Kari Koivula

Prior residency as a key determinant of social dominance in the willow tit (Parus montanus)

SummaryWe studied how age, body size and prior residency affected social dominance in the willow tit (Parus montanus) groups. The contribution of each variable was experimentally tested in unisexual two-bird trials, in which the birds were matched for all variables except the one studied. Large birds were dominant over smaller ones (Fig. 1). The effect of body size was more prominent in males than in females. Age had no influence on dominance. Residents became dominant more often than newcomers (Fig. 2). Adulthood or larger body size did not override the advantage of prior residency (Fig. 2). Therefore, the proximate reason for the age-dependent dominance seen in natural willow tit flocks is most likely the prior residency advantage of the adults. Factors connected with fighting ability (body size and age) seem to be less important than the time of establishment of rank, which may reflect the importance of resource value differences between residents and newcomers in this context. The advantage of residency might make it advantageous to be a member of a flock even as a subordinate, rather than being solitary.

Do subordinates expose themselves to predation? Field experiments on feeding site selection by Willow Tits

To examine the relationship between social dominance and exposure to predation risk, we allowed natural groups of Willow Tits Parus montanus to choose between two nearby feeding sites differing in safety. The birds preferred to feed at the safe site. Socially dominant individuals used safe sites more than did subordinates. However, adult females mated with alpha males, used safe sites more than expected from their rank, possibly as a result of protection offered by their mates. Subordinates increased their use of safe sites after experimental removal of dominants, implying that the greater risk-taking of subordinates in natural flocks is due to interference from dominants. The observed pattern of risk-taking at feeders may influence survival also in natural conditions, because dominants survive better than subordinates, as documented earlier for this population.

TIME AND FOOD DEPENDENCE IN WILLOW TIT WINTER SURVIVAL

Timing of mortality was studied in Willow Tits (Parus montanus) during three winters, 1991–1993 and 1995–1996, near Oulu, northern Finland. The aim of this study was, by investigating monthly winter survival rates, to examine (1) whether late winter is the most difficult time for birds, and (2) whether temperature, especially in late winter, correlates with survival, as previously proposed. We also sought information on sex and age differences in timing of mortality, rarely studied among passerines. After color-marking, individuals were resighted monthly from September to May. Capture–recapture models were applied to separately estimate survival and resighting probabilities. Survival estimates given by three methods of varying complexity (simple counts, Jolly-Seber, Cormack-Jolly-Seber) were compared. The three different methods yielded similar survival estimates. Because simple counting does not account for emigration, consistent results from different analyses support the earlier view that Willow Tits are sedentary. Adults survived better than yearlings in all winters and exhibited more constant survival rates. Two-year data also suggest that winter and summer survival rates were similar in adults, but not in yearlings, for which mortality was greatest in winter. Males survived better than females in one winter. Temperature per se did not correlate with survival, but deviations to colder than long-term average temperature seemed to be associated with low survival probability, which could reflect occasional food limitation. A feeding experiment was conducted to test the winter food limitation hypothesis. Fed birds survived winter better than did control birds, significantly so in yearlings and males, but not in adults or females. Previously, late winter has been suggested as being a difficult time for birds. However, no support was found for this hypothesis. Small passerines wintering at high latitudes seem to be able to cope with high energy demand and the limited foraging time of winter.

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

SummaryBrood 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.

Habitat deterioration and efficiency of antipredator strategy in a meadow-breeding wader, Temminck's stint (Calidris temminckii)

Abstract Many populations of waders breeding on open shores and shores with short vegetation especially on the Baltic coasts have recently become endangered. The declines have taken place simultaneously with human-induced loss and deterioration of habitats due to eutrophication and overgrowth. To investigate mechanisms by which habitat changes could affect breeding success and ultimately population dynamics, we studied an endangered coastal population of Temmincks stint. We hypothesized that the rate of nest predation has become higher because the nest defence strategy (early detection of predator and early departure from the nest), which originally evolved in open habitats, is less effective on shores with reduced visibility. As predicted, nests survived better on wide than on narrow shores. Predation made a major contribution to this trend, although successful and predated nests did not differ in concealment at a microhabitat scale. The better the visibility from the nest, the longer was the flushing distance, but only in response to alarm calls or behaviour of other species, not when they were absent. Temmincks stints seem to obtain information about an approaching predator visually from sentinels. Therefore, it is essential that there is at least moderate visibility around the nest. We conclude that habitat characteristics – visibility from the nest and sentinel birds – affect the effectiveness of the nest defence strategy of Temmincks stint. These should be taken into account when seeking causes and mechanisms for declines of Temmincks stint and other waders of open and shortly vegetated shores.

