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.     

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.     

The function of threat display in wintering great tits

It is generally accepted that threat displays have signal function and serve to repel opponents, but why they should have such an effect is a matter of debate. Using videotaped interactions from 2 years and nine flocks each year, we analysed the pattern of occurrence of agonistic displays and attacks in captive flocks of wintering great tits, Parus major. One currently influential perspective is to view threat interactions as sequences of distinct moves and countermoves, in which stronger threats can reliably indicate aggressive motivation because they carry a cost of eliciting counterattacks from certain opponents. We found little evidence for this kind of mechanism operating in great tits. Another possibility is that social dominance is important in ensuring reliable communication, for instance because an individual needs both to display and to attack to maintain its rank. It would be hard to test the importance of this kind of mechanism directly, but our observations were consistent with such an idea. Dominants won the majority of bouts of interaction and had higher rates of display and attack than subordinates, but a subordinate could temporarily overcome a dominant through intense aggression. We also found that the level of displaying in one bout provided information about the rate of aggression in following bouts. As a general interpretation of great tit aggressive behaviour, we suggest that particular displays form part of a graded signal of motivational state, so that a greater rate of displaying or a greater proportion of intense displays, together with a greater rate of attacking, correspond to a greater aggressive motivation. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Early learning affects social dominance: interspecifically cross-fostered tits become subdominant

Social dominance influences the outcome of competitive interactionsover limited resources, and may hence be important for individualfitness. Theory thus predicts that its heritability will below and that non-genetic determinants of dominance should prevail.In this field experiment we reciprocally cross-fostered greattits (Parus major) to blue tits (Parus caeruleus) to investigatethe impact of early social experience on dominance status incompetition over food during winter. Controlling for potentialeffects of age, size, sex and site-related dominance, we showthat cross-fostered birds of both species were subdominant toconspecific immigrants, while controls originating from unmanipulatedbroods were dominant to conspecific immigrants. Furthermore,blue tits reared by blue tit parents but with at least one greattit broodmate had lower dominance status relative to conspecificimmigrants than did controls. Although great tits generallydominated blue tits, cross-fostered birds of both species initiatedmarginally more fights against the other species than did theirrespective controls, suggesting faulty species recognition.Since both social parents and broodmates strongly influencethe dominance behavior of offspring later in life, we concludethat social conditions experienced at an early age are crucialfor the determination of subsequent social dominance.     

Differential response by males and females to manipulation of partner contribution in the great tit (Parus major)

1.  In birds with bi-parental care, handicapping is often assumed to decrease the amount of parental care of the handicapped partner. We discuss how handicapping could alter the shape of the handicapped bird's survival–effort curve (theoretical curve relating the survival of a parent to its effort) and show that the optimal response could yield a decrease, no response or even an increase in effort of the handicapped bird.
2.  Male or female great tits Parus major (L.) were handicapped during the nestling period by clipping a number of feathers in order to study the effects on parental care and body condition.
3.  Handicapped males significantly decreased their feeding rates, while handicapped females did not. Condition of handicapped females significantly deteriorated, while condition of handicapped males did not change during the experiment. Females with a handicapped partner fully compensated for their partner's decrease in work rate, while males with a handicapped partner did not show any compensation and even tended to decrease their feeding rates.
4.  Using an inverse optimality approach, we reconstructed the theoretical curve relating the survival of a parent to its effort on the basis of the experimental effects. The handicapped male's survival–effort curve appeared to be slightly steeper than that of handicapped females. This suggests that handicapped males suffer more from an increase in effort than handicapped females.     

Clutch size and malarial parasites in female great tits

life-history models predict an evolutionary trade-off in theallocation of resources to current versus future reproduction.This corresponds, at the physiological level, to a trade-offin the allocation of resources to current reproduction or tothe immune system, which will enhance survival and thereforefuture reproduction. For clutch size, life-history models predicta positive correlation between current investment in eggs andthe subsequent parasite load. In a population of great tits,we analyzed the correlation between natural clutch size of femalesand the subsequent prevalence of Plasmodium spp., a potentiallyharmful blood parasite. Females that showed, 14 days after hatchingof the nestlings, an infection with Plasmodium had a significantlylarger clutch (9.3 eggs ± 0.5 SE, n = 18) than uninfectedfemales (8.0 eggs ± 0.2 SE, n = 80), as predicted bythe allocation trade-off. Clutch size was positively correlatedwith the prevalence of Plasmodium, but brood size 14 days afterhatching was not. This suggests that females incur higher costsduring laying the clutch than during rearing nestlings. Infectionstatus of some females changed between years, and these changeswere significantly correlated with a change in clutch size aspredicted by the trade-off. The link between reproductive effortand parasitism may represent a possible mechanism by which thecost of egg production is mediated into future survival andmay thereby be an important selective force in the shaping ofclutch size     

