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 SELECTION OF TITS PARUS SPP. BY SPARROWHAWKS ACCIPITER NISUS

Selection of the Wytham Wood population of tits by Sparrowhawks during the tit post-fledging period was studied over three years. Within three weeks of the median date of tit fledging, juvenile Blue Tits were selected more frequently than juvenile Great Tits but thereafter juvenile Great Tits were selected more frequently; it is suggested that the former was due to greater vulnerability of Blue Tits in the two to three weeks after fledging, and the latter due to changes in availability due to immigration/emigration of juvenile tits. When juvenile tits killed by hawks were compared on the basis of brood and physical characteristics with their cohorts in the entire population and in those surviving to the following year, only one factor, fledging date, was found to have affected selection by hawks. It is believed that this was because hawks selected late fledged young still in family parties in preference to early fledged young which had already become independent of their parents and were foraging in the better cover of the forest canopy. When adults killed by hawks were compared with all adults available for selection on the basis of species, sex, timing of nesting and whether or not they had been born in the wood, the only selection trend found was that male Great Tits were taken more often than females due, possibly, to greater risk of exposure for males while foraging or during territorial behaviour. When compared with availability, juveniles were selected more frequently than adults in only one year, possibly a result of yearly differences in the number of prey available per hawk, the rate of non-predator related juvenile mortality or unknown hawk hunting strategies. The overall finding of relative non-selectivity by hawks was attributed to the surprise factor associated with the hunting methods used by Sparrowhawks in woodland.     

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.     

Habitat and laying date of Great and Blue Tit Parus major and P. caeruleus

Using 19 year's data from nine study areas at Ghent we investigated the effect of habitat on laying date, and found it to be quite different for Great and Blue Tits. In Great Tits we found a clear gradient in laying dates from urban over suburban to rural habitats. Blue Tits laid earliest in a suburban habitat and latest in a rural habitat. All other areas, including urban, suburban and rural ones, formed one intermediate group. The overall average laying date for the Great Tit, 18.9 April, was slightly earlier than that of the Blue Tit, 19.4 April. In urban areas Great Tits laid significantly earlier than Blue Tits, in optimal oak habitat and in one suburban area Blue Tits laid earlier, whereas in the five remaining areas no significant differences between the two species were found.
We argue that the differences in laying dates, although correlated to temperature, are probably caused by differences in the timing of food availability. We suggest that differences in laying dates of Great and Blue Tits are caused by a different response to environmental variations through differences in feeding ecology.     

Post-fledging recovery and dispersal of ringed Eurasian Sparrowhawks Accipiter nisus

Ring recoveries were used to explore the effect of early experience on the subsequent survival and dispersal of young Sparrowhawks in two areas in south Scotland. Young raised on high grade territories were recovered in greater proportion than young on low grade territories, implying that they had survived better after leaving the nest. Recovery rate tended to decline with increasing elevation of the nesting territory, implying that young from the high-ground territories survived less well than those from low-ground territories. Recovery rate showed no relationship with brood-size or sex composition, or with maternal age (yearling or older).
After the post-fledging period, dispersal distances were greater in females than in males and showed significant increases with increasing elevation of nesting territory (one area only) and with lateness of laying (or fledging) date. Regardless of laying date, young males from yearling mothers also dispersed further than young from older mothers, and young males from high elevation territories further than those from lower territories. No significant relationships emerged between dispersal distances and grade of natal territory, brood size or sex composition.     

The timing of breeding and offspring size in Great Tits Parus major

The relationship between the timing of breeding, offspring weight and offspring recruitment was investigated in two populations of Great Tits Parus major. In two of six seasons a significant negative correlation was found between laying date and fledgling weight, in three seasons the opposite was true and in one season no significant correlation existed between these variables. Offspring recruitment was quantified in five seasons, and late-nesting birds were found to produce significantly fewer recruiting offspring in two of these, the same two seasons when late fledglings had significantly lower weights. No significant correlation existed between the number of recruits and laying date in the three seasons when late fledglings were as heavy as or heavier than early fledglings. The potential detriment to reproductive success of lateness thus was offset when late-nesting parents managed to produce heavy young. Between-year variations in the seasonal pattern of reproductive success has previously been suggested to contribute to the maintenance of variability in the timing of breeding in Great Tits. In this paper we suggest that producing heavy offspring late in the season might be a proximate factor influencing the seasonal patterns of reproductive success. Producing heavy offspring late in the season is probably not a "strategy" but rather an effect of chance differences between years in the seasonal availability of food or the seasonality of other factors important for the nesting birds.     

