Female choice and the quality of parental care in the great tit Parus major

Summary Previous work on great tits suggested that female mate choice was based on the characteristics of the male rather than the quality of his territory. The aim of this paper was to see whether females were deriving any direct benefits as a result of such choice by comparing male plumage colouration (the size of the central black breast-stripe) with the quality of parental care provided by both members of the pair. Males with large stripes were more likely to defend their broods because their nest attentiveness was higher increasing the chances of predator detection. Males with large stripes produced heavier fledglings primarily as a result of faster growth early in the nestling period. Previous work on great tits has shown that heavier fledglings have a higher survival probability than lighter ones. The results of this study can more easily be explained by differences in parental quality between males rather than as a result of variation in female or territory quality. Therefore, female great tits could increase their reproductive success by pairing with males with a large stripe because such males appear to be better quality parents. This suggests that female choice in the great tit may concern, at least in part, the quality of male parental care.     

Determinants of brood defence in the great tit Parus major L.

Summary Great tits (Parus major) tending nestlings reacted defensively to a live predator (Glaucidium perlatum; domestic cat) and the playback of a mixed species mobbing chorus, or to the latter alone. Defensive behaviour, mainly mobbing, reflected the risk taken and is assessed by five measures. Multivariate and contingency analyses revealed that at least 11 of 16 contextual independent variables affected the risk taken. Incremental effects are due to: Age of young, sex of the defending bird, the expected number of neighbouring mobbers, low temperature, wet canopy, the raptor's distance from cover, coniferous forest, advancing season. A decremental effect is exerted by a large brood that is older. Annual differences in defence arise probably from demographic factors such as fecundity, which in turn affect the parent's benefit-cost ratio (number of young of the same sex as the parent/residual reproductive value of the parent).While the effects of annual fecundity, age of young and season were predicted on the basis of this benefit-cost ratio, the failure to verify an incremental effect of brood size runs counter to established theory. We conclude that parents gear their defence efforts to energy investment, past or future, and are mal-adapted to brood size as a promotor of risk taken. The influence of the habitat is poorly understood. At least three factors (age and number of young, parent's sex) act additively on part of the response. Despite the large number of variables examined, about 43% of the total response variance remains unexplained.While four defence measures are determined by at least 10 contextual factors, a fifth measure, the male's minimum distance from the raptor, is determined by one other factor, the appearance of the male. The latter leads us to assume an additional, social rôle of brood defence.Risk-assessment by great tits leading to risk-aversive defence behaviour is governed by evolved restraints rather than by momentary constraints. Examples are provided by the effects of weather and cover.     

Interspecific observational memory in a non-caching Parus species,the great tit Parus major

Scatter-hoarding passerine birds have become a model system for research on spatial memory capacity. This research has focussed on two families, the Corvidae (crows, jays, nutcrackers, etc.) and the Paridae (titmice and chickadees). Corvids are considered to have highly developed cognitive skills that sometimes have been compared with the great apes. Even though pilfering, or stealing of caches made by others, is common among scatter-hoarding birds, the ability to memorize positions of caches made by others has only been demonstrated in some species of corvids. In parids, the ability to memorize positions of caches made by others has not been demonstrated. In a laboratory experiment, we allowed caged great tits to observe caching marsh tits and found that they remembered caching locations both 1 and 24 h after observation. This is the first time observational spatial learning of this type explicitly has been demonstrated in a parid. This ability is surprising since the great tit is not itself a food hoarder, meaning that it may not have the special memory adaptations in the brain that specialized food hoarders possess. Also, the memorization process in an observing pilferer may differ from the memorization that hoarders make of own caches. For example, the typical close inspection of the cache that hoarding parids do after caching will usually not be possible from a distance. Also, the visual perspective of the observing scrounger may be different from that of the hoarder, meaning that some understanding of allocentric space may be required.     

