Experimental evidence for density dependence of reproduction in great tits

1. Density dependence of avian reproduction has often been analysed using correlations between annual mean reproductive output and population density. Experiments are necessary to prove that density is the cause of the observed patterns, but so far, three out of four experimental studies do not support a direct causal effect of density on reproduction.
2. This paper presents experimental evidence that reproductive decisions in great tits, Parus major L., are causally affected by breeding density. The breeding density of great tits was manipulated by providing nest-boxes at different densities in an ecologically homogeneous area.
3. Within years the densities in the high and low density plots differed approximately 8-fold. During the 11 years of the experiment, clutch size, nestling mass and the proportion of birds starting a second brood were all lower in the high density plot. In 5 years with equal breeding densities in both parts, clutch size did not differ between the plots. The patterns found were consistent with the density effects as predicted from the non-experimental data.     

Climate change and breeding parameters of great and blue tits throughout the western Palaearctic

Increasing evidence suggests that climate change has consequences on avian breeding phenology. Here, variations in laying date and clutch size of great tit Parus major and blue tit Parus caeruleus within and between breeding populations through the western Palaearctic are examined in relation to climatic fluctuations, measured by the winter North Atlantic Oscillation (NAO) index. Within and across breeding sites, laying date was related to winter‐NAO index such that great and blue tit females lay earlier after warmer, moister winters (positive values of winter NAO‐index). The present study shows that for most populations there is an advancement of laying date, but the rate of change with respect to NAO significantly differed geographically across the western Palaearctic and did not differ between species. However, clutch size of great and blue tits was not affected by climatic fluctuations, presumably because the whole season is being shifted, but not in relation to food supplies. These combined analyses for the two species controlled for potentially confounding variables such as latitude, longitude, elevation and habitat of each study site.     

Health impact of blood parasites in breeding great tits

Hypotheses of hemoparasite-mediated sexual selection and reproductive costs rely on the assumption that avian blood parasite infections are harmful to their hosts. To test the validity of this assumption, we examined the health impact of Haemoproteus blood parasites on their great tit (Parus major) host. We hypothesised that if blood parasites impose any serious health impact on their avian hosts, then infected individuals must differ from uninfected ones in respect to hemato-serological general health and immune parameters. A 3-year study of two great tit populations, breeding in contrasting (urban and rural) habitats in south-east Estonia, revealed that Haemoproteus blood parasites affected the health state of their avian hosts. Infected individuals had elevated lymphocyte hemoconcentration and plasma gamma-globulin levels, indicating that both cell-mediated and humoral immune response mechanisms are involved in host defence. The effect of parasites on cell-mediated immunity was both age- and sex-specific, as infection status affected peripheral blood lymphocyte counts only in males, and among these, the magnitude of response was greater in old individuals than yearlings. Heterophile hemoconcentration and plasma albumin levels were not affected by infection status, suggesting that blood stages of Haemoproteus infection do not cause a severe inflammatory response. Parasitism was not related to hematocrit values, indicating that Haemoproteus infection does not cause anemia. In two years, infected individuals were heavier than uninfected ones in the urban but not in the rural study area. This suggests, that under certain circumstances (possibly related to reproductive tactics), breeding great tits may avoid losing body mass in order to save resources for an anti-parasite immune response. Received: 16 February 1998 / Accepted: 22 May 1998     

Strong evidence for selection for larger brood size in a great tit population

We measured the selection pressure on brood size in a recentlyestablished population of great tits (Parus major L.) in thenorthern Netherlands by manipulating brood size in three years(1995: n = 51, 1997: n = 66, 1998: n = 51), and we estimatedfitness consequences in terms of local survival of both offspringand parents. Enlarged broods had highest fitness; the offspringfitness component was positively affected by manipulation andthe parental fitness component was unaffected. Parental survivaland the probability that parents produced a second clutch werenot affected by the treatment. However, parents that had raisedenlarged broods produced their second clutch later in the season.Clutch size, brood size, and laying date of birds recapturedin the next breeding season were largely independent of thetreatment. We conclude that there is strong evidence for selectionfor larger brood size and reject the individual optimizationhypothesis for this population because the number of young inthe nest predicts fitness independently of the manipulationhistory.     

