Social rank in winter flocks of Willow Tits Parus montanus

The social hierarchies in winter of ten flocks of Willow Tits Parus montanus were studied when the birds were foraging naturally and when visiting feeders. All the flocks consisted of one adult mated pair together with two juvenile males and two juvenile females (probably pairs). All flocks studied had a stable composition and the hierarchies remained constant throughout the study period. The hierarchies were linear and unilateral. The adults of each sex dominated the respective juveniles and within each age group the male dominated the female. The dominance relationships between the age and sex groups were not consistent. Although the males dominated all the females in six flocks, in one flock the adult female dominated both the juvenile males, but only one of them in three other flocks. The degree of aggression between flock-members was 0.8 encounters per hour, and males initiated 94% of all attacks. Body-weight explained 77% of the variation in dominance rank. It is suggested that the dominance rank of a male is also a function of his seniority, while the rank of a juvenile female is correlated with the rank of her mate.     

Social rank and antipredator behaviour of Willow Tits Parus montanus in winter flocks

Field experiments on three free-ranging Willow Tit winter flocks, each consisting of one adult pair and two male and two female juveniles (first-year birds), were performed to examine whether preferences for feeding site and antipredator behaviour are related to social rank. The dominance structure was the same in all flocks; adult male > juvenile male 1 > juvenile male 2 > adult female > juvenile female 1 > juvenile female 2.
The proportion of time spent scanning for predators was positively correlated with distance from cover, and adults scanned relatively more than juveniles at the same controlled distance from cover, especially in the afternoon. Given a choice between feeders placed 1 m, 3 m, 5 m, 10 m and 20 m from the forest edge, the tits preferred feeders close to cover. Low-ranking individuals used feeders farther from cover indicating that higher ranked tits prevented them from using the feeders close to cover by means of social dominance. When only the 10m and 20 m feeders were baited, only low-ranked juveniles visited the feeders, subordinate females slightly more than males. The subordinate juveniles increased their use of exposed feeders at low ambient temperature, suggesting that they are prepared to take greater risks during cold periods.
The sequence of return to a feeder, after a life-like stuffed predator model mounted 1 m from a feeder opening was removed, was positively correlated with dominance status, revealing that subordinates take the greatest predation risks.     

Advantages of social foraging of Willow Tits Parus montanus

The mean number of Willow Tits Parus montanus in single-species flocks was significantly larger than in mixed-species flocks of Willow and Coal Tits P. ater. Both flock size and the tendency of Willow Tits to join mixed-species flocks were negatively correlated with ambient temperature, probably because each bird, when the metabolic rate of the birds increased, could allocate more time to foraging due to improved predator detection by many eyes. The vigilance time of Willow Tits decreased with flock size and was determined by the total number of individuals in a flock rather than by the number of Willow Tits in mixed-species flocks of Willow and Coal Tits.     

Territory acquisition during winter by juvenile Willow Tits Parus montanus

Willow Tits Parus montanus live within exclusive territories in non-kin winter flocks with a dominance hierarchy in which males dominate females and adults dominate juveniles. Because they live in habitats that are saturated with dominant territory owners, the options of subordinate juveniles surviving the winter are: (1) to become a dominant owner of a vacated territory, (2) to stay in an occupied territory and hope for ownership or (3) to leave the area and hope for territory acquisition elsewhere. Removal experiments during December to February revealed that particularly the most dominant of the juvenile pairs left their own flock territory and replaced birds experimentally removed from a neighbouring flock, if by doing so they increased their own dominance position. It is suggested that juvenile Willow Tits updated their information about the composition and dominance structure of adjacent winter flocks by regularly visiting these territories and attempted to become dominant owners during winter, thereby increasing their chances of breeding.     

