Pair bond and breeding success in Blue Tits Parus caeruleus and Great Tits Parus major

Data from 939 nests of the Blue Tit Parus caeruleus and 1008 nests of the Great Tit P. major from nestboxes provided in superabundance in mixed forest study sites between 1976 and 2001 were analysed to examine the effects of mate retention on breeding success and the relationship between mate fidelity and site fidelity. Most birds retained their former partner (76% in Great Tits and 65% in Blue Tits). The probability of a pair divorcing was affected by male age in Great Tits, divorce being more likely in pairs with first‐year males. Great Tit pairs breeding together for a second season bred earlier, but had no higher breeding success than pairs breeding together for the first time. In Blue Tits laying date and start of incubation tended to be earlier in pairs breeding together for a second season, but hatching and fledging dates were not earlier than in other pairs. Great Tit pairs breeding together for two consecutive seasons bred earlier in the second season than in the first, but breeding success did not differ significantly between years. In both species, breeding performance did not differ between pairs that divorced after a season and pairs that stayed together. Thus breeding success did not determine whether a pair divorced or bred together again. Neither Blue Tits nor Great Tits improved their breeding performance through divorce. Blue Tit females even had fewer fledglings in the year after divorce than in the year before. Mate retention affected breeding site fidelity. Blue Tit females had greater breeding dispersal distances between consecutive years when re‐mating than when breeding again with the same mate. In Great Tits both males and females dispersed more when re‐mating than when retaining the former partner, suggesting that mate retention increased the chance of retaining the breeding site. In both species, breeding dispersal distances did not differ between pairs that divorced and pairs in which one mate disappeared. Because no major advantage of mate retention was evident, we suggest that mate retention evolved under different conditions than those found in study sites with high breeding densities and a superabundance of artificial nesting sites.     

The effect of old age on the reproduction of Great Tits Parus major and Blue Tits P. caeruleus

The results from two data sets show that in Great Tits Parus major and Blue Tits P. caeruleus reproduction is influenced by age. In both species, laying dates become earlier while clutch- and brood-size increase between the ages of 1 and 2, but there is no change in nesting success or post-fledging juvenile survival. Great Tits aged 5 or older are 'old' in that laying starts later and nesting success, brood-size and post-fledging juvenile survival decrease. Blue Tits become 'old' 1 year earlier than Great Tits: females aged 4 or older lay later, have a lower nesting success and smaller brood-size, and their young show a decreased post-fledging survival. The effect on laying date, in the Blue Tit, becomes apparent only for females aged 6 and older. It is concluded that ageing in small passerines is to be expected more generally, but that no extrapolations can be made as to the timing of the effect from one species to another.     

Seasonal decline in provisioning effort and nestling mass of Blue Tits Cyanistes caeruleus: experimental support for the parent quality hypothesis

A common feature of many birds breeding in seasonal environments is that fitness‐related parameters such as nestling mass or survival decline as the breeding season progresses. Consequently, there is a tendency for early breeders to have better reproductive performance than individuals breeding later in the season. This variation could be caused by factors associated with the date of laying, such as changing environmental conditions (date hypothesis), or by differences in parental quality between early and late breeders (parent quality hypothesis). To evaluate the relative importance of both hypotheses, we manipulated hatch dates of Blue Tits Cyanistes caeruleus by exchanging clutches with different incubation stages and assessed the impact on nestling mass, nestling diet and provisioning rates. Mean nestling mass declined significantly over the season. This was the combined result of differences in parental quality, which dominated in the early part of the season, and the influence of hatching date (date effect per se), which prevailed later in the season. Nestling diet composition was apparently uninfluenced by the manipulation, suggesting that deteriorating food supplies are the primary reason for the seasonal variation in the nestling diet. Counter to the date hypothesis, delayed parents did not feed their young less than control pairs did, but in fact exhibited higher provisioning rates. Our results suggest that in this population, parental quality seems to constrain post‐hatching reproductive performance and such intrinsic limitations may help to explain why certain individuals breed later.     