Brood size, sibling competition, and the cost of begging in great tits (Parus major)

Evolutionary theory of parent-offspring conflict explains beggingdisplays of nestling birds as selfish attempts to influenceparental food allocation. Models predict that this conflictmay be resolved by honest signaling of offspring need to parents,or by competition among nestmates, leading to escalated beggingscrambles. Although the former type of models has been qualitativelysupported by experimental studies, the potential for a beggingcomponent driven by scramble competition cannot be excludedby the evidence. In a brood-size manipulation experiment withgreat tits, Parus major, we explored the scramble componentin the begging activity of great tit nestlings by investigatingthe mechanisms of sibling competition in relation to brood size.While under full parental compensation, the feeding rate pernestling will remain constant over all brood sizes for bothtypes of models; the scramble begging models alone predict anincrease in begging intensity with brood size, if begging costsdo not arise exclusively through predation. Great tit parentsadjusted feeding rates to brood size and fed nestlings at similarrates and with similar prey sizes in all three brood-size categories.Despite full parental compensation, the begging and food solicitationactivities increased with experimental brood size, whereas nestlingbody condition deteriorated. These findings support a scramblecomponent in begging and suggest that the competition-inducedcosts of food solicitation behavior play an important role inthe evolution of parent-offspring communication.     

Social stability and daily body mass gain in great tits

This paper presents the results of an experiment on the influenceof changes in flock composition and social rank on the patternof daily mass gain in captive wintering great tits. We createdflocks of three male great tits and scored dominance when rankshad stabilized after a few days. We then moved the dominantbird from each flock into a neighboring flock, thus creatinga certain amount of social instability and unpredictable resourceaccess. Both before and after moving the dominants, we collectedindividual body mass data three times a day. When the hierarchieshad stabilized in the second flocks, individual ranks were scored.After the dominant birds were moved, the rate of daily massgain increased for the average individual. There was also adecrease in average morning body mass, so the birds startedthe day at a lower mass and ended at a slightly higher massthan previously. Dominants were on average heavier than subordinates,but we found no statistically significant effect of rank changeon body mass. The pattern of daily mass gain was linear overtime both before and after treatment. We conclude that an impressionof unpredictable resource access can be created by disturbingthe social structure in a flock, with similar effect as fromrestricting individual access to feeders. Within a flock, instabilityof the social hierarchy thus seems to have an influence on bodymass management in addition to that of environmental stochasticity.     

Reproductive investment and parasite susceptibility in the Great Tit

1. Reproduction and parasite defence are assumed to be costly activities for hosts, and therefore trade-offs might exist between reproduction and parasite defence.
2. Brood sizes were manipulated in a population of Great Tits ( Parus major L.) to assess trade-offs between reproduction and parasite defence. Blood samples were taken from males and females during the late nestling phase, and parasite prevalence was compared among parents raising enlarged, reduced and control broods.
3. A higher prevalence of protozoan blood parasites dominated by Haemoproteus majoris was found among parents with enlarged broods, as compared with parents with control and reduced broods, respectively. Brood size manipulation did not affect parasite prevalence between the sexes or between age classes differently.
4. Mean parasite intensity (number of parasites per microscope field) among adult birds only infected with H. majoris was higher for parents with experimentally enlarged broods than controls and reduced broods, respectively.
5. These results support the hypothesis of a trade-off between parasite defence and reproduction.     

The function of extrapair paternity in blue tits and great tits: good genes or fertility insurance?

DNA fingerprinting of an island population of blue tits andgreat tits in southeast Norway revealed that extrapair paternityaccounted for 36% (17/47) and 27% (15/55) of broods and for7% (31/466) and 8% (33/408) of young in the two species, respectively.Cuckolded males did not differ from noncuckolded males withrespect to morphology, age, or survival. There was no seasonalpattern in the frequency of extrapair paternity, and males showedno individual consistency in paternity loss over multiple broods.Extrapair offspring did not grow faster, they did not fledgewith a higher body mass, and they did not show a higher localsurvival rate than their half siblings. Hence, there was noevidence of any association between extrapair paternity andmale phenotypic or genotypic quality. Extrapair offspring wererandomly distributed among broods, with the only exceptionsof one blue tit and two great tit broods in which all young(six to nine) were sired by an extrapair male. This patternis best explained by a small proportion of males (2%–4%)being infertile and by most females performing a few extrapaircopulations as insurance against laying infertile eggs. We concludethat the results suggest a role for fertility insurance butthat alternative functional explanations to extrapair paternityin these populations cannot yet be ruled out.     