Patterns of per-brood and per-offspring provisioning efforts in the Willow Tit Parus montanus

Nestling feeding behaviour of Willow Tits Parus montanus was studied in natural and artificially size-altered broods at Oulu, northern Finland. Per-brood provisioning effort was positively and per-offspring effort negatively correlated with brood size. Brood size manipulations (about + 30% relative to control broods) did not significantly affect the patterns of per-offspring provisioning effort, i.e. parents of enlarged broods could increase their per-brood effort significantly as compared with control broods. Thus, Nurs refined trade-off model of optimal nestling feeding was supported. On the other hand, no measurable extra costs (in physical condition, body mass or size) to parents of enlarged broods were detected; however, the young of enlarged broods had lower body masses than those of control and reduced broods confirming earlier results in the same population. Thus, in broods above the modal size parents primarily tried to ensure their own rather than their offsprings survival. At the beginning of the nestling period, when females brooded the young, male contribution to provisioning was higher. At the end of the nestling period male load sizes were larger but female visiting rates slightly higher, resulting in sexually equal total feeding effort. This was expected for a monogamous passerine with biparental care. Feeding effort increased with offspring age owing to the increased load sizes, increased female feeding rates and prolonged working days. However, the temporal increase in female visiting rates may be explained by the decreased brooding effort. A temporal increase in feeding effort is consistent with the corresponding increase in nest defence intensity, found in our earlier studies.

The dawn chorus is linked with female fertility in the Willow Tit Parus montanus

We studied the dawn chorus in the Willow Tit Parus montanus, a monogamous and territorial passerine. We expected that if the song is part of sperm competition, a male should invest most in singing during the period when his female is fertile. A total of 57 pairs of Willow Tits was observed during the spring of 1992. To express singing effort, two variables characterising the dawn chorus were measured for each morning: proportion of time devoted to singing and the onset of singing. Time singing peaked about ten days before commencement of laying, and decreased afterwards. The earliest singers were observed during the laying-period. Time singing differed between male age classes; old males sang more than yearlings. Our results indicate that the dawn chorus in Willow Tits functions as interand/or intrasexual communication used in sperm competition. Since adult (high quality) males tended to invest more in singing than yearlings, it is also possible that the dawn chorus is honest advertisement of male quality.

Song Rate Correlates With Paternal Care and Survival in Willow Tits: Advertisement of Male Quality?

Dawn song in birds may advertise male quality in terms of parental care and/or survival. Males who sing most may also be the best parents, and eager singing may also reflect good chances of a males or his mates survival. These relationships were studied in the willow tit Parus montanus, a species with a simple and relatively non-variant song. Song output was expressed as the proportion of time a male spent singing at dawn. Later in the breeding season, two aspects of parental care, nest defence and feeding effort, were measured in the same males. High song output was associated with more intense nest defence behaviour (eager singers attacked the predator model more often and approached closer to it) and high nestling feeding effort. However, reproductive success was not associated with song output, although males with high song rates tended to be more likely to produce recruits. Males who survived to the next year had higher song intensity than males who did not survive, while female survival was not correlated with song output of her mate. The results suggest that dawn song in the willow tit advertises male quality. Some aspects of honest advertisement and female choice are discussed.

Correlation between two components of parental investment: nest defence intensity and nestling provisioning effort of willow tits

Nest defence intensity and nestling provisioning effort of female willow tits (Parus montanus) were significantly correlated at the end of nestling period: well-fed young were defended most intensely. Increased effort was rewarded, since broods with the highest female per-offspring provisioning rates were the most likely to produce local recruits. This suggests that the feeding ability is an important cue for parental investment decisions, at least in a species like the willow tit which has adopted the clutch adjustment strategy. Thus, the most valuable broods would not necessarily be the largest ones, but the ones in which the original number of young could be fed most adequately. However, no associations were found between the level of parental effort and offspring weight, size or condition, nor did the broods producing recruits differ from other broods in timing of breeding or number and size of offspring. The female behaviour may suggest that they invest the most time, energy and risk in the young whose chances of joining the winter flock are the best. The first well-fed young also gain an advantage of prior residency in joining the flock. The first to join normally obtain higher social status, and hence better winter survival, than latecomers. The corresponding patterns in male parental investment behaviour were weak or absent, which suggested that the male effort was affected by the female behaviour. Males seemed to invest in nestling provisioning in such a way as to supplement the female effort. During nest defence action males also seemed to invest in protection of females against predation.

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.