The effect of nest ectoparasites on parental provisioning in a north-African population of the Blue Tit Parus caeruleus

Bird nests are often heavily infested with several haematophagous ectoparasite species that drain energy and other essential resources needed for the development of the rapidly growing chicks. The nutritional requirements of altricial chicks can only be satisfied by the parents frequently bringing prey to the nest. In a 3-year study, we tested experimentally whether a parasite-induced change in the nutritional budgets of the chicks in an Algerian population of the Blue Tit, could be compensated by a response in parental food provisioning behaviour. We found that nestling body mass and size did not differ between heat-treated 'parasite-free' nests and heavily infested control nests. However, after controlling for potential confounding variables (laydate, clutch size, chick age, year), we found that broods of heavily infested control nests were more frequently visited and fed than broods that received antiparasite treatment. The results are discussed in the framework of theory related to behavioural responses of hosts to parasites.     

Breeding time,food supply and fitness components of Blue Tits Parus caeruleus in Mediterranean habitats

We analysed the relationship between the timing of food availability and within-season variation of both reproductive success and nestling body size of Blue Tits Parus caeruleus in Mediterranean habitats. Synchronization between food supply and reproduction was expected to be positively related to fitness components. We measured deviation from maximum food supply using a parameter that we called “time-lag”, which quantifies the degree of synchronization between the date of maximum food requirements by the nestlings and the date of maximum caterpillar supply in the habitat. This parameter was expected to be related to reproductive success as measured by the number and body-condition of fledglings. The predictions were that time-lag should be negatively correlated with the proportion of nestlings raised to fledging and the size of the fledglings. These predictions have been tested in different types of habitat. The results demonstrate that caterpillar supply during a critical nestling period can have a strong influence on fitness components. As predicted, synchronization with caterpillar supply is positively related to the number and body size of fledglings. Since there is large between-habitat variation in the timing of food supply, the key issue seems to be the adjustment to local patterns of food availability.     

Habitat quality, nestling diet, and provisioning behaviour in great tits Parus major

Variation in early nutrition is often a strong predictor of offspring condition and fitness. In the case of woodland passerine birds, nestling diet is determined by the spatiotemporal distribution of prey items such as caterpillars during the nestling period, and is usually quantified as differences in provisioning behaviour between habitats. However, the habitat level does not account for variation between individual territories, the level at which competition and selection are assumed to operate. Here we use nestbox cameras and Radio Frequency Identification technology (RFID) to simultaneously assess variation in both nestling diet (components) and provisioning rates (quantity) among a sample (n=22) of different quality great tit Parus major territories selected from a larger breeding population (n=310 fledged broods) in a single year. Caterpillars were by far the most numerous item provisioned to nestlings (mean=75% of prey items), as expected given the known importance of this food source for this species. Broods raised close to an oak tree, or far from the woodland edge, were provisioned the highest proportion of caterpillars. Provisioning rates declined seasonally and there was a weak association between low provisioning rates and caterpillar rich diets. During the first week of the nestling stage, nestling condition was unrelated to the proportion of caterpillars in the diet, provisioning rates and oak proximity. Condition at fledging was slightly improved in broods fed a higher proportion of caterpillars in the diet and in broods raised close to an oak tree. However, in our data early breeding was the only predictor of recruitment success, although power was low for this test. Analyses of long‐term data (41 years) from the same population confirmed a relationship between oak proximity and fledgling mass, but not recruitment success. Our results suggest that territory level environmental variation can affect offspring condition, probably through observed changes in nestling diet, but that such variation does not necessarily produce discernable effects on offspring fitness.     