Within-season divorce in Blue Tits (Cyanistes caeruleus)

Divorce between breeding seasons, i.e. mate change while the old breeding partner is still alive, has been well-studied in several bird species. It can be viewed either as the result of reproductive decisions of individuals to maximize their own fitness or as the outcome of intra-sexual competition for mating opportunities. In contrast, divorce within a breeding season—also referred to as rapid mate switching—has received much less attention. Within-season divorce may occur after sudden changes in environmental conditions, such as those causing the disappearance or devaluation of nesting sites or territories, or when better breeding partners become available. Within-season divorce can be initiated by a member of the pair, or it can be the result of a take-over by an unpaired individual that competes for access to the resource. During a field experiment investigating the effects of limiting nesting sites on reproductive behaviour in Blue Tits, we recorded several cases of within-season divorce. The rate of divorce was not related to the experimental nest-site limitation, and pairs that changed their partner suffered reduced reproductive success compared to faithful pairs. Although there were no differences in the timing of breeding, clutch size or hatching success, pairs with a new partner also suffered a reduced fledging success, which was partly explained by complete brood failures. This study highlights that the pair bond prior to egg laying can be unstable when conditions force individuals to compete for a new partner or nest site and indicates the importance of the correct timing of divorce within the breeding cycle.     

A meta-analytic study of the effects of female age on laying-date and clutch-size in the Great Tit Parus major and the Pied Flycatcher Ficedula hypoleuca

The effects of age (1 year-old vs older females) on laying-date and clutch-size of the Great Tit Parus major (in eight independent study areas) and the Pied Flycatcher Ficedula hypoleuca (five areas) were meta-analysed using both published and unpublished records. To standardize comparisons between areas and species, the standardized difference between the means of the two age groups was used as a measure of the magnitude of the age-effect (effect size).
In Great Tits the laying-date of 'young' females was 0.30 standard deviation units later and clutch-size 0.31 units smaller than that of 'old' females. In the Pied Flycatcher the corresponding figures were 0.45 and 0.81, respectively. All the effect size measures were highly significant but for clutch-size the effect size of the Pied Flycatcher was significantly greater than that of the Great Tit. In the Great Tit the effect size of clutch-size was heterogeneous across the different study areas.
It was estimated that the following numbers of unpublished studies showing null results would have to have accumulated in file-drawers before we could say that the results concerning effect sizes are due to sampling bias: for Great Tits 121 studies of laying-date and 196 studies of clutch-size; and for Pied Flycatchers 45 studies of laying-date and 139 studies of clutch-size.     

Summer dispersal and survival of juvenile Great Tits in southern Sweden

Summary Movements and survival of 506 first-brood Great Tit nestlings that fledged in mid-June in an oak wood in southern Sweden were studied by intensive trapping in that wood and in several neighbouring woods between 17 July and 10 September 1977. A total of 1177 captures of 508 individuals was made.Two periods of summer dispersal can be distinguished: the first period, one to one- and-one-half months after fledging, lasts longer in females and results in females moving farther away from their birthplace than males; and the second period in early September. There was no effect of brood-size, fledging date or size on dispersal movements. There is, however, a nest effect in that siblings tend to be more alike in the distance moved than non-siblings.The summer mortality rate is constant but high (13% per week). It is not affected by brood size or fledging date, but during the first month after fledging large individuals survive better than smaller ones. The summer mortality rate of dispersers (birds moving between woods) is not higher than that of non-dispersers.The results contradict two hypotheses proposed to explain when and how postfledging mortality occurs. The mortality rate is not higher during the first month after fledging, as Perrins' and Lack's hypothesis predicts. Since only 22% of the young are still alive at the beginning of September, autumn territorial behaviour cannot be the main factor causing juvenile losses, as proposed by Kluyver.There is some circumstantial evidence that Great Tits compete for food during the summer, and that food therefore could be in short supply, as suggested by Perrins.The observed differential dispersal of adults and young, and of male and female juveniles, may be the result of the dominance relationships in the family flock and later in the summer flocks, with subordinate individuals moving farthest.     