The influence of predation risk on threat display in great tits

In wintering birds, conflicts over food are often resolved bythreat displays. For displays to be effective, there oughtto be a cost associated with displaying. We investigated whetherincreased vulnerability to predators due to reduced vigilancecould be such a cost. Conflicts ought then to be resolved usingfewer or less intense displays in conditions of high risk. We also looked for differences between dominants and subordinatesin their reaction to risk. Because there is considerable evidencethat subordinate wintering birds forage in riskier places thandominants, one might expect dominants to be less successfulin conflicts under high predation risk. In our experiment,nine flocks of four or five wintering male great tits were keptin outdoor aviaries. In the predation risk treatment, a stuffedpygmy owl was briefly shown before birds were allowed accessto a feeder. In the control treatment the owl did not appear.The predator presentation caused a reduction in the amountof aggression shown by subordinates, whereas for dominants there was no statistically significant change. Dominants were at leastas successful in subduing subordinates under high risk as underlow risk. A possible interpretation is that our experimentreflected a natural foraging situation for great tits, whereephemeral resources can appear unpredictably. In such situations,dominants may need to be bold to gain priority of access even under increased risk of predation, whereas a subordinate wouldgain little by risking a conflict with small chances of winning.     

The relation between dominance and exploratory behavior is context-dependent in wild great tits

Individual differences in personality affect behavior in novelor challenging situations. Personality traits may be subjectto selection because they affect the ability to dominate others.We investigated whether dominance rank at feeding tables inwinter correlated with a heritable personality trait (as measuredby exploratory behavior in a novel environment) in a naturalpopulation of great tits, Parus major. We provided clumped resourcesat feeding tables and calculated linear dominance hierarchieson the basis of observations between dyads of color-ringed individuals,and we used an experimental procedure to measure individualexploratory behavior of these birds. We show that fast-exploringterritorial males had higher dominance ranks than did slow-exploringterritorial males in two out of three samples, and that dominancerelated negatively to the distance between the site of observationand the territory. In contrast, fast-exploring nonterritorialjuveniles had lower dominance ranks than did slow-exploringnonterritorial juveniles, implying that the relation betweendominance and personality is context-dependent in the wild.We discuss how these patterns in dominance can explain earlierreported effects of avian personality on natal dispersal andfitness.     

Clutch and egg size in ladybird beetles

There was a significant positive correlation between the number of ovarioles and clutch size in the 8 species of cocinellid studied. There was no intraspecific correlation between egg size and clutch size under standard conditions. These tactics are discussed in terms of the allocation of resources to reproduction.      

Is clutch size individually optimized?

Brood size manipulations were carried out to test whether clutchsize variation in individual great tits (Parus major) controlledfor laying date was tuned to their phenotypic quality and/orlocal food abundance (individual optimization hypothesis; IOH).Broods with different original clutch sizes, but equal hatchingdates, were manipulated to a common brood size. A third broodwas kept as a control. Under the IOH, we expected a positiveassociation between reproductive success and original clutchsize. Fledgling production varied in an inconclusive way aftermanipulation, with data from 1 out of 3 years favoring the IOH.The effect of manipulation on the probability of a second clutchwas consistent with the IOH in another 1 out of the 3 years.When fitness accrued to second broods was also taken into considerationin terms of annual fledgling production, results from 2 outof 3 years tended to support the IOH. There was no effect ofthe manipulation on fitness (estimated as the number of recruitsplus parents breeding in the next season). Both the clutch component(local recruitment) and the parental component (survival tillnext breeding season) varied inconclusively with respect tothe IOH. On the basis of fitness measurements, the IOH couldnot be confirmed as an explanation for clutch size variationin this population. In 2 out of 3 years one of the three fitnesscomponents measured varied in accordance with the IOH. Overallthe evidence for the IOH in this data set is therefore weak.     

