In vertebrates, darker individuals are often found to be more active and willing to take risks (representing characteristics of a ‘proactive’ coping style), whereas lighter individuals are instead ...more cautious and less active (representing characteristics of a ‘reactive’ coping style). It is thus generally expected that melanin‐based coloration and proactivity form a suite of positively integrated traits at the among‐individual level. Here, we use a multigenerational pedigree of free‐living great tits (Parus major) to partition variation in, and the correlation between, melanin‐based breast stripe (‘tie’) size and exploration behaviour (a proxy for coping style) into its among‐ and within‐individual components. We show that both traits harbour heritable variation. Against predictions, tie size and speed of exploration were negatively correlated at the among‐individual level due to the combined influences of permanent environmental and additive genetic effects. By contrast, the two traits were weakly positively correlated within individuals (i.e. individuals increasing in tie size after moult tended to become more explorative). The patterns of among‐individual covariance were not caused by correlational selection as we found additive and opposite selection pressures acting on the two traits. These findings imply that testing hypotheses regarding the existence of a ‘syndrome’ at the among‐individual level strictly requires variance partitioning to avoid inappropriate interpretations as the negative ‘unpartitioned’ phenotypic correlation between exploration and tie size resulted from counteracting effects of within‐ and among‐individual correlations. Identifying sources and levels of (co)variation in phenotypic traits is thus critical to our understanding of biological patterns and evolutionary processes.
Negative density dependence of clutch size is a ubiquitous characteristic of avian populations and is partly due to within‐individual phenotypic plasticity. Yet, very little is known about the extent ...to which individuals differ in their degree of phenotypic plasticity, whether such variation has a genetic basis and whether level of plasticity can thus evolve in response to selection. Using 18 years of data of a Dutch great tit population (Parus major), we show that females reduced clutch size with increasing population density (slopes of the reaction norms), differed strongly in their average clutch size (elevations of the reaction norms) at the population‐mean density and that the latter variation was partly heritable. In contrast, we could not detect individual variation in phenotypic plasticity (‘I × E’). Level of plasticity is thus not likely to evolve in response to selection in this population. Observed clutch sizes deviated more from the estimated individual reaction norms in certain years and densities, implying that the within‐individual between‐year variance (so‐called residual variance) of clutch size was heterogeneous with respect to these factors. Given the observational nature of this study, experimental manipulation of density is now warranted to confirm the causality of the observed density effects. Our analyses demonstrate that failure to acknowledge this heterogeneity would have inflated the estimate of ‘I × E’ and led to misinterpretation of the data. This paper thereby emphasizes the fact that heterogeneity in residuals can provide biologically insightful information about the ecological processes underlying the data.
Many ecologists mark their free living study animals with the aim to collect knowledge on individual life histories. Yet, marking animals may affect life histories and it is important to quantify ...such effects. Literature on this subject is relatively rare, especially when it concerns the effect of bird rings. This is partly because control groups are often missing, since the rings were not applied with the goal to measure their effect on life histories. From studies in captivity there is evidence that the colour of rings may affect partner choice and in the field certain outstanding colours of rings may affect predation risk. Here we use data of an ongoing study of Great Tits Parus major to analyse whether the colour of rings fitted in different combinations to nestling Great Tits (day 14, n = 9818) affected their life histories in terms of natal dispersal, first year local recapture rate, breeding performance and second year local recapture rate. We measured reflectance spectra to quantify the brightness of the colour rings, and we used a human panel to judge the conspicuousness of the rings against a grey background. We found support for a positive effect of the conspicuousness, as judged by human observers, of the colour ring combination on local recapture rate, but not on natal dispersal suggesting that survival was affected. No effect was detected on breeding performance. The brightness of the rings did not explain variation in life history components. Although the effect of conspicuousness was statistically weak, the effect size is of potential biological importance. Life expectancy of individuals without conspicuous rings is estimated to be 28% shorter than for individuals with three conspicuous rings. Our result could have a considerable impact on life history studies where fitness measures are based on colour-marked bird populations. Colour of the colour rings will affect the variance of fitness estimates and may even bias the mean of fitness estimates when specific colours are used in specific periods, in specific environments or to mark particular categories of birds like year classes.
In seasonal environments variation in food abundance in the non‐breeding season is thought to affect songbird population dynamics. In a unique tit‐sea buckthorn berry system we can estimate the berry ...abundance and both the tit consumption and population dynamics. Six hundred nest boxes were available to great and blue tits Cyanistes caeruleus for breeding in spring and roosting in winter. We followed the dynamics including the recapture histories of individually marked great tits from 2008 to 2014. In each year we estimated 1) the winter sea buckthorn berry availability, 2) an index of berry consumption in December based on the colour of the faeces of roosting birds, 3) the number of breeding great and blue tits, 4) both recapture probability and the return rate of the great tits and 5) immigration rates. December berry abundance positively predicted the number of breeding pairs of both species in the subsequent season and great tit return rates in the second half of the winter. There was support for a sex specific berry effect on the adult return rate in the great tit: female return rate was associated less strongly to berry abundance than male return rate. This skewed the sex ratio of the local breeders in the following breeding season. Intriguingly, annual berry consumption in December was not related to berry abundance, and individuals consuming more berries tended to have slightly lower return rates. Reproductive rate was not related to berry abundance. There was hardly support for a relation between immigration rates of first year breeders and berry abundance. Taken together these results imply that berry stock not only affected population size but also the population composition through sex specific exchange with the surroundings. Since population density covaried with berry abundance, density dependent effects provide an alternative explanation for the patterns observed.