The effect of nestboxes on breeding populations of birds in broadleaved temperate woodlands

Removal of nestboxes from a semi-mature, broadleaved woodland caused a decrease in the number of breeding pairs of Great Tits, but did not significantly reduce breeding densities of Blue Tits. Breeding densities of warblers were unaffected by changes in the population of Great Tits. In the absence of nestboxes, intraspecific competition for natural nestholes was more intense amongst Great Tits than Blue Tits. Great Tits tended to occupy larger holes on or near the trunk, while Blue Tits used smaller holes on side branches. Nesting success for both species was lower in natural cavities than nestboxes, due to a higher percentage of total nest failures. The reintroduction of nestboxes did not produce an immediate rise in the breeding population of Great Tits; possible reasons for this are discussed.     

The effect of nestboxes on breeding populations of birds in broadleaved temperate woodlands

Removal of nestboxes from a semi-mature, broadleaved woodland caused a decrease in the number of breeding pairs of Great Tits, but did not significantly reduce breeding densities of Blue Tits. Breeding densities of warblers were unaffected by changes in the population of Great Tits. In the absence of nestboxes, intraspecific competition for natural nestholes was more intense amongst Great Tits than Blue Tits. Great Tits tended to occupy larger holes on or near the trunk, while Blue Tits used smaller holes on side branches. Nesting success for both species was lower in natural cavities than nestboxes, due to a higher percentage of total nest failures. The reintroduction of nestboxes did not produce an immediate rise in the breeding population of Great Tits; possible reasons for this are discussed.     

Male Blue Tits Cyanistes caeruleus choose early-leafing tree species during spring dawn chorus

Capsule Blue Tits sang their dawn song in trees that provided greater concealment.

Aims To determine if dawn singing Blue Tits select trees that increase their concealment.

Methods We compared the timing of leaf growth initiation in Blue Tit song-post trees to the average timing of leaf growth initiation for other tree species within 25 m of the song-post.

Results Most Blue Tits (96%, n = 23) sang from tree species that begin leaf growth earlier than the average tree available within 25 m. If males singing in Hawthorns Crataegus monogyna were excluded, 92% (n = 13) sang from earlier leafing trees.

Conclusion Dawn singing Blue Tits select perches that offer greater concealment.     

Reproduction and survival of polygynous and monogamous Blue Tit Parus caeruleus

In a seven-year study of Blue Tits in optimal habitat near Antwerp, Belgium, 45 polygynous broods involving 22 males out of 667 successful first broods were found. In another 43 nests no male was found, although a major effort was made to trap all adults. The estimated proportion of polygynous males is 3.4%, if only confirmed cases are considered, but 10.8% if all possible cases are included. One male was paired simultaneously to three females.
Primary females (laying earliest in a triangle) were as successful as monogamous ones. Secondary (laying later in a triangle) and deserted females (nests in which no male was trapped), although still quite successful, raised fewer young and in one plot had a lower probability of recruiting offspring.
Both in males and females, the frequency of polygyny was independent of age. Adult survival did not differ between monogamous and polygynous males. Among females no effect of pairing status on survival was found in one plot, but in a second plot monogamous females survived better than others. It is concluded that in any study of Blue Tits in optimal habitat one could expect to find polygyny.     

The significance of clutch overlap in Great Tits Parus major

We studied the effects of manipulation of the size of first broods in the Great Tit Parus major on the size and breeding success of second clutches and its relation to the degree of clutch overlap. The rearing of first brood fledglings always overlapped with the laying of the second clutch and in most cases also with the incubation period of the latter. The degree of clutch overlap depended on the size of the first brood, being less when the first brood was large. Clutch overlap also increased with season. Mechanisms affecting the timing of laying of second clutches are discussed. A large first brood imposed reproductive costs. It affected the size of the second clutch by causing it to be delayed; second clutch size decreases with season. It affected the post-fledging survival of second brood young as, in this population, this decreases with fledging date. The breeding success of second clutches was, however, not affected by the size of the first brood, but instead by the weight of the female when rearing the first brood.     

