Seppo Rytkönen

The design of artificial nestboxes for the study of secondary hole-nesting birds: a review of methodological inconsistencies and potential biases

Abstract. The widespread use of artificial nestboxes has led to significant advances in our knowledge of the ecology, behaviour and physiology of cavity nesting birds, especially small passerines. Nestboxes have made it easier to perform routine monitoring and experimental manipulation of eggs or nestlings, and also repeatedly to capture, identify and manipulate the parents. However, when comparing results across study sites the use of nestboxes may also introduce a potentially significant confounding variable in the form of differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. However, the use of nestboxes may also introduce an unconsidered and potentially significant confounding variable due to differences in nestbox design amongst studies, such as their physical dimensions, placement height, and the way in which they are constructed and maintained. Here we review to what extent the characteristics of artificial nestboxes (e.g. size, shape, construction material, colour) are documented in the ‘methods’ sections of publications involving hole-nesting passerine birds using natural or excavated cavities or artificial nestboxes for reproduction and roosting. Despite explicit previous recommendations that authors describe in detail the characteristics of the nestboxes used, we found that the description of nestbox characteristics in most recent publications remains poor and insufficient. We therefore list the types of descriptive data that should be included in the methods sections of relevant manuscripts and justify this by discussing how variation in nestbox characteristics can affect or confound conclusions from nestbox studies. We also propose several recommendations to improve the reliability and usefulness of research based on long-term studies of any secondary hole-nesting species using artificial nestboxes for breeding or roosting.

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.

PLUMAGE BRIGHTNESS OF PREY INCREASES PREDATION RISK: AN AMONG‐SPECIES COMPARISON

The risk to passerine birds of predation by the Sparrowhawk (Accipiter nisus) was analyzed with respect to differences in plumage brightness, body mass, and density of prey species, while taking into account phylogenetic relatedness of species. Data were collected from published sources in five separate areas along a south–north geographical gradient in Finland and consisted of 2214 prey remains collected from Sparrowhawk nests. Prey plumage brightness was the most important factor determining vulnerability to predation. In adults, male brightness was more important than female brightness in explaining prey vulnerability. Prey abundance did not affect the relationship between predation vulnerability and plumage brightness, because both rare and common species with bright plumage suffered higher predation. Prey species with large body mass were more vulnerable to predation in northern Finland, suggesting that in northern harsh conditions, Sparrowhawks may prefer large prey in order to fulfill their daily energy requirements. Our analyses provide evidence that predation may impose viability costs on sexually selected traits such as plumage brightness and body size.

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.

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.

Determinants of distribution and prevalence of avian malaria in blue tit populations across Europe: separating host and parasite effects.

Although avian malarial parasites are globally distributed, the factors that affect the geographical distribution and local prevalence of different parasite lineages across host populations or species are still poorly understood. Based on the intense screening of avian malarial parasites in nine European blue tit populations, we studied whether distribution ranges as well as local adaptation, host specialization and phylogenetic relationships can determine the observed prevalences within populations. We found that prevalence differed consistently between parasite lineages and host populations, indicating that the transmission success of parasites is lineage specific but is partly shaped by locality‐specific effects. We also found that the lineage‐specific estimate of prevalence was related to the distribution range of parasites: lineages found in more host populations were generally more prevalent within these populations. Additionally, parasites with high prevalence that were also widely distributed among blue tit populations were also found to infect more host species. These findings suggest that parasites reaching high local prevalence can also realize wide distribution at a global scale that can have further consequences for host specialization. Although phylogenetic relationships among parasites did not predict prevalence, we detected a close match between a tree based on the geographic distance of the host populations and the parasite phylogenetic tree, implying that neighbouring host populations shared a related parasite fauna.

Nest defence in great tits Parus major: support for parental investment theory

Abstract. Parental investment (PI) theory predicts that parents should invest more in large than small broods and high- than low- quality offspring. These predictions were tested in relation to nest defence behaviour of great tits, Parus major, in northern Finland. We manipulated future benefits as a function of investment in current offspring using food supplementation, which resulted in manipulated broods having a greater number of larger-sized fledglings than control broods. Supplemented parents engaged in riskier nest defence behaviour than did non-supplemented parents. The result was independent of parental condition, suggesting that the expected costs of provisioning did not contribute to the result. Thus, great tit parents appear able to adjust their nest defence behaviour to the expected fitness value of their offspring, as predicted by the PI theory. On the other hand, the large manipulation effect on brood value was possible only because the breeding success of northern great tits is exceptionally low (50–60% of eggs produce fledglings, cf. 90% in mid-latitudes). This is due to food limitation during the nestling period, suggesting that great tit clutch adjustment strategies are not adapted to the harsh northern environment.

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.

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.