Male quality,reproduction, and survival in the great tit (Parus major)

Summary Three possible measures of male quality (social dominance, song, and size), reproduction, and survival were studied in a single population of great tits. Winter dominance position on a feeder was related to strophe length (number of phrases per strophe), inversely related to positive drift (decrease of the singing rate of the phrases in a strophe), but not related to song repertoire size. Neither winter dominance position nor song were related to size (wing length, tarso-metatarsus length, weight).Singing capacity was not correlated with individual reproductive success in a single breeding season, using a rather limited data set. However, better singers (males which sing longer strophes, show less positive drift, and have larger song repertoires) survived better and had a higher individual lifetime reproductive success (on the basis of a male's recruited offspring of all breeding seasons). Our results show that there exist measurable differences whereby birds that are dominant in winter sing better, survive longer, and produce more surviving offspring during their life time. We suggest that differences in male quality are the common cause (direct and indirect) of all these effects.     

Sound degradation as a distance cue in great tit (Parus major) song

Summary We report an experiment designed to test the ideas that: 1. male songbirds can use cues from the distortion of song by environmental factors (degradation) to estimate the distance of another singing male; 2. song degradation is assessed by reference to an internal standard. Great tits respond more strongly to undegraded than to degraded songs when both are played at the same amplitude and from the same position in the territory. This difference in response is shown only if the playback song is familiar to the test bird; familiar songs being those sung either by the test bird or neighbours of the test bird. We interpret these results as evidence that cues from song degradation can be used to estimate the distance of a singing conspecific and that degradation assessment is only possible if the bird has an internal representation of the song (because either it and/or a neighbour sings the song). We discuss the implications of these results for Morton's (1982) ranging hypothesis, and for the distinction between learning and performance in bird song. Our results partially support the ranging hypothesis, but question the nature of unrangeable songs sensu Morton. The finding that birds can assess the degree of degradation of songs that they do not sing, supports the idea that birds learn more songs than they sing.     

Dominance,prior occupancy,and winter residency in the great tit (Parus major)

Summary This study reports an aviary experiment aimed at determining what affects social dominance in the great tit (Parus major), especially why older birds (adults) in nature normally dominate younger ones (juveniles). When birds were matched with respect to age, prior residency determined dominance. Without a difference in prior residency older birds dominated younger ones. However, when juvenile birds had a prior residency advantage over adult birds, they often became dominant. This was especially so when the juvenile bird was large relative to the adult bird. When a resident juvenile male was also consorted by a female, he became dominant over an adult male on most occasions. An experiment where the dominant bird was removed and later returned to the aviary failed to produce more than one shift in dominance. However, the proportion of reversals in dominance interactions increased with separation time. It is argued that the fact that dominance depends on prior residency selects for winter residency in the great tit.Offprint requests to: H.G. Smith     

Feeding frequency and parental division of labour in the double-brooded great tit Parus major

Summary We studied the relative contribution of each sex and total effort expended in feeding nestlings in the great tit Parus major in relation to artificially altered brood size. A recent model suggests that feeding frequency should reflect the optimal trade-off between parental and fledgling survival, the former being negatively, the latter positively, influenced by high feeding frequencies. In both sexes weight loss was linearly related to feeding frequency. Since fledgling survival increases with nestling weight, the conditions of this model are fulfilled. However, in contrast to the predictions of the model, the total feeding frequency for both sexes combined did not differ between control and enlarged broods, but was lower for reduced ones. This outcome was not the result of a physiologically related inability of the parents to increase their delivery rate. Instead, we suggest that parents with enlarged broods could not find sufficient amounts of prey large enough to be economically worth transporting to the nest. Differences in brood-provisioning rates between the sexes may arise because costs and benefits of feeding nestlings may differ. Females lost more weight than males during the nesting period, but maintained a relatively higher weight during the incubation period. The relationship between weight loss and feeding frequency was similar for both sexes. Male and female brood-feeding frequency was related to brood size in a similar way. This is discussed in light of the great tit's mating system and the fact that the great tit is facultatively double-brooded.     