Advanced breeding dates in relation to recent climate warming in a Mediterranean montane population of Blue Tits Cyanistes caeruleus

Trends in the onset of breeding, clutch size and numbers of hatchlings and fledglings are examined for a Mediterranean montane population of Blue Tits (Cyanistes caeruleus) subject to recent warming in springtime monitored during 20 years. Blue Tits advanced their breeding dates in relation to mean air temperatures in April and, as a consequence, laid larger clutches. However, increases in the numbers of hatchlings and fledged young over time were not statistically significant after accounting for variables of influence. The entire breeding season seems to have been displaced towards earlier dates by adjusting breeding time to increased temperatures in prebreeding time, to which Blue Tits have been more responsive than Pied Flycatchers (Ficedula hypoleuca) in the same area. The alternative hypothesis, that interference competition with Pied Flycatchers for nestboxes and caterpillars, the main common food base of nestlings, has been the driving force behind the advancement of laying of the Blue Tit population, was not supported. However, the significant advance of breeding dates in Blue Tits has not been sufficient to overcome the precipitous decline in reproductive fitness with the advancement of the season.     

Differences in the onset of laying between island and mainland Mediterranean Blue Tits Parus caeruleus: phenotypic plasticity or genetic differences?

Blue tits Parus caeruleus breeding in deciduous or in evergreen woodlands on the mainland of Europe start to lay 3–4 weeks earlier than Blue Tits on the island of Corsica breeding in evergreen woodlands. A similar difference in average laying date between the two Blue Tit populations has been found in a relatively small sample of captive birds held in outdoor aviaries on the mainland. To provide data on the habitat contrast in Corsica for comparison with that on the mainland, a new study area was started in one of the few deciduous woodlands on Corsica. In addition, new data on laying dates of captive Blue Tits were gathered to increase the sample size of independent pairs in aviaries. This study shows that the habitat effect (deciduous ν evergreen woodland) on the average laying date of Blue Tits is relatively small in comparison with the geography effect (European mainland ν Corsica). The laying date differences between mainland and insular Blue Tits persisted whatever the type of habitat (deciduous, evergreen or outdoor aviaries on the mainland). The data support the hypothesis that the observed differences in the onset of laying between the mainland and insular Blue Tits were mainly influenced by genetic effects and relatively little by non-genetic maternal effects of by genotype—environment interactions.     

STRUCTURE AND FORAGING PATTERNS OF FLOCKS OF TITS AND ASSOCIATED SPECIES IN AN ENGLISH WOODLAND DURING THE WINTER

Blue Tits were the commonest and most frequent members of mixed-species insectivorous flocks during the winter at Wytham Wood, Oxford. Six common flocking species (Blue Tit, Longtailed Tit, Great Tit, Coal Tit, Marsh Tit and Golderest) made up nearly two-thirds of the birds in the study area. A tentative interspecific social dominance hierarehy was constructed: Great Tit > Blue Tit > Marsh Tit > Coal Tit > Long-tailed Tit and Goldcrest. Blue Tits permitted conspecifics to approach them more closely than did other flock members. Blue Tits most frequently led the flocks, but no more than would be predicted by their abundance. Great Tits, and Marsh Tits and Coal Tits to a lesser extent, defended territories during the winter. Two of the six common species frequented the inner parts of branches (Great Tit, Coal Tit), two the outer parts of branches (Blue Tit, Marsh Tit), and two the twigs (Long-tailed Tit, Goldcrest). Members of each of these species-pairs showed marked differences in height of foraging and/or species of tree frequented. Because of their abundance, the impact of Blue Tits outside of their most highly frequented foraging zones may exceed that of species concentrating in these other zones (e. g., twigs high in trees). The Coal Tit foraged most diversely, the Great Tit least diversely. Species that foraged diversely in one of the three foraging categories (species of tree, substrate, height) usually foraged less diversely than most other species in the other dimensions. Species usually overlapped least in the zones exploited (inner parts of branches, etc.). Great Tits overlapped with other species less than did any other common flock member, and Coal Tits were the next lowest in this regard.     

