycatcher, F. albicollis, have indicated that the white forehead patch of males is a sexually selected trait. We studied the white forehead patch of male pied flycatchers (n = 487) in a Norwegian population over seven years. Males with large forehead patches were in general more brightly colored, but patch height was not correlated to body mass, body size, or parasite loads. Conditions during the nestling period did not seem to influence patch height as an adult. Patch height increased slightly from the first to the second year as adults, but then remained relatively constant at higher ages. Patch height was not related to survival. Year‐to‐year changes showed that males who increased in patch height also increased in body mass, suggesting that expression of the forehead patch may be partly condition dependent. However, changes in body mass explained only a small proportion of the variance in patch height between males. Thus, patch height would not be a good indicator of male quality. Furthermore, patch size was also not related to male ability to feed nestlings, indicating that females would not obtain direct benefits by choosing males with large patches. However, patch height could be a Fisher trait, but this requires heritability and there was no significant father‐son resemblance in patch height. Comparisons of the males visited by each female during the mate sampling period indicated that chosen males did not have larger forehead patches than rejected males. Experimental manipulation of patch height did not affect male mating success. These results indicate that females do not use patch size as a mate choice cue. Finally, patch height did not predict the outcome of male contests for nestboxes, suggesting that the forehead patch is not an intrasexually selected cue of status. Norwegian pied flycatchers have smaller forehead patches than both Spanish pied flycatchers and Swedish collared flycatchers. We suggest that this pattern may be explained by the lack of sexual selection on the forehead patch in the Norwegian population as compared to the other populations, where the patch is apparently sexually selected. We discuss possible reasons for these population divergences, such as female choice on an alternative secondary sexual character (general plumage color) and speciation among Ficedula flycatchers.

Random settlement of female pied flycatchers, Ficedula hypoleuca: significance of male territory size

Abstract In a study of the pied flycatcher the availability of nest sites, and hence the territory size of males, was manipulated. The number of females that a male attracted was positively correlated with his territory size; males in a low-density plot attracted more females than males in a high-density plot. A partial correlation analysis showed that territory size was the single most important determinant of male mating success, at least late in the season. If females actively choose between males on the basis of territory size or male density, males in the low-density plot should be preferred over those in the high-density plot. However, the relative number of females settling in the plots was the same before and after manipulation of male density. The results are consistent with an hypothesis of random female settlement where females arrive randomly in relation to area and mate with the first male they encounter. It is argued that when females are constrained by time and energy, random settlement is the best pairing strategy they can adopt, as high-quality males in high-quality territories are most likely to be detected by prospecting females. The results suggest that male pied flycatchers defend territories to gain access to females.