The influence of brood size upon metabolic rate and body temperature in nestling Blue tits Parus caeruleus and House sparrows Passer domesticus

Metabolic rate and body temperature in nestling Blue tits and House sparrows were measured in broods of different size and age. Surface-volume ratio effects were found in both poikilothermic and homoiothermic Blue tits at ambients of 15°C but not at 20°C. The possibility of incipient hyperthermia amongst young nestlings maintaining sub-adult body temperatures is discussed. For the House sparrows heat retention by the nest was of greater importance than the surface-volume effect. Differences in nest structure between the two species are described and related to the thermal requirements of nestlings in large as against small broods.     

Foraging Blue Tits Parus caeruleus may trade-off calorific value of food items and distance from cover

Capsule Flowers of an invasive plant species are more visited by native birds than flowers of ornithophilous endemic plants.

Aims To describe the bird guild and its behaviour visiting the century plant Agave americana in an insular environment and to determine which factors are affecting visitation rates.

Methods We noted number and species of birds visiting inflorescences on Tenerife, Canary Islands. We used multimodel inference of generalized linear models to analyse the factors affecting the number of visits and the visitor species richness.

Results Eighty-one per cent of inflorescences were visited by eight native bird species. All species fed on nectar and only the Atlantic Canary fed also on pollen. Foraging behaviour varied among species. Visitation rate increased with density and diversity of birds and flower characteristics and decreased through the day. The number of species visiting the inflorescences increased with diversity and density of birds in the surroundings and decreased through the day.

Conclusion The native bird community uses the invasive century plant as a feeding resource at a higher rate than it uses endemic ornithophilous plants. This could have negative effects for the pollination of endemic plants, but positive effects for birds.     

Genetic and environmental components of growth in nestling blue tits (Parus caeruleus)

We investigated the effect of brood‐size mediated food availability on the genetic and environmental components of nestling growth in the blue tit (Parus caeruleus), using a cross‐fostering technique. We found genetic variation for body size at most nestling ages, and for duration of mass increase, but not of tarsus growth. Hence, nestling growth in our study population seems to have the potential to evolve further. Furthermore, significant genotype–environment interactions indicated heritable variation in reaction norms of growth rates and growth periods, i.e. that our study population had a heritable plasticity in the growth response to environmental conditions. The decreasing phenotypic variance with nestling age indicated compensatory growth in all body traits. Furthermore, the period of weight increase was longer for nestlings growing up in enlarged broods, while there was no difference to reduced broods in the period of tarsus growth. At fledging, birds in enlarged broods had shorter tarsi and lower weights than birds in reduced broods, but there was no difference in wing length or body condition between the two experimental groups. The observed flexibility in nestling growth suggests that growing nestlings are able to respond adaptively to food constraint by protecting the growth of ecologically important traits.     

Habitat structure modulates nestling diet composition and fitness of Blue Tits Cyanistes caeruleus in the Mediterranean region

Capsule: The nestling diet of Blue Tits Cyanistes caeruleus is influenced by parental effort and habitat type, and consequently has an impact on breeding success.

Aims: In a three-year study, we compared the nestling diet of Blue Tits in two Mediterranean forests (pinewood and oakwood) and tested its implications for breeding success.

Methods: Adults were captured at the nest to obtain morphological measurements, and provisioning behaviour was recorded when chicks were 11 days old. Nestling tarsus length and body mass were measured on day 13 after hatching.

Results: Caterpillars constituted the largest proportion of nestling diet in both habitats, however, higher numbers and biomass of noctuid, as well as higher numbers of tortricid larvae, were provided to nestlings in the pinewood. Furthermore, females provided tortricids more often than males, whereas males supplied more geometrid larvae and spiders. We found a more generalist diet for nestlings raised in the pinewood. Also, a greater number of young fledged when their diet included more tortricids and was more generalist, and Blue Tit nestlings raised on a diet with a higher number of spiders were in better body condition.

Conclusion: Differences in nestling diet between habitat types contribute to explain variance in breeding performance and therefore demonstrate diverse foraging behaviour strategies among populations.     