Will Great Tits become Little Tits ?

The mean size of Great Tits Parus major in our study at Ghent shows a significant decreasing trend in the period 1962 to 1975. Since it has been shown that in the Great Tit the heritability of tarsus length, a measure for size, is rather high, we believe the observed trend to be a directional micro-evolutionary change. After investigating several possible causes for the size decrease, we suggest that the observed change is caused by a shift in the equilibrium between selective pressures favouring large and small individuals. Before our study, breeding numbers of Great Tits were limited through the lack of suitable nest-sites. We think that more of the large males could reproduce in mat situation. When we provided nestboxes in surplus this selective pressure was relaxed, and since small females must produce more surviving offspring, this further shifts the balance.
We argue that during the evolutionary history of the Great Tit there was no lack of suitable nest-sites in natural habitats, that by providing nest boxes we restore the natural situation, and diat therefore the decrease in size should level off.     

Prey selection and foraging performance of breeding Great Tits Parus major in relation to food availability

We studied the nestling diet and the foraging performance of Great Tits in relation to prey abundance in the field. Numerous experimental studies present data on foraging decisions in captive Great Tits. Little is, however, known about prey selection in the field in relation to the food available and the consequences this has for the food delivery rate to nestlings. Since the foraging performance of the parents is one of the main determinants of fledging weight and juvenile survival, foraging behaviour is an important part of Great Tit reproduction. During the early breeding season up to 75% of the prey biomass delivered to the nestlings were spiders, which is in contrast with other studies. Only when caterpillars reached a size of 10–12 mg (approximately the average size of the spiders caught at that time) did the Great Tits change their preferences and 80–90% of the delivered prey masses were caterpillars, as reported by other authors. This 'switching' between prey occurred within a few days. It was not related to the changes in abundance but to size of caterpillars. The rate at which caterpillars were delivered to the nestlings (in mg/nestling/h) was strongly correlated with the caterpillar biomass available (in mg/m of branches) and nestling growth rate was significantly influenced by the mass of available caterpillars. The results provide evidence why perfect timing of breeding is so important for the Great Tit, and contribute to the understanding of the causal link between food supply, growth and breeding success.     

Nesting and post-fledging predation risk influence diel patterns of songbird fledging

Among stages of avian ontogeny, the act of nest departure or fledging is an abrupt transition into a new environment and a major leap toward independence for offspring. In altricial birds, the timing (i.e. time of day) of fledging is notable in that many species tend to fledge early in the morning. Past studies have proposed nest predation as a key factor driving birds to fledge earlier in the morning (the ‘survival hypothesis’), whereby offspring avoid peak times of nest predation that occur later in the day. A natural extension of this hypothesis is the predation of offspring post-fledging, whereby offspring are also timing their fledging with future survival prospects outside of the nest. However, few studies have investigated fledging behaviour in the context of both nesting and post-fledging predation. To help fill this knowledge gap, we investigated factors driving the timing and duration of fledging across six songbird species in the context of offspring predation: daily nest mortality, post-fledging mortality and diel patterns of nest predation risk. We found that > 60% of songbirds fledged early in the morning, whereas the peaks in nest predation risk occurred several hours post-fledging. Furthermore, species under greater risk of nest predation fledged earlier in the day and in closer succession to their siblings. Parameters of post-fledging mortality were poor predictors of fledging timing, but individuals from broods of species under higher risk of post-fledging mortality fledged in closer succession to their siblings. These results provide evidence in support of the survival hypothesis, and suggest that songbirds fledge in the morning to avoid peak times of nest predation risk that occur later in the day (~ 8 h after civil dawn). Such results corroborate past research highlighting predation on dependent offspring as a key factor driving variation in life histories across animal taxa; however, estimates of post-fledging mortality suggest that nest predation alone does not fully explain variation in fledging behaviour among species. Future research is therefore needed to investigate the contribution of other factors, such as energetics, parent–offspring conflict and diel patterns of post-fledging survival, which may help to mediate diel patterns of fledging within and among songbird species.