Inbreeding depression along a life-history continuum in the great tit

Inbreeding resulting from the mating of two related individuals can reduce the fitness of their progeny. However, quantifying inbreeding depression in wild populations is challenging, requiring large sample sizes, detailed knowledge of life histories and study over many generations. Here we report analyses of the effects of close inbreeding, based on observations of mating between relatives, in a large, free-living noninsular great tit (Parus major) population monitored over 41 years. Although mating between close relatives (f > or = 0.125) was rare (1.0-2.6% of matings, depending on data set restrictiveness), we found pronounced inbreeding depression, which translated into reduced hatching success, fledging success, recruitment to the breeding population and production of grand offspring. An inbred mating at f = 0.25 had a 39% reduction in fitness relative to that of an outbred nest, when calculated in terms of recruitment success, and a 55% reduction in the number of fledged grand offspring. Our data show that inbreeding depression acts independently at each life-history stage in this population, and hence suggest that estimates of the fitness costs of inbreeding must focus on the entire life cycle.     

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     

Effect of an ectoparasite on lay date, nest-site choice, desertion, and hatching success in the great tit (Pants major)

Ectoparasites are common in most bird species, but experimentalevidence of their effects on life-history traits is scarce.We investigated experimentally the effects of the hematophagoushen flea (Ceratophyllus gallinae) on timing of reproduction,nest-site choice, nest desertion, clutch size, and hatchingsuccess in the great tit (Parus major). When great tits wereoffered a choice on their territory between an infested anda parasite-free nest-box, they chose the one without parasites.When there was no choice, the great tits in a territory containingan infested nest-box delayed laying the clutch by 11 days ascompared with the birds that were offered a parasite-free nestingopportunity. The finding that there was no difference in phenotypictraits related to dominance between the birds nesting in infestedboxes and birds nesting in parasite-free boxes suggests thatthe delay is not imposed by social dominance. Nest desertionbetween laying and shortly after hatching was significandy higherin infested nests. There was no difference between infestedand parasite-free nests in clutch size, but hatching successand hence brood size at hatching were significantly smallerin infested nests. Nest-box studies of great tits have beenseminal in the development of evolutionary, ecological, andbehavioral theory, but recently a polemic has arisen in theliterature about the validity of the conclusions drawn fromnest-box studies where the naturally occurring, detrimentalectoparasites are eliminated by the routine removal of old nestsbetween breeding seasons. Our study suggests that this criticismis valid and that the evaluation of the effects of ectoparasitesmay improve our understanding of behavioral traits, life-historytraits, or population dynamics     

Impaired flight ability--a cost of reproduction in female blue tits

When prey are attacked by predators, escape ability has an obvious influence on the probability of survival. Laboratory studieshave suggested that flight performance of female birds mightbe affected by egg production. This is the first study of changesin take-off ability, and thus potentially in predation risk,during reproduction in wild birds. We trapped individual maleand female blue tits repeatedly during the breeding season.Females were 14% heavier and flew 20% slower (probably as aconsequence of a lower ratio of flight muscle to body mass)during the egg-laying period than after the eggs had hatched.However, flight muscle size did not change to compensate for changes in body mass over this period. In contrast, males showedno changes in either body mass, muscle size, or flight abilityover the same period. Furthermore, the impairment of flightin females increased with the proportion of the clutch thathad been laid, an effect that was independent of body mass and muscle size. This indicates that egg production causes additional physiological changes in the female body that produce impairedlocomotor performance. We suggest that courtship feeding offemale blue tits by their mates might reduce predation riskduring the period when female take-off ability is impairedby reducing the time females have to spend foraging and thusreducing the time they are exposed to increased predation.