Influence of fine-scale habitat structure on nest-site occupancy,laying date and clutch size in Blue Tits Cyanistes caeruleus

Most birds have specific habitat requirements for breeding. The vegetation structure surrounding nest-sites is an important component of habitat quality, and can have large effects on avian breeding performance. We studied 13 years of Blue Tit Cyanistes caeruleus population data to determine whether characteristics of vegetation structure predict site occupancy, laying date and number of eggs laid. Measurements of vegetation structure included the density of English Oak Quercus robur, European Beech Fagus sylvatica, and other deciduous, coniferous and non-coniferous evergreen trees, within a 20-m radius of nest-boxes used for breeding. Trees were further sub-divided into specific classes of trunk circumferences to determine the densities for different maturity levels. Based on Principal Component Analysis (PCA), we reduced the total number of 17 measured vegetation variables to 7 main categories, which we used for further analyses. We found that the occupancy rate of sites and the number of eggs laid correlated positively with the proportion of deciduous trees and negatively with the density of coniferous trees. Laying of the first egg was advanced with a greater proportion of deciduous trees. Among deciduous trees, the English Oak appeared to be most important, as a higher density of more mature English Oak trees was associated with more frequent nest-box occupancy, a larger number of eggs laid, and an earlier laying start. Furthermore, laying started earlier and more eggs were laid in nest-boxes with higher occupancy rates. Together, these findings highlight the role of deciduous trees, particularly more mature English Oak, as important predictors of high-quality preferred habitat. These results aid in defining habitat quality and will facilitate future studies on the importance of environmental quality for breeding performance.     

Does loss of mass during breeding correlate with reproductive success? A study on Blue Tits Parus caeruleus

Substantial loss of mass of female Blue Tits Parus cueruleus during breeding is commonly explained by three not mutually exclusive explanations: shrinking of gonadal tissues, cost of reproduction and adaptation to save energy during flight. This study showed that loss of body mass was inversely correlated with reproductive success of individual females. Female tarsus length and timing of breeding correlated with loss of body mass, whereas ectoparasite loading had no significant effect on body mass. Loss of body mass during the rearing of young could be a useful measure of cost of reproduction by accounting for individual variation in female quality.     

Foraging behaviour of Blue Tits Parus caeruleus in a patchy environment under contrasting levels of natural food supply

We analysed the foraging behaviour of free-ranging Blue Tits Parus caeruleus in open holm oak Quercus ilex woodlands of western Spain during winter. Such woodlands are patchy for foraging tits because of the scattered distribution of trees and the patterns of abundance of canopy arthropods within and among trees. Results were compared with those obtained in spring of the same year, when we found that the foraging behaviour and spatial distribution of Blue Tits were largely unaffected by food availability (Pulido and Díaz 1997). Patch (tree) residence time was highly variable both within and among individual birds, and it was uncorrelated with either previous travel time or patch quality. Contrary to a priori expectations, the behaviour of tits did not conform to short-term energy maximizing rules in winter, in spite of a 2.5-fold decrease in food supply from spring to winter and a likely 2-fold increase in bird requirements. Instead, birds tended to fly towards patches that were further away than locally available. Overall, we conclude that energy intake rate was not the fitness-related currency that birds were trying to maximize while foraging.     

Weight loss of the female during the first brood as a factor influencing second brood initiation in Great Tits Parus major and Blue Tits P. caeruleus

We tested the hypothesis that the weight lost by female Great and Blue Tits Parus major and P. caeruleus while raising their first brood influences their ability to start a second brood. The evening weight of female parents was recorded when the nestlings were 5 and 13 days old, in different years and habitats. Several predictions were tested: (1) both species lose weight while raising nestlings and Great Tit females which start a second brood lose less weight than females which do not; (2) differences in the average weight lost between years and areas correlate with differences in the proportion of second broods; (3) the relative weight loss in Blue Tits, which only rarely undertake second broods, is higher than in Great Tits in which second broods are more common. Other factors also are related to the probability of undertaking a second brood: more second broods are undertaken by more successful females, adult females and females that lay earlier.
The comparison of Great and Blue Tits suggests that the two species use different reproductive strategies.     

