Bird predation by tawny owls (Strix aluco L.) and its effect on the reproductive performance of tits

The density of great tit Parus major L. and blue tit Parus caeruleus L. was artificially increased by placing nest-box colonies for these species in the vicinity of the nests of breeding tawny owls during 1993–1997. Bird prey composition in the owl nests, the proportion of parents disappearing from the breeding tit populations and the reproductive performance of the widowed parents were analysed. The frequency of predation on tits by tawny owls was greater in areas where tit density had been artificially increased. Owls preyed more on tits during the feeding period of owlets than during the incubation period and more in years when snow covered the ground during the incubation period than when it did not. Mortality due to predation was male biased and more females lost their mates in populations breeding near tawny owl nests. Reproductive performance of the widowed parents was lower and their body weights were lighter at the end of the nestling period than those found in birds rearing youngs with their mates. Predation by owls increased the between-year turnover in the breeding tit population: widowed parents did not return to the nesting site for the next breeding season.     

Evidence of a link between survival and pair fidelity across multiple tit populations

Although they have the potential to strongly influence individual fitness and the dynamics and productivity of populations, the survival consequences of pairing outcomes and the influence of current pairing outcomes on those in the future have rarely been addressed. Previously, we have shown that pair fidelity increases both survival and future pair fidelity in a population of great tits Parus major. The aim of this study was to explore the generality of our previous findings by evaluating the influence of current paring outcomes on survival and on future pairing outcomes in two different species and in different populations. We addressed our aims within a multievent capture–mark–recapture (MECMR) statistical framework, which accounts for differences in recapture rates and uncertainty in the assignment of pair status (i.e. whether an individual is breeding with the same partner or not). We applied the framework to breeding records of two great tit populations and one blue tit Cyanistes caeruleus population. We detected survival benefits (i.e. increased survival) of pair fidelity in all three populations. These were similar in both great tit populations, but higher for male great tits than for male blue tits. We found that age‐dependence in the rate of pair fidelity was shared between different populations and species, but did not detect any influence of current pair status on future pair status. Our study highlights the importance of considering survival when studying the fitness benefits of pair fidelity. Some of the differences in pair fidelity rates and survival benefits of pair fidelity are likely the result of long‐term and short‐term demographic and environmental factors in the population. We advocate the use of the MECMR framework used here for further exploration of these differences.     

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.     

Interspecific aggression declines seasonally in breeding great tits Parus major

During the breeding season, great tits show aggression to protect their nest from intra‐ and interspecific intruders. Aggression is a labile trait that can be plastically expressed as a result of individual differences (e.g., personality), seasonal gradients in the costs and benefits of aggression, or other environmental components (e.g., number of competitors). Competitors may try to take over great tit nests, because the number of suitable nesting sites is limited, and great tits may guard high quality territories. Taking over a great tit nest may be especially fruitful in early phenological stages (egg laying) when great tits frequent their nests less often. However, great tits may compensate for this vulnerability by being more aggressive toward intruders during early nesting stages, a pattern that has already been established in an intraspecific context. Previous studies have shown that interspecific intruders were most likely to die from great tit aggression during great tit egg laying, suggesting great tits may also be more aggressive during this phase in an interspecific context. Here, I tested this hypothesis with simulated territorial intrusions in great tit territories using taxidermized blue tits Cyanistes caeruleus (hereafter called blue tit models). Great tit aggression (number of calls and approach distance toward blue tit model) was assayed during egg laying, incubation, and chick rearing in the breeding season of 2014. Although sample size was low due to a high fraction of non‐responders (n = 44 out of 89 assays across 26 out of 35 individuals), I found that great tits showed a seasonal decline in aggressiveness, which is congruent with intraspecific results on this study species. I discuss my findings in the context of differential adjustment to climate change between interspecific competitors.     