Winter survival and breeding success of dominant and subordinate Willow Tits Parus montanus

We studied the effect of winter rank on survival rate and reproductive success in Willow Tits Parus montanus, a resident passerine living in dominance-structured flocks during the nonbreeding season, in 6 years. Winter survival was dependent on both the birds' age and rank. Adults survived better than first-year birds, and within first-year males, dominants survived better than subordinates. In other sex and age classes, rank did not contribute to survival. Although first-year males were in excess among nonbreeders, no connection existed between breeding status and rank. Female rank did not explain the variation in the start of laying, clutch size, number of fledglings or recruit production. We conclude that social status in Willow Tits affects individual fitness mainly through rank-dependent survival. Acquiring a high rank position seems to be most important for first-year birds, especially first-year males.     

Age Differences in the Response of Willow Tits (Parus montanus) to Conspecific Alarm Calls

Predation is an important mortality factor in wintering birds. To counter this, birds produce alarm calls in the presence of predators which serve to warn conspecifics. In social hierarchical bird flocks, adults survive the winter better than juveniles and therefore survival strategies probably vary with social status. This study examined the differential responses to alarm calls by free-living willow tits, Parus montanus, in dominance-structured winter flocks in Finland. To explore the age-dependent differences in response to conspecific alarm calls, a series with three alarm calls was played to focal adults and juveniles while they sat in the middle section of a spruce branch. Immediately after the playback, juvenile willow tits moved more often, flew longer distances and changed branches more often than did adults. Previous mammal studies have shown that juveniles are more likely to flee than adults after hearing conspecific alarm calls. The current study demonstrates that similar age-dependent responses to conspecific alarm calls occur in birds also. These findings reflect an increased vulnerability to predators or lack of experience of young birds.     

Seeing without being seen: a removal experiment with mixed flocks of Willow and Crested Tits Parus montanus and cristatus

This paper tests the hypothesis that foraging site selection reflects a trade-off between the various needs for concealment from predators, to find food, and for the individual to maintain some view of its surroundings. After removal of Crested Tits Parus cristatus (the dominant species in mixed flocks), Willow Tits P. montanus did not decrease their foraging heights as expected but remained in the most exposed parts of young pines. In contrast, after removal of Willow Tits, Crested Tits increased their foraging height from the sheltered lower canopy to sites previously occupied by Willow Tits. When flock size was reduced, the birds maintained the same high levels of vigilance without concealing themselves in dense vegetation. I suggest that flock members may benefit from foraging in sites that afford good anti-predator vigilance.     

Dawn chorus and female behaviour in the Willow Tit Parus montanus

We studied the effect of female behaviour on the dawn chorus of Willow Tits Parus montanus. Ten females were temporarily kept in their nest holes in order to delay their dawn emergence. Simultaneously, their mates' singing behaviour was measured. We compared the control dawn behaviour (during which the females emerged when they wanted) with that during the experimental delay. The dawn song by the males lasted longer when the females' emergences were delayed. Our results show that the duration of the dawn chorus is directly related to the female's behaviour. We suggest that at least one function of the dawn song is intrapair signalling, which is often linked to sperm competition.     

Age-Related Microhabitat Segregation in Willow Tit Parus montanus Winter Flocks

It is expected that through flexibility in behaviour, flock living birds respond to the asymmetries in resource access derived from dominance relationships. We analysed the microhabitat use of willow tits in winter flocks and assessed possible factors which shape habitat segregation between adults and juveniles in different temperature regimes. When foraging in mild conditions (ambient temperature > 0°C), flocks split up into subgroups with adults foraging in inner parts of trees more often than juveniles. However, no differences were recorded in the vertical position occupied in trees. In harsh conditions (< ? 4°C), flocks re‐united and juveniles further moved to outer parts of trees, increasing horizontal segregation between age classes. In mild conditions, vigilance behaviour was not related to the position of birds in trees, but in harsh conditions, scanning frequency was higher in outer parts of trees only for adults. In mild weather, juvenile position in trees was associated with body size and mass. The foraging microhabitat segregation detected in harsh conditions fits the age‐related hoarding distribution previously described in the same population. This supports the hypothesis that hoarded food is important in determining future foraging habitat use. Adult preference and intraspecific competition for safer or richer inner parts of trees as foraging sites during harsh conditions seems to determine the habitat segregation between adults and juveniles. Furthermore, we suggest that in mild weather, when foraging in the absence of adults, juveniles balance the costs of using a potentially dangerous microhabitat with the benefits of building energetically cheap and large food reserves through hoarding. The expected patterns of microhabitat segregation may differ in parids, depending on whether predation risk or other factors such as food availability are the main factors controlling habitat quality.     