In seasonal environments, the main selection pressure on the timing of reproduction (the ultimate factor) is synchrony between offspring requirements and food availability. However, reproduction is ...initiated much earlier than the time of maximum food requirement of the offspring. Individuals should therefore start reproduction in response to cues (the proximate factors), available in the environment of reproductive decision making, which predict the later environment of selection. With increasing spring temperatures over the past decades, vegetation phenology has advanced, with a concomitant advancement in the reproduction of some species at higher trophic levels. However, a mismatch between food abundance and offspring needs may occur if changes in the environment of decision making do not match those in the environment of selection. Date of egg laying in a great tit (Parus major) population has not advanced over a 23-year period, but selection for early laying has intensified. We believe that this is the first documented case of an adaptive response being hampered because a changing abiotic factor affects the environment in which a reproductive decision is made differently from the environment in which selection occurs.
1. To elucidate the links between avian brood size, parental effort and parental investment, we measured daily energy expenditure (DEEfem), condition (residuals of mass on tarsus) and feeding rate in ...female great tits Parus major L. rearing broods in which the number of young was either reduced, unmanipulated or enlarged. 2. Female condition was negatively correlated with manipulation when measured at the nestling age of 8 days (measured during the day), which suggests a shift in allocation from self-feeding to chick-feeding. However, there was no detectable manipulation effect on condition measured at the nestling age of 12 days (measured during the night). Either female condition was only affected by manipulation in the early nestling phase or the females adjusted their diurnal mass trajectory in response to brood size manipulation. More detailed data are required to verify this point. There were no indications of a fitness cost associated with the condition during the day, but condition at night was positively related to winter survival. Since manipulation only affected condition during the day, there was no link between manipulation and winter survival. 3. The duration of the working day was not affected by manipulation and female feeding rate tended to flatten off with manipulated brood size. Similarly, brood reduction resulted in a lower DEEfem, whilst brood enlargement had no effect. This suggests that females worked at an energetic ceiling when rearing an unmanipulated brood. However, the level of this `ceiling' in DEEfemwas not fixed: it differed between years. This leads us to conclude that the observed ceiling was imposed by extrinsic factors (e.g. available foraging time) and not by an intrinsic factor such as maximum energy assimilation rate. We hypothesize that time limitation was the cause for the observed ceiling in energy expenditure and that the annual variation in the level of this ceiling was due to annual variation in ambient temperature. 4. A cost of reproduction was previously demonstrated in this population: brood enlargement caused a reduction in the incidence of second clutches. However, since DEEfemdid not differ between control and enlarged broods, we judge it unlikely that daily energy expenditure is a general predictor for parental investment.
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.
Jan Tinbergen, galardonado con el primer Premio Nobel de Economía en 1969, participó a finales de ese mismo año en una reunión de economistas que la OIT había convocado con el fin de fijar las ...prioridades en materia de investigación del recién creado Programa Mundial del Empleo. En su ponencia Tinbergen examinó la influencia de la política comercial internacional en la capacidad de creación de empleo. En su opinión, … Seguir leyendo
Jan Tinbergen, galardonado con el primer Premio Nobel de Economía en 1969, participó
a finales de ese mismo año en una reunión de economistas que la OIT había
convocado con el fin de fijar las prioridades en materia de investigación del recién
creado Programa Mundial del Empleo. En su ponencia Tinbergen examinó la influencia
de la política comercial internacional en la capacidad de creación de empleo. En
su opinión
Seasonal variation in reproductive success is a common feature of most organisms. To understand the evolution of breeding seasons and reproductive strategies of individual animals, it is necessary to ...assess the extent to which seasonal variation in reproductive success is causally related to seasonal variation in the environment ('timing' hypothesis), to differences in quality between early and late breeders or their territories ('quality' hypothesis), or to a combination of both. We manipulated timing of breeding in the Great Tit Parus major, a small passerine, to test these hypotheses. A group of experimentally delayed birds was created by removing first clutches, inducing birds to lay a replacement clutch. Reproductive success of delayed pairs was compared with control pairs that bred early and with pairs that bred late. We conclude that seasonal declines in reproductive success at the nestling stage and survival of adult females were caused by differences in quality between early and late breeders. Recruitment of fledglings into the breeding population and the occurrence of second clutches were causally related to the timing of breeding. The seasonal decline in clutch size was caused by a combination of timing and quality effects. We attempted to assess the relative importance of timing and quality in the seasonal decline in reproductive success, expressed as lifetime production of recruits. We tentatively conclude that 87% of the seasonal decline in lifetime reproductive success could be attributed to a timing effect per se, whereas quality differences between early and late breeders accounted for the remaining 13%.
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