Population dynamics of an expanding passerine at the distribution margin

Individuals may be maladapted to novel environments at the species’ distribution margin. We investigated population dynamics in a marginal habitat where reproduction has been proven poor. Survival, population growth rate (λ) and its components, breeding and natal dispersal were studied in great tits Parus major breeding at the northern margin of its distribution in northern Finland. We used long term capture–mark–recapture data sets. Study area size and population density were used to explain adult survival rates. The average annual estimates of adult survival rose from 0.371 to 0.388 between the periods of 1971–1984 and 1999–2009. The estimates are slightly lower than estimates of small passerines in Europe. Low local survival rate of fledglings (0.050–0.055) probably reflects intensified emigration from this low quality area. Temporal variation in λ was large (0.498–1.856). Despite of low adult survival and recruitment rates, the mean estimates of λ (1.008 and 1.033) indicate an overall stability in the population size. Indeed, our results suggest that the immigration has an important role in the population dynamics of northern great tits. Thus the population is demographically and genetically dependent on core habitats which may cause adaptive problems due to intensive gene flow. Given those limitations, options for evolution of local adaptations in northern distribution margins are discussed.     

Climate change and annual survival in a temperate passerine: partitioning seasonal effects and predicting future patterns

Predicting climate change impacts on population size requires detailed understanding of how climate influences key demographic rates, such as survival. This knowledge is frequently unavailable, even in well‐studied taxa such as birds. In temperate regions, most research into climatic effects on annual survival in resident passerines has focussed on winter temperature. Few studies have investigated potential precipitation effects and most assume little impact of breeding season weather. We use a 19‐year capture–mark–recapture study to provide a rare empirical analysis of how variation in temperature and precipitation throughout the entire year influences adult annual survival in a temperate passerine, the long‐tailed tit Aegithalos caudatus. We use model averaging to predict longer‐term historical survival rates, and future survival until the year 2100. Our model explains 73% of the interannual variation in survival rates. In contrast to current theory, we find a strong precipitation effect and no effect of variation in winter weather on adult annual survival, which is correlated most strongly to breeding season (spring) weather. Warm springs and autumns increase annual survival, but wet springs reduce survival and alter the form of the relationship between spring temperature and annual survival. There is little evidence for density dependence across the observed variation in population size. Using our model to estimate historical survival rates indicates that recent spring warming has led to an upward trend in survival rates, which has probably contributed to the observed long‐term increase in the UK long‐tailed tit population. Future climate change is predicted to further increase survival, under a broad range of carbon emissions scenarios and probabilistic climate change outcomes, even if precipitation increases substantially. We demonstrate the importance of considering weather over the entire annual cycle, and of considering precipitation and temperature in combination, in order to develop robust predictive models of demographic responses to climate change. Synthesis Prediction of climate change impacts demands understanding of how climate influences key demographic rates. In our 19‐year mark‐recapture study of long‐tailed tits Aegithalos caudatus, weather explained 73% of the inter‐annual variation in adult survival; warm springs and autumns increased survival, wet springs reduced survival, but winter weather had little effect. Robust predictions thus require consideration of the entire annual cycle and should not focus solely on temperature. Unexpectedly, survival appeared not to be strongly density‐dependent, so we use historical climate data to infer that recent climate change has enhanced survival over the four decades in which the UK long‐tailed tit population has more than doubled. Furthermore, survival rates in this species are predicted to further increase under a wide range of future climate scenarios.     

Teleconnections and local weather orchestrate the reproduction of tit species in the Carpathian Basin