Cost of reproduction: parental survival and production of recruits in the Willow Tit Parus montanus

Summary Brood sizes of the Willow Tit were altered experimentally by subtracting or adding two nestlings in 1986 and 1987 in the vicinity of Oulu, northern Finland. The manipulated broods were within the normal range observed in natural conditions. Unaltered broods were used as controls. Data from natural broods from 1978–1985 were available for comparison. When the nestlings were 13 days old they were ringed and weighed and their tarsus, wing, and tail lengths were measured. On the same day the parents were caught, weighed, and measured. In 1986 there were no differences in nestling mortality between the reduced, control, or enlarged broods; i.e. parents were able to fledge the two extra young. In 1987 starvation was most pronounced in the enlarged broods. This resulted in the number of fledglings being practically the same in each manipulation category. Especially the body weight, but also the other indices of body size, decreased as a function of the brood size category, suggesting that there may be quality differences between the young reared in different experimental groups. In 1986 there was a non-significant trend towards lower body weight of the parents attending reduced, control, or enlarged broods, in that order. In 1987 the differences were much smaller. These results were not due to size differences between the groups, so possibly the increased reproductive effort of raising extra young was responsible for the trend observed in 1986. There were no significant differences in parental survival associated with the manipulation category, although the trend in the females was consistent with the hypothesis of reproductive cost. It is possible that environmental conditions in 1986 were so favourable that the tits were not unduly stressed even when attending two extra young. Correlative data from 1978–1985 did not support the cost hypothesis either. A non-significant trend towards reduced post-fledging survival and recruitment of the young was observed with increased brood size. The average fitness value of parents, incorporating parental survival and number of recruits, showed that the success of the adults raising enlarged broods may be lower than that of others. It seems that the reproductive cost, if it exists, decreases individual fitness value by reducing the chances of recruiting descendants into the next generation. The reproductive stress may be insufficient to reduce the subsequent survival of parents. More data are however needed to confirm these results.     

Pair bond and breeding success in Blue Tits Parus caeruleus and Great Tits Parus major

Data from 939 nests of the Blue Tit Parus caeruleus and 1008 nests of the Great Tit P. major from nestboxes provided in superabundance in mixed forest study sites between 1976 and 2001 were analysed to examine the effects of mate retention on breeding success and the relationship between mate fidelity and site fidelity. Most birds retained their former partner (76% in Great Tits and 65% in Blue Tits). The probability of a pair divorcing was affected by male age in Great Tits, divorce being more likely in pairs with first‐year males. Great Tit pairs breeding together for a second season bred earlier, but had no higher breeding success than pairs breeding together for the first time. In Blue Tits laying date and start of incubation tended to be earlier in pairs breeding together for a second season, but hatching and fledging dates were not earlier than in other pairs. Great Tit pairs breeding together for two consecutive seasons bred earlier in the second season than in the first, but breeding success did not differ significantly between years. In both species, breeding performance did not differ between pairs that divorced after a season and pairs that stayed together. Thus breeding success did not determine whether a pair divorced or bred together again. Neither Blue Tits nor Great Tits improved their breeding performance through divorce. Blue Tit females even had fewer fledglings in the year after divorce than in the year before. Mate retention affected breeding site fidelity. Blue Tit females had greater breeding dispersal distances between consecutive years when re‐mating than when breeding again with the same mate. In Great Tits both males and females dispersed more when re‐mating than when retaining the former partner, suggesting that mate retention increased the chance of retaining the breeding site. In both species, breeding dispersal distances did not differ between pairs that divorced and pairs in which one mate disappeared. Because no major advantage of mate retention was evident, we suggest that mate retention evolved under different conditions than those found in study sites with high breeding densities and a superabundance of artificial nesting sites.     

