Willow Tits Research Articles

Pseudo Concorde fallacy in the willow tit?

The predictions of optimal parental investment theory were tested by studying nest defence decisions of willow tits,Parus montanus. As predicted, parental defence intensity increased with offspring age, female parents defended early and middle-season broods more vigorously than late ones and the annual proportion of mobbers correlated significantly with annual means of clutch and brood sizes. Against the predictions, defence intensity was not adjusted to the quality (weight, size and condition) or quantity of offspring, which was also confirmed by brood size manipulation experiments. In this respect, willow tits seemed to commit the so-called Concorde fallacy. A suggested explanation for these non-adaptive responses is that a trait to determine the brood value continuously is not needed, since breeding success is high and predictable every year. In addition, in this population parents with reduced and enlarged broods had the same fitness as those with control broods. Therefore, parents behaved optimally when they did not change their defence intensity according to the manipulations. Thus, this lack of response to unnatural variation in a minor component of lifetime reproductive success could be considered a pseudo Concorde fallacy. These results suggest that parents are able to predict the forthcoming breeding success early in the season, and that this prediction holds quite precisely, with parental investment levels being relatively equal regardless of clutch and brood size.

Food Caching By Willow and Crested Tits: A Test of Scatterhoarding Models

In coniferous forests of Central Finland, Willow (Parus montanus) and Crested Tits (P. cristatus) store seeds in a scattered distribution within their territory during the autumn. Individuals cache and recover food items while moving together as members of mixed—species flocks. The purpose of this study was to test certain predictions of scatter—hoarding models (Stapanian and Smith 1978, Clarkson etal. 1986), which predict how the animal should hoard food items from a superabundant source to maximize the number of caches recovered. Our field experiments gave support to most of the predictions of the models. Individual tits stored seeds closer to the food source when food had been available for first time (new feeder with seeds not previously available) than to feeders with seed available for 1 mo. Individual birds of both species carried large food items (hulled sunflower seeds) father than small items (spruce seeds) before caching them. Crested Tits distributed their catches uniformly around the feeder while most of the Willow Tits favored certain sectors around the feeder. Subdominant individuals carried seeds the same distances as did dominant birds when the food source was ephemeral, but farther from a continuously available food source.

Differences in mate guarding between age classes in the willow tit, Parus montanus

Differences in mate guarding between age classes in the willow tit, Parus montanus

Differences in mate guarding between age classes in the willow tit, Parus montanus

Differences in mate guarding between age classes in the willow tit, Parus montanus

Alarm calling by willow tits, Parus montanus, as mate investment

The response of captive willow tits to a life-like, stuffed sparrowhawk, Accipiter nisus, was studied during four consecutive winters in Central Norway. Birds from 31 flocks, usually consisting of one adult mated pair, with two first-year males and two first-year females (non-kin), were studied in an outdoor aviary. When the sparrowhawk, kept 4 m away out of sight of the tits, was suddenly presented to individual birds, all the tits froze and some gave alarm calls. Adult birds gave alarm calls more frequently than juveniles, adult males more than juvenile males, and adult females more than juvenile females. Adult males gave an alarm call more frequently when they could see their mates than when they could not and alarm calling may therefore be a form of investment in the mate.

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.

Causes of divorce in the monogamous willow tit, Parus montanus, and consequences for reproductive success

Abstract. Divorce rates were measured in a population of willow tits, in which the size of some broods was manipulated. The population, with annual survival rates of ca 63 and 64% for females and males, respectively, had a relatively low divorce rate of 12·0%. The experiments on altered breeding success did not support the hypothesis that poor success is likely to initiate divorce. Most variation in reproductive success of the pairs differing in breeding history was attributable to the age of the female, yearling females being least successful. Yearling birds split up more often than older birds. Divorced females usually paired with older, and divorced males with younger, mates than their previous ones. With a new mate, divorced females bred successfully and advanced their laying date more than widowed females indicating that divorce was beneficial. The success of divorced males was low and some even remained unpaired in the season after splitting up. Divorce thus seems to be a strategy chosen by females to increase future reproductive success. The decision seems to be initiated when a female can obtain a higher-ranked mate among the birds in her own or in a neighbouring winter flock.

