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.     

Recognition of individual neighbors by song in the song sparrow,a species with song repertoires

Summary Previous theory and research have suggested that bird species with song repertoires in general, and song sparrows (Melospiza melodia) in particular, cannot readily discriminate between the songs of neighbors and strangers. In a recent study (Stoddard et al. 1991) we showed that song sparrows can in fact discriminate neighbors from strangers on the basis of song. In this study we sought to demonstrate that song sparrows can make the finer discrimination between individual neighbors and that they can do so on the basis of a single song type. We compared the response of territorial males to song playback of neighbors and strangers at three locations: the neighbor's regular boundary, the opposite boundary, and the center of the territory. The birds showed strong neighbor-stranger discrimination at the regular boundary but not at the opposite boundary, nor in the center of the territory. The differences in song discrimination between different boundary locations indicate that song sparrows associate particular songs with particular territories, effectively discriminating between individual neighbors on the basis of song. Song repertoires themselves do not interfere with neighbor recognition to the extent originally postulated. As speakers are moved inside the territory from the border, however, the degree of discrimination diminishes. We believe that differences in speaker placement may have contributed to the variability in neighbor-stranger discrimination observed in previous studies of the song sparrow and perhaps other repertoire species as well. This interpretation is consistent with data from another song sparrow population showing that half the territory takeovers are by immediate neighbors.     

Reproductive strategy and singing activity: blue tit and great tit compared

The costs and benefits of bird song are likely to vary among species, and different singing patterns may reflect differences in reproductive strategies. We compared temporal patterns of singing activity in two songbird species, the blue tit (Cyanistes caeruleus) and the great tit (Parus major). The two species live side by side year round, and they have similar breeding ecology and similar rates of extra-pair paternity. However, they differ in two aspects of reproductive strategy that may have an influence on song output: blue tits are facultatively polygynous and have a fairly short breeding season with almost no second broods, whereas great tits are socially monogamous but more commonly raise second broods. We found that great tit males continued singing at high levels during the egg-laying and incubation periods, while monogamously paired blue tit males strongly reduced singing activity after the first days of egg-laying by their female. Since males of both species sang much more intensely shortly before sunrise than after sunrise, at midday or in the evening, this difference was most conspicuous at dawn. No differences in singing activity were found within species when testing for male age. We suggest that in contrast to blue tits, great tit males continued singing after egg-laying to defend the territory and to encourage the female for a possible second brood.     

Individual recognition of song repertoires in two wood warblers

We examined recognition of individual neighbors in hooded warblers (Wilsonia citrina) and Kentucky warblers (Oporornis formosus) by comparing responses to playbacks of neighbors' songs from appropriate and inappropriate boundaries of the subjects' territories. Kentucky warblers, which sing one song type each, responded more strongly to the songs of neighbors broadcast from incorrect than from correct boundaries. Hooded warblers use their repertoires of three to nine song types in two distinct modes of singing: repeat mode involves repeated presentation of one song type; mixed mode involves presentation of two or more song types in irregular sequence. Playbacks of neighbors' mixed-mode songs from appropriate and inappropriate boundaries indicated capabilities of individual recognition similar to those reported previously for repeat-mode songs (Godard 1991) and to those of Kentucky warblers. Repertoires of moderate size, therefore, have no pronounced influence on eventual recognition of individual neighbors in hooded warblers.     

The conflict between feeding and territorial defence in the great tit

Summary A field experiment investigated whether feeding and territorial defence competed for time in the activity budget of territorial male great tits during the spring. Feeding tables were placed on the territories of five males. Five other territorial males also fed at these tables, while ten additional males had no access to any feeding tables. A standardised intrusion using playback of territorial song and a stuffed mount of a male great tit was conducted on each of the twenty territories, and the response of the resident male measured. All of the males with access to a feeding table responded more vigorously to the intrusion than the males who received no extra food. These results are not attributable to the fact that males defended their territories more vigorously because of the addition of feeding tables, since males that gained extra food outside the boundaries of their own territories also defended more vigorously than birds with no access to a feeding table. The results support the conclusion that the provisioned males defended more vigorously because they could afford to take more time out from feeding.     