Age‐dependent responses to alarm calls depend on foraging activity in Willow Tits Poecile montanus

Survivorship in animals depends on both foraging activities and avoidance of predation, and thus behavioural decisions often reflect a trade‐off between predation risk and foraging efficiency. In this experimental study, we compared behavioural responses of free‐living adult and juvenile Willow Tits Poecile montanus to a conspecific alarm call in two treatments. The alarm call was played back when a focal bird was either not feeding, or feeding on a sunflower seed on the middle part of a spruce branch. When feeding at the time of the alarm call, juveniles more often stayed motionless or moved shorter distances than adults. Our results suggest that in hierarchical groups, juveniles are forced to take greater risks to maintain access to food or lack experience to optimize between food and safety.     

Brood size manipulations within the natural range did not reveal intragenerational cost of reproduction in the Willow Tit Parus montanus

To test for the existence of a reproductive cost, we manipulated brood sizes (-2 and +2 nestlings) over 6 years in a northern population of Willow Tits Parus montanus breeding in natural holes. Possible effects were sought in subsequent survival and fecundity of the parents. Parents given extra chicks made more feeding visits than did parents with reduced and control broods. However, this was not reflected in differences in parental body-weight between groups at the end of the nestling period. Brood size manipulation did not significantly affect female or male survival. In 4 out of 6 years, there was a weak and nonsignificant effect on male survival, consistent with a cost of reproduction. Female and male fecundity in the year following the experiment was not affected by the manipulations. Thus, the data do not give evidence of an intragenerational cost of reproduction in the Willow Tit. Parents appeared unwilling to increase their breeding effort to a level which jeopardized their own survival or future breeding success. It is possible that, because of the time constraints in northern latitudes, females work under their capacity and lay smaller clutches than would otherwise be most profitable. Thus, no costs to the parents would be expected as a consequence of manipulations. These results suggest that the current reproductive rate is not maintained by reproductive cost in the Willow Tit. However, the results do not rule out the possibility that selection has operated outside the current range of reproductive rates during evolutionary history of the species.     

Gender and Personality Differences in Response to Social Stressors in Great Tits (Parus major)

In response to stressors, animals can increase the activity of the hypothalamic-pituitary-adrenocortical axis, resulting in elevated glucocorticoid concentrations. An increase in glucocorticoids results in an increase in heterophils and a decrease in lymphocytes, which ratio (H/L-ratio) is an indicator of stress in birds. The physiological response to a stressor can depend on individual characteristics, like dominance rank, sex and personality. Although the isolated effects of these characteristics on the response to a stressor have been well studied, little is known about the response in relation to a combination of these characteristics. In this study we investigate the relationship between social stress, dominance rank, sex and exploratory behaviour as a validated operational measure of personality in great tits (Parus major). Great tits show consistent individual differences in behaviour and physiology in response to stressors, and exploratory behaviour can be classified as fast or slow exploring. We group-housed four birds, two fast and two slow explorers, of the same sex that were previously singly housed, in an aviary and compared the H/L-ratio, lymphocyte and heterophil count before and after group housing. After experiencing the social context all birds increased their H/L-ratio and heterophil count. Females showed a stronger increase in H/L-ratio and heterophil count than males, which seemed to be related to a higher number of agonistic interactions compared to males. Dominance rank and exploration type did not affect the H/L-ratio or heterophil count. Contrary to our expectations, all birds increased their lymphocyte count. However, this increase was slower for fast than for slow explorers. Our study suggests that personality and sex related differences, but not dominance rank, are associated with changes in an individual''s physiological response due to a social context.     