Selection on the genetical and environmental components of tarsal growth in juvenile willow tits (Parus montanus)

AbstractWe studied selection on tarsus length among first year willow tits Parus montanus in relation to environmental and genetical influences on growth. The main environmental influence on growth was a cohort effect. We also found a substantial heritable component of phenotypic variation for tarsus length (h2 = 0.61), and crossfostering in one year showed no shared environment effect which could account for parent/offspring resemblance. The deteriorating conditions for growth later in the season did not confound our heritability estimates as the time of laying was not correlated to parent size, and no maternal effects operating through egg size were found. We tested for selection during the summer dispersal phase by comparing tarsus length among fullgrown pulli (age 14 days, controlled by repeated measurements of the same individuals later during breeding and the ensuing winter) and the tarsus length of the first year cohort in autumn composed of a mixture of locally born birds remaining within our study area after the dispersal phase and immigrants born outside the study site. Following a season with poor nestling growth, birds with short tarsi were selected against when underweight, suggesting that growth condition is the target of selection. Such selection on those individuals which show the strongest environmental influence on phenotypic variation will reduce the potential for an evolutionary response to selection.

Low Temperatures Affect the Photoperiodically Induced LH and Testicular Cycles Differently in Closely Related Species of Tits ( Parus spp.)

Photosensitive great tits ( Parus major) and willow tits ( P. montanus) were exposed to long days (20L:4D) under three different temperature conditions (+4, + 10, and + 20°C) in early winter. The two species showed significant differences in their LH and testicular reaction patterns to low temperatures. Testes showed pronounced growth cycles under all temperature regimes. For the willow tit, testes in birds kept at +20°C reached maximum size about 2 weeks earlier than testes in birds living under the two lower temperature regimes, whereas in the great tit testes reached maximum size at about the same time in all three groups. Low temperatures delayed the onset of testicular regression in both species. Plasma levels of LH did change with time in both species. However, the patterns of the induced LH-cycles in the three great tit groups differed significantly from each other, whereas this was not the case for the willow tits. The LH cycle was especially pronounced in great tits kept at +20°C. The initial LH peak in great tits kept at +4 and +10°C was about 50% lower than in great tits kept at +20°C. These results are discussed in relation to species differences in winter ecology and establishment of breeding territories.

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.

Separation of caches between individual willow tits hoarding under natural conditions

Abstract. Individual variation in foraging and hoarding locations of willow tits, Parus montanus, was studied for 3 years during the main hoarding period in the autumn. Caching locations of different individuals were separated vertically; this was especially clear between males in the same flock. Spatial separation of caches for individuals could make hoarding evolutionarily stable even if specific caching locations are not remembered. Pine, which is the most important foraging tree during the winter, was preferred for hoarding more than for foraging during the autumn. Seeds were hoarded more frequently than the other major component in the diet, larvae. These characteristics may be adaptations for large-scale long-term hoarding of the kind that has been reported from field studies of boreal tits.

The disappearance of caches that have been stored by naturally foraging willow tits

The disappearance of caches that have been stored by naturally foraging willow tits

The role of naturally stored food supplies in the winter diet of the boreal Willow Tit Parus montanus

The average amount of food that was stored during an autumn by Willow Tits Porus montanus was estimated by sampling hoarding intensities and analysing the energetic contents of the stored items. Measurements of retrieval and losses of the stored food made it possible to calculate how much of the stored food that actually was used by the hoarder. Also the chemical composition of the stored food was analysed, and a comparison of the energy contents of the food eaten with the energy requirements of the birds made it possible to assess the importance of hoarded food in the winter diet. According to the calculations, hoarded food can be assumed to account for almost all food that was consumed during December, whereas it was estimated to constitute around 24% of the food eaten from January to March. During colder winters, with higher energetical stress, it is possible that stored supplies could be more important during this part of the winter, perhaps of the magnitude of 45% of the total food consumed.

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

We 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.