Brood defence in the great tit (Parus major): the influence of life-history and habitat

Summary We examined the extent to which parental investment, as measured by brood defence, is determined by life-historical selection in a shortlived bird, the great tit (Parus major). Pairs tending first (n=20) and second (n=21) broods in the same Scots pine woods in 1983 were used to test predictions of a cost/benefit model of brood defence based on the species' average demography in coniferous forest. Furthermore, the differences in demography between pine and deciduous forest permitted us to test whether habitat-specific life-history would affect the seasonal pattern of defence. In the model, benefit was defined as the brood's potential contribution to a parent's fitness, and the cost as the potential loss if the defender dies in the act of defence. Univariate and multivariate procedures were applied to six measures of defence response to a live owl (Glaucidium perlatum) plus a taped mobbing chorus. Results proved three of the model's predictions to be false. In coniferous forest, neither the overall strength nor the individual variance of defence behaviour differed among first and second broods, nor was there any consistent difference in defence strength between pairs living in coniferous (n=54) or deciduous (n=84) forest as revealed by comparisons within each brood. These failures could be reconciled with the model by assuming that selection in the past had acted via the average demography of both types of habitat. The model received direct support from defence strength increasing with age of young and, more forcefully, becoming more influenced by brood size in second broods, regardless of habitat.A difference in the strength of defence by the male and female suggests two more functions of behaviour: In first broods, the male risks more than the female as measured by five of six variables. This suggests that defence is facultatively linked to the need for territorial protection from predators all the year round. In the female's presence, the male, taking an additional risk, approaches the owl to half the distance of that of a single, though paired, male, suggesting an additional, social role of defence behaviour.Taken together, anti-predator defence in the great tit serves to protect the brood, the home range and, in the male, the female mate. The magnitude of the benefits envisaged varies among the sexes.     

Territorial defence in the great tit (Parus major): Do residents always win?

Summary I removed resident pairs of great tits from their territories for short periods and released them after replacement pairs had occupied the spaces. When two pairs are manipulated in this way into occupying the same territory an escalated contest ensures. Contests between residents and replacements are longer, more likely to involve physical fights and longer fights or displays than contests between established neighbours or residents and intruders. The degree of escalation between residents and replacements is an increasing function of replacement time. It rises to a peak and then diminishes again. The probability that the replacement will defeat the former resident is an increasing function of replacement time, reaching 90% after several days. These results are consistent with the hypothesis that territorial residents win in contests against intruders because of an asymmetry in payoff rather than an asymmetry in resource holding potential or an arbitary convention. A possibly asymmetry in payoff in the great tit is the cost of defending the territory against neighbours. The cost is higher for replacements than for residents.     

Parental feeding rate in relation to begging behavior in asynchronously hatched broods of the great tit Parus major

Summary Variations in the begging behaviour of the nestlings of altricial birds can provide the parents with information about the nestlings' nutritional needs and thus influence the parental feeding rate. In a series of four experiments, the stimulus situation encountered by great tits on their feeding visits to the brood was manipulated to explore its effect on feeding rate.A higher feeding rate was observed under the following conditions: (1) after a period of food deprivation, as compared with both normal conditions and satiation through artificial feeding; (2) in periods when recorded begging calls were played during feeding visits, as compared with control periods; (3) after temporary removal from the nest of heavier, as compared with lighter, siblings. The lighter nestlings in the brood benefitted more —in terms of gain in weight — from the increase in parental feeding rate following the playing of begging calls than did the heavier nestlings. Differences in weight spread within broods did not affect the amount of food the parents brought.We conclude that parental feeding rate is affected not simply by the begging of the hungriest nestling, but rather by the behaviour of all the nestlings in the brood, which makes possible an adjustment of the feeding rate to the average hunger level of the brood. The effects of hatching asynchrony and sibling competition on parental feeding rate are discussed.     