Phylogeography of the Eurasian Willow Tit (Parus montanus) based on DNA sequences of the mitochondrial cytochrome b gene

The phylogeographic relationships of the trans-Palearctic Willow Tit assemblage were studied by obtaining sequence data from the mitochondrial cytochrome b gene from 34 specimens representing nine subspecies from across the species range. Four distinct genetic groups were identified: Parus montanus weigoldicus, P. m. affinis, P. m. songarus, and a clade containing six Eurasian subspecies (ssp. baicalensis, borealis, montanus, restrictus, rhenanus, and sachalinensis). P. m. weigoldicus, P. m. affinis, and P. m. songarus were reciprocally monophyletic and separated from each other and other subspecies by uncorrected genetic distances between 1.9 and 5.8%. The remaining six subspecies were closely related and shared mitochondrial haplotypes, despite marked morphological and acoustical differences, suggesting a rapid evolution of distinct vocalization patterns. The current classification splitting the species into the songarus and montanus subspecies groups is not concordant with our phylogeny. Also, the four regiolect groups, Lowland, Alpine, Siberian, and Sino-Japanese, are not fully mirrored in the phylogeny. Our data suggest that the mono-frequency song type may be ancestral and was retained over a long evolutionary time in certain populations, but was altered or camouflaged by learning processes in others.     

To breed or not to breed: causes and implications of non-breeding habit in the willow tit Parus montanus

Causes and consequences of non-breeding in willow tits were studied in northern Finland during 1986–1992. The breeding status was sex and age biased; males and yearling birds were in excess among the non-reproducers. Due to sex bias in the population it appeared detrimental for males to lose a mate, especially shortly before breeding. Lack of a mate was a important factor for males not reproducing (37% of non-breeding males) than for females (14%). Most of the non-breeding birds maintained a pair bond which only rarely broke up for the next breeding season (divorce rate 5.5%). This implies that parental incompatibility is not a possible explanation for pairs not reproducing. Males that did not breed tended to survive better than reproducing ones, whereas such a relationship was not found for females. It is possible that this sex-related difference in survival cost is attributable to quality differences among non-breeding individuals. It was especially low-quality yearling females, with low survival prospects, that were responsible for the discrepancy. The proportion of non-breeding females in the population correlated highly with clutch size and subsequent juvenile survival. It is therefore suggested that for most of these females non-breeding is a phenotypic response to low offspring value in the prevailing circumstances (inter-generational tradeoff). However, it is uncertain whether willow tits in a northern population can use breeding density as an indicator of changing survival prospects of their descendants, as suggested by Ekman and Askenmo (1986) for southern Sweden.     

Initial winter flock formation in the Willow Tit Parus montanus. Do immigrating juveniles assess the quality of territorial adults?

In the Willow Tit Parus montanus , forming of winter flocks starts shortly after the juveniles have become independent of their parents. In the early phase of flock formation the juveniles roam about prospecting for suitable winter territories. They visit several occupied territories in succession and seek contact with the territory holders. I describe the behaviour of juveniles during such encounters with adult conspecifics. I suggest that, by approaching adults closely and behaving specifically, juveniles gain important information on the adults' innate qualities; this may affect the settling juveniles' future survival.     

The significance of clutch overlap in Great Tits Parus major

We studied the effects of manipulation of the size of first broods in the Great Tit Parus major on the size and breeding success of second clutches and its relation to the degree of clutch overlap. The rearing of first brood fledglings always overlapped with the laying of the second clutch and in most cases also with the incubation period of the latter. The degree of clutch overlap depended on the size of the first brood, being less when the first brood was large. Clutch overlap also increased with season. Mechanisms affecting the timing of laying of second clutches are discussed. A large first brood imposed reproductive costs. It affected the size of the second clutch by causing it to be delayed; second clutch size decreases with season. It affected the post-fledging survival of second brood young as, in this population, this decreases with fledging date. The breeding success of second clutches was, however, not affected by the size of the first brood, but instead by the weight of the female when rearing the first brood.