Temporal patterning of within-song type and between-song type variation in song repertoires

In many songbird species, males sing a repertoire of distinct song types. Song sparrows typically are described as having repertoires of about a dozen song types, but these song types are themselves quite variable and some songs are produced that appear intermediate between two types. In this study we quantify the similarity between successive songs produced by song sparrows in order to determine if differences between song types are emphasized or deemphasized in bouts of continuous singing. In spite of the high degree of variation within song types and similarity between song types observed in this species, we show that transitions from one song type to the next are distinct as compared to transitions within sequences of the same type (Figs. 4 and 5). Variation does not accumulate across sequences of the same song type, and the average variation observed within a continuous sequence of the same song type is significantly less than is predicted from the total variation recorded for that type across many different bouts (Fig. 6). These results support the view that song sparrows include two levels of variation in their singing: differences between song types as is commonly observed in species with song repertoires, and an independent level of variation observed for songs of the same type.     

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.     

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.     

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.     

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.     

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     

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.     

Yolk androgens in great tit eggs are related to male attractiveness,breeding density and territory quality

Females can adaptively adjust phenotype of their offspring via deposition of various compounds into eggs, including androgens and other hormones. Here, I investigated how egg yolk androgens (testosterone and androstenedione) related to environmental conditions and parental traits in the great tit (Parus major) across three breeding seasons. Male and female traits studied included age, condition and multiple feather ornaments, both carotenoid- and melanin-based (carotenoid and UV chroma of yellow breast feathers, area of black breast band and white cheek immaculateness). Yolk mass increased with laying temperature, laying date and area of male black breast band. Concentration of androgens increased with breeding density, territory quality and carotenoid chroma of male yellow breast feathers and was higher in mates of 1 year old as compared to older males. Yolk androgens were not related to any of the female traits analysed. These patterns were thus consistent with (1) social and environmental effects on yolk mass and composition and (2) both positive and negative differential allocation strategies of resource allocation in females. Overall, male traits were the most important predictors of egg yolk characteristics in this socially monogamous songbird.     

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.     

Does song reflect age and viability? A comparison between two populations of the great reed warbler Acrocephalus arundinaceus

Song complexity is often regarded as a sexually selected trait that reflects the overall quality of a male. In many passerine species, old males possess larger song repertoires than younger males. This may be either because individual males improve their performance as they get older (longitudinal increase) or because poor singers have reduced viability and, hence, are underrepresented in old age classes (cross-sectional increase). We studied the age dependence of repertoire size and other song traits in a German and a Swedish great reed warbler (Acrocephalus arundinaceus) population. We found marked differences between longitudinal and cross-sectional approaches, as well as between the two study populations. In the German population, we found that syllable switching, a measure of immediate versatility and strophe length, increased with age in a cross-sectional analysis. This was not because birds improved with age (longitudinally) but because syllable switching was positively correlated with male longevity. However, in Sweden, syllable switching seemed to be unrelated to age and longevity. In the Swedish population, individual males increased their repertoire size as they got older (longitudinal increase), but this did not happen in the German population. Hence, two populations, even when belonging to the same subspecies, may differ in whether or not they show delayed song maturation.     

Habituation in the green frog, Rana clamitans

Habituation and its role in the dear enemy effect was investigated in a population of green frogs, Rana clamitans. Green frogs have a prolonged breeding season, and males defend territories centered around suitable oviposition sites. We tested the prediction that male green frogs will habituate to broadcasts of synthetic conspecific stimuli. Our results indicate that male green frogs can discriminate familiar from unfamiliar stimuli. We suggest that habituation helps to mediate the territorial interactions between male green frogs. Strangers present a greater threat than familiar neighbors. By habituating to the advertisement vocalizations of their near neighbors, males avoid costly interactions with individuals that are not a major threat to their territories. Received: 20 July 1998 / Accepted after revision: 6 September 1998