Island biogeography and the reproductive ecology of great tits Parus major

Island biogeography theory has contributed greatly to both theoretical and applied studies of conservation biology (e.g., design of nature reserves, minimum viable population sizes, extinction risk) and community composition. However, little theoretical and empirical work has addressed how island isolation and size affect reproductive ecology. We investigated the reproductive ecology of great tits (Parus major) on one offshore and one nearshore island, as well as on the Danish mainland. Tits breeding on the offshore island bred later, laid smaller clutches, and laid larger eggs than those on the nearshore island and mainland. In addition, the level of ectoparasite infestation in nests was highest on the offshore island, intermediate on the nearshore island, and lowest on the mainland. These insular effects may occur due to lower food abundance on islands, to density-dependent effects, or to effects related to low genetic diversity within island populations. Whatever the cause, the results emphasize that future studies of forest fragmentation/population isolation should consider not only gross measures of reproductive success, but also fine-scale measures such as clutch size, timing of breeding, and parasite prevalence. Received: 10 November 1997 / Accepted: 9 March 1998     

Gene flow and immigration rate in an island population of great tits

It is generally recognized that immigration and gene flow cannot be equated, but few detailed quantitative comparisons of these processes have been made over the entire lifetime of individual animals. We analyzed data collected in a longterm study of an island population of great tits Parus major, and tested two assumptions frequently made in population genetic studies. (1) The assumption that there is no difference in reproductive output between immigrant and resident breeding birds was refuted for females but not for males. Lifetime production of local recruits (LRS) was reduced by 37% in immigrant females, because female immigrants tended to leave the island after breeding, and thus reproduced for fewer years. Female LRS was density dependent, but the effect of density was independent of status (resident/immigrant). (2) The assumption that mating was random with respect to status was also refuted: assortative mating was found, even when temporal and spatial aggregation of immigrants was controlled for. Thus both assumptions were refuted, and gene flow was lower than immigration rate.     

Effects of habitat fragmentation on provisioning rates, diet and breeding success in two species of tit (great tit and blue tit)

The aim of this study was to examine the effects of forest fragmentation on the ability of parent birds to provide their young with an adequate food supply. To examine whether prey population densities of the great tit (Parus major L.) and the blue tit (P. caeruleus L.) vary between study areas in different forest size classes we compared provisioning rates and chick diet and related these parameters to breeding success. We filmed 217 nests over two breeding seasons and collected data on frass fall as a general estimate of caterpillar availability. Nests which were attended by none or one parent only during filming (n = 46) were excluded from the analyses. In both years and for both species feeding rates were highest in the smallest fragments and lowest in the large forest. There was also a suggestion that differences in feeding rates between areas vary between years. We found no consistent tendency for prey size to change with forest size, although both species brought slightly smaller prey items to the nest in the smallest forest fragments and feeding rates correlated negatively with prey size. Caterpillars were the main item fed to nestlings, in both species. We found no evidence to suggest that either frass fall or the proportion of caterpillars in the diet varied with forest size. There was also no correlation between mean frass fall and the total number of caterpillars brought to the nests, in either species. Breeding success, as measured by clutch size, brood size, fledging weight and fledging success, did not differ between the small fragments and the large forest, in either species. There was also no relationship between provisioning rate (as concerns volume of prey fed to nestlings and the quality of chick diet) and breeding success parameters. In conclusion, this study does not suggest suboptimal foraging or breeding conditions in small fragments compared to a nearby large forest, for either species. Received: 23 June 1997 / Accepted: 29 December 1997     

Body mass of female great tits (Parus major) at egg laying

Based on automated weighing of nest-boxes, we found that female great tits (Parus major) usually maintained their body mass during the egg-laying period. A drop in temperature (3°C or more) during laying was followed by a drop in body mass, and female body mass also depended slightly but significantly on mean temperature on the other days. Day to day change in body mass depended negatively on the actual body mass. Moreover, the repeatability of the morning body mass was high (r _I =0.90, n=24). Thus, female body mass seems to be regulated around a female specific, but temperature-dependent, level during laying. The lack of strong correlation between female average body mass and clutch size makes it improbable that body mass at laying plays a leading, proximate role in the control of clutch-size of great tits.