The unsung songs of great tits (Parus major): learning neighbours' songs for discrimination

Summary Many territorial birds are capable of recognising neighbours from their songs. In this paper we investigate the timing of learning of neighbours' songs for recognition. This is the first time that the timing of discrimination learning has been studied explicitly by experiment in the field. We show that discrimination learning is not limited to early life and contrast this with the timing of song learning for performance. We also show that the ability to recognise new neighbours is inversely related to the number and similarity of neighbour's songs which have previously been experienced. This suggests pro-active memory interference and provides the first field evidence for memory constraints on discrimination.     

Optimal foraging and territorial defence in the Great Tit (Parus major)

Summary We describe an experiment designed to investigate the trade-off between foraging and territorial vigilance in the Great Tit. Captive territorial male Great Tits were observed while foraging in a large indoor aviary. They obtained food from two operant patches in which the supply of food was gradually depleted during a visit. We predicted that during control sessions the birds would switch between patches in such a way as to maximise their overall feeding rate. In experimental sessions, we introduced briefly a rival male as an intruder at the start of the test. The foraging male could see the rival only when travelling between patches and not while feeding within patches. We predicted that during experimental sessions birds would switch between patches more often than in control tests, sacrificing food intake for territorial vigilance. Three of the four males tested behaved in approximately the predicted manner. We discuss the use of an inverse optimality argument to provide a calibration of feeding against the benefit resulting from territorial vigilance.     

Habituation and song repertoires in the great tit

Summary This paper is concerned with the idea that the song repertoires of passerine birds are an evolutionary adaptation to reduce habituation in listeners. In an experiment involving 16 territorial males I played either a single song or a repertoire of songs for 15 two-min trials through a single loudspeaker near the edge of the territory. In a second experiment with 10 birds I played the songs through one of two loudspeakers in different parts of the territory, alternating between loudspeakers on successive trials. The birds tended to habituate more rapidly to single song playback than to repertoires. In the second experiment the overall level of habituation was lower and the difference between the two treatments was less marked.Two features of song repertoire organisation are consistent with the habituation hypothesis (1) the avoidance of low recurrence intervals in switches between song types, and (2) the fact that within repertoire variability is as great or greater than between repertoire varability.The main problem with the habituation hypothesis is that habituation by listeners does not seem to be adaptive, so it is not clear why they should habituate. I suggest a hypothesis. Intruders may assess the density of birds in an area by listening to songs, so that habituation may be a mechanism by which this density assessment is achieved. Repertoires could be a mechanism by which resident birds cheat, through increasing the apparent density of singing birds.     

Parental effects on carotenoid-based plumage coloration in nestling great tits, Parus major

Carotenoid pigments have attracted much interest in behavioural and evolutionary ecology because of their dual function in immune physiology and as color signals. In vertebrates, carotenoids must ultimately be obtained from the diet, and the mechanisms and magnitude of this environmental dependence are central for understanding carotenoid signal functions and evolution. In the present cross-fostering experiment with great tits Parus major, we investigate pre- and postnatal parental effects (egg yolk carotenoids, parental coloration) on nestling size and carotenoid coloration, using HPLC analysis of egg yolk carotenoids, and a reflectance-based measure of ‘chroma’ that reflects the plumage pigment concentration. Both rearing environment and origin influenced offspring size and plumage chroma. Maternal allocation of carotenoids to eggs had a weak positive effect on nestling plumage chroma, whereas we found no prenatal maternal effects (egg size or yolk carotenoid concentration) on size. Nestling plumage chroma was also significantly predicted by the chroma of the rearing father, but not by the color of the rearing mother or either of the original (genetical) parents. Thus, both prenatal maternal effects and postnatal paternal effects influence the carotenoid-based plumage coloration of nestling great tits. Future studies will reveal if parental effects have long-term consequences for plumage development and associated fitness components.     

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

Summary 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.