Kimmo Lahti

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.

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.

Presence of conspecifics, time of day and age affect willow tit food hoarding

Abstract Willow tits, Parus montanus , spend the winter in flocks and hoard food especially in the autumn to be used later. For caching to be an evolutionarily stable strategy, the hoarder should be more likely to retrieve its caches than conspecifics. The effects of individual differences, the time of day and, especially, the presence of conspecifics and heterospecific birds on caching behaviour of free-living willow tits were examined with a logit model. The presence of conspecifics had the strongest influence on caching: if there were other willow tits near the observed individual, it was less likely to cache. Willow tits stored food at a greater intensity in the morning than in the afternoon. Yearlings stored more intensively than adults. Individuals differed in their caching intensities. However, there were no significant interactions between the factors investigated, indicating, for example, that individual birds responded similarly to the factors studied. The presence of hetero-specific Paridae or non-Paridae had no effect on the caching rate in the willow tit. It is possible that conspecifics are perceived as potential kleptoparasites (immediate threat) or pilferers (long-term threat) and therefore avoided when hoarding.

Dominance, Daily Activity and Winter Survival in Willow Tits: Detrimental Cost of Long Working Hours?

Predation risk can vary both in space and in time. Dawn and dusk may present greater risk to diurnal birds, since their vision is not well adapted to dimness and both nocturnal and diurnal predators may be active at these times. If the birds are not time-limited, selection should favour activity patterns where crepuscular activity is avoided. Individuals in dominance-structured groups may differ in their time-limitation. The roost-entering and emerging times of willow tits (Parus montanus) spending the winter in flocks with a social hierarchy were examined. In early winter, dominants initiated their daily activities later and roosted earlier than subordinate individuals. As a result, the duration of roosting was longer for dominants than for subordinates. By the next breeding season, significantly more subordinates than dominants had disappeared (presumably died), and the individuals which disappeared had the longest days. There were no sex differences in daily activity in early winter, but in late winter, males emerged earlier in the morning than females. A feeding experiment revealed that the birds were restricted by food availability: the fed birds reduced their roosting period less than simultaneous controls. The greater change in the controls is understandable, because the days grew longer (and the roosting period thus shorter) during the experiment. We suggest that the longest active birds were forced to extend their day in order to obtain sufficient food, which perhaps resulted in greater mortality through predation.

Seasonality in daily body mass variation in a hoarding boreal passerine

We studied the body mass variation from autumn to winter, in a free-living population of willow tits (Parus montanus), a food-hoarding passerine living year-round in boreal forests. Our aim was to find out whether this population exhibits ‘winter fattening’ as part of the annual body mass cycle. ‘True winter fattening’ is considered to be a strategic response to winter conditions. The strategy includes an increase in both the morning mass and the daily mass increase, as winter approaches. A multivariate approach was used to find which predictors (year, date, age, sex, body size, temperature and snow depth) explained the mass variation in birds measured twice per day. Morning mass variation was explained by sex, age, wing length and snow depth. Independently, date explained morning mass variation only in adult males. None of the predictors explained the variation observed in daily mass increase in any age or sex class. Therefore, we failed to detect winter fattening in our study population of willow tits. Response to increasing night length is not due to higher absolute intake, but to higher energy acquisition rate and decreased night-time energy consumption. The results suggest that willow tits at high latitudes manage increasing energy demands on a short-term basis and respond flexibly to changing conditions by adjusting foraging efficiency and especially night-time energy expenditure.

Is the social hierarchy always linear in tits

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Sex-specific mortality costs of dispersal during the post-settlement stage promote male philopatry in a resident passerine

The costs and benefits of dispersal that select for sex-biased dispersal are still poorly understood. Many studies examine fitness consequences of dispersal after first breeding, while dispersal costs are most likely paid before first breeding during the movement, settlement, and post-settlement stages. We studied survival correlates of dispersal between flock settlement and first breeding during the first winter of juvenile willow tits (Poecile montanus), a small passerine that has female-biased natal dispersal, but shows no dispersal-associated survival differences after first breeding. This resident food-hoarding species winters in small stable non-kin territorial flocks. We collected capture-recapture data by following flocks from autumn to the following spring. We compared monthly survival and return rates of juveniles that were born and recruited within the study area (philopatric juveniles) and juveniles that originated from outside the study area (immigrant juveniles). Among males, survival was highest for philopatric juveniles whereas survival of females was higher among immigrant juveniles, providing one explanation for the female-biased natal dispersal observed in the species. Philopatric males may benefit from prior residency either through increased site familiarity and knowledge of winter food resources and/or by gaining higher social ranks during flock establishment. However, rank data provided little support for the latter hypothesis. Other mechanisms such as increased ability to find high-quality flocks and mates may be important for female survival. Our results provide further evidence that dispersal costs are paid mainly before first breeding and that sex-specific costs of dispersal play a role in the evolution of sex-biased dispersal.Significance statementThis paper shows that female-biased dispersal can be a consequence of sex-specific costs and benefits of dispersal during the post-settlement stage of the dispersal process which is a very poorly understood stage in dispersal theory. By examining correlates of dispersal before rather than after first breeding as it is usually done, our study aids in understanding the selection pressures modifying dispersal strategies. Our results have wide applicability because there are many resident taxa similar to the willow tit (our study species) that have a settlement stage and a prolonged non-reproductive phase before their first reproduction.