Ontogeny of the Vocal Repertoire in the Willow Tit Parus montanus

Results of a long-term study of the Willow Tit Parus montanus population in the birch region at Venabu, Ringebu (61?39'N, 10008'E) in Central Norway, are presented. After fledging, the juvenile Willow Tits were dependent on their parents for another 2-3 weeks. During that period, they developed their vocabulary, which was nearly complete before independence. The syntax and ontogeny of eight main vocalizations (including the song) are described and illustrated by spectrograms. Non-aggressive calls developed before those with an aggressive content, the latter being postponed to the end of the period of parental dependence, or immediately thereafter. This pattern is probably adaptive. For comparison, also the corresponding adult vocalizations are described and illustrated by spectrograms. Besides the main sounds, a special puretone note, which evidently is restricted to the Willow Tits at Venabu and maintained in the deme by learning, is described. It is suggested that subsong has an important function in the process of vocal development.

Sex-role reversal in willow tit nest defence

The roles of sexes in the nest defence behavior of monogamous willow tit (Parus montanus) were studied near Oulu, northern Finland, in 1988–1990. The relative roles of the sexes changed during the breeding cycle: females defended their nests more vigorously before hatching and males defended more after hatching. This sexual asymmetry was studied by means of the cost/benefit model of optimal parental investment (PI). Because of the monogamous breeding system, sexual differences in future benefits were not a likely explanation for the asymmetry. This was also supported by preliminary results of DNA-fingerprinting analyses. Behavioral observations — sex-role reversal, high correlation between the mates, and equal variations in male and female behavior — indicated the same. In the beginning of the breeding cycle, higher female defence was related to renesting potential. The extra costs of renesting are considerably higher for females than for males, and therefore, females appeared to benefit more than males in keeping the first clutch alive while renesting was still possible. Such a female strategy was possible in this northern population, since the time for renesting was rather short and never lasted beyond the hatching of the first nests. After this point, the male can only invest in the present clutch. By the end of the breeding cycle, increased defence of the nest by males may be a consequence of males being larger and/or in better condition than females. Therefore, in the latter half of the breeding cycle, the sexual differences in nest defence did not indicate any sexual difference in PI. The asynchronous arrival of the parents at the nest also affected individual responses, indicating that defending the nest is a dynamic process.

Is the Coal Tit Parus ater Really the Most Subordinate of the Scandinavian Tits?

were usually successful in their supplanting attacks, but the dominance order between the two species was not absolute and the number of interspecific interactions were few. As emphasised by the authors, it is necessary to consider how applicable the results from birds held in captivity are to the natural situation. In this paper I present the results of interspecific encounters between five tit species visiting an outdoor feeder in central Norway. In general, the following social rank order was observed: Great Tit P. major > Blue Tit P. caeruleus > Marsh Tit P. palustris > Coal Tit > Willow Tit. The Coal Tits showed a statistically significant tendency to be dominant over the larger Willow Tits, although not in all cases. Nevertheless, it seems that the Coal Tit's status in the social hierarchy requires consideration in the current discussion on the interspecific competition for food and foraging sites among species belonging to the tit guild of the coniferous forest.

The throat badge as a status signal in juvenile male willow titsParus montanus

Status signalling in juvenile male Willow TitsParus montanus was studied by observing the outcome of interactions of birds with different size of their black throat badge, and by manipulating the size of their badge. The bird with the largest throat badge was the dominant one in all dyads studied (N=25), and the relative rank position of the males was positively correlated with the size of their badge. The badge size explained about 95% of the variation in the birds' dominance rank. When confronting juvenile males with a stuffed Willow Tit mounted close to a feeder placed in an aviary, there was found a negative correlation between the size of the badge of the males and the time from they were released till they visited the feeder. After subordinate birds from seven earlier dyads got their throat badge artificially increased, the originally dominant bird reacted submissively and tried to avoid the bird with a larger dyed badge, that temporary had increased its rank position. Our data support the status signalling hypothesis.

Characteristics of Cache Sites Most Likely to Be Robbed by Willow Tits (Parus montanus)

Characteristics of Cache Sites Most Likely to Be Robbed by Willow Tits (Parus montanus)

Low Rate of Loss of Willow Tit Chaches May Increase Adaptiveness of Long-Term Hoarding

Low Rate of Loss of Willow Tit Chaches May Increase Adaptiveness of Long-Term Hoarding