Sex ratio of Parus major and P. caeruleus broods depends on parental condition and habitat quality

Brood sex ratio was studied in 88 families of Parus caeruleus (blue tit) and 95 families of P. major (great tit) in deciduous and mixed forest habitats differing in food availability. As a food specialist, the blue tit is expected to be more sensitive to the nutritional differences between the habitats than a food generalist such as the great tit. A shift of brood sex ratio towards males was detected for great tits in the high quality habitat, but there was no significant impact of parental condition or the number of nestlings. In contrast, brood sex ratio of blue tits was not affected by habitat quality. In blue tits, male condition correlated positively with a male-biased sex ratio. Habitat quality, however, affected the body mass differences of male and female blue tit siblings, and nestlings developed differently. The low quality habitat had a negative effect on the sexual dimorphism of siblings in male-biased broods, and the condition of offspring was bad. Nevertheless, sexual dimorphism cannot explain the differences between great and blue tits with respect to the correlation of sex ratio and individual condition.     

The effect of egg size and habitat on starling nestling growth and survival

In spite of the fact that hatchling size and energy reserves in birds are affected by egg size, many studies have failed to find an effect of egg size on offspring fitness. One possibility is that this is because they have been performed in areas with high food availability and that effects of egg size on offspring fitness are most apparent in areas of low food availability. To investigate this, egg size,␣offspring mass and survival of European starlings (Sturnus vulgaris) were measured in an agricultural landscape with a low but variable amount of pasture, the preferred foraging habitat of parent starlings. Offspring mass was related to egg size, but egg size explained a declining proportion of the variation in nestling mean mass as nestlings grew older. Offspring survival during the early, but not during the late nestling period was related to egg size. Throughout the nestling␣period, survival was related to the mass of the nestlings. It is suggested that the effect of egg size on␣offspring survival is through the effect of egg size on offspring mass, this effect declining as offspring grow older. Offspring survival during the early part of the nestling period was related to egg size when availability of pasture was low, but not when it was high. However, the interaction was not significant. Selection for␣larger egg size is discussed in relation to the structuring␣of starling populations into sources and sinks. Received: 22 September 1997 / Accepted: 22 January 1998     

The effect of nest temperature on growth and survival in juvenile Great Tits Parus major

1. For birds, maintaining an optimal nest temperature is critical for early‐life growth and development. Temperatures deviating from this optimum can affect nestling growth and fledging success with potential consequences on survival and lifetime reproductive success. It is therefore particularly important to understand these effects in relation to projected temperature changes associated with climate change.
2. Targets set by the 2015 Paris Agreement aim to limit temperature increases to 2°C, and, with this in mind, we carried out an experiment in 2017 and 2018 where we applied a treatment that increased Great Tit Parus major nest temperature by approximately this magnitude (achieving an increase of 1.6°C, relative to the control) during the period from hatching to fledging to estimate how small temperature differences might affect nestling body size and weight at fledging and fledging success.
3. We recorded hatching and fledging success and measured skeletal size (tarsus length) and body mass at days 5, 7, 10, and 15 posthatch in nestlings from two groups of nest boxes: control and heated (+1.6°C).
4. Our results show that nestlings in heated nest boxes were 1.6% smaller in skeletal size at fledging than those in the cooler control nests, indicating lower growth rates in heated boxes, and that their weight was, in addition, 3.3% lower.
5. These results suggest that even fairly small changes in temperature can influence phenotype and postfledging survival in cavity‐nesting birds. This has the potential to affect the population dynamics of these birds in the face of ongoing climatic change, as individuals of reduced size in colder winters may suffer from decreased fitness.