Variation in climatic conditions is an important driving force of ecological processes. Populations are under selection to respond to climatic changes with respect to phenology of the annual cycle (e.g. breeding, migration) and life‐history. As teleconnections can reflect climate on a global scale, the responses of terrestrial animals are often investigated in relation to the El Niño‐Southern Oscillation and North Atlantic Oscillation. However, investigation of other teleconnections and local climate is often neglected. In this study, we examined over a 33‐year period the relationships between four teleconnections (El Niño‐Southern Oscillation, North Atlantic Oscillation, Arctic Oscillation, East Atlantic Pattern), local weather parameters (temperature and precipitation) and reproduction in great tits Parus major and blue tits Cyanistes caeruleus in the Carpathian Basin, Hungary. Furthermore, we explored how annual variations in the timing of food availability were correlated with breeding performance. In both species, annual laying date was negatively associated with the Arctic Oscillation. The date of peak abundance of caterpillars was negatively associated with local temperatures in December–January, while laying date was negatively related to January–March temperature. We found that date of peak abundance of caterpillars and laying date of great tits advanced, while in blue tits clutch size decreased over the decades but laying date did not advance. The results suggest that weather conditions during the months that preceded the breeding season, as well as temporally more distant winter conditions, were connected to breeding date. Our results highlight that phenological synchronization to food availability was different between the two tit species, namely it was disrupted in blue tits only. Additionally, the results suggest that in order to find the climatic drivers of the phenological changes of organisms, we should analyze a broader range of global meteorological parameters.     

Impact of an exceptional winter flood on the population dynamics of bearded tits (Panurus biarmicus)

Mark–recapture data are used to investigate the impact of an exceptional winter flood on the population dynamics of an isolated bearded tit Panurus biarmicus population in north-west England. Adult numbers increased threefold (from 60 to 180 individuals) between 1992 and 2000, but declined by 94% during 2000/2001. This large reduction in numbers was caused by severe over-winter mortality associated with a prolonged flood of the Phragmites litter layer, the main winter foraging habitat of bearded tits, followed immediately by cold weather. At the end of the flood, bearded tits were 20% lighter than during previous winters. Population changes in other years were accurately predicted by annual variation in recruitment, and there was evidence that recruitment was higher following the introduction of artificial nest boxes and was density dependent. This study highlights the potential threat to bearded tits in Europe of predicted increases in the frequency and extent of winter flooding, and the potential utility of measures like artificial nest boxes that aim to promote recruitment. Recent extensions of methodology now permit a detailed exploration of animal population dynamics from mark–recapture data alone.     

State uncertainty models and mark‐resight models for understanding non‐breeding site use by Piping Plovers

Conservation of beach‐nesting medium‐distance migrants has focused on breeding areas because protection of nests is more tractable than protection of non‐breeding habitat. As breeding ground management has encountered diminishing returns, interest in understanding threats in non‐breeding areas has increased. However, robust estimates of non‐breeding demographic rates and abundance are generally lacking, hindering the study of limiting factors. Estimating such rates is made more difficult by complex population dynamics at non‐breeding sites. In South Carolina, endangered Piping Plovers Charadrius melodus start arriving in July and some depart prior to December (the autumn‐only population) while others remain through at least March (the wintering population). State uncertainty capture‐mark‐recapture models provide a means for estimating vital rates for such co‐occurring populations. We estimated the proportion of the population entering the study area per survey (entry probability) and proportion remaining per survey (persistence rate) for both populations during autumn, and abundance of the wintering population, at four sites in South Carolina in 2006/7 and 2007/8, taking advantage of birds previously colour‐ringed on the breeding grounds. We made fairly precise estimates of entry and persistence rates with small sample sizes. Cumulative entry probability was ~50% by the end of July and reached 95% for both populations by October. Estimated stopover duration for birds in the autumn‐only population was 35 days in year 1 and 42 days in year 2. We estimated a wintering super‐population size of 71 ± 16 se birds in the first year and 75 ± 16 in the second. If ringing programmes on the breeding grounds continue, standardized resighting surveys in the non‐breeding period and mark‐recapture models can provide robust estimates of entry and persistence rates and abundance. Habitat protection intended to benefit non‐breeding Piping Plovers at our coastal sites should be in effect by late summer, as many birds are resident from July to the end of winter.     

Absence of seasonal variation in great tit offspring sex ratios

When the timing of breeding affects the reproductive value of sons and daughters differently, parents are expected to increase their fitness by changing the offspring sex ratio during the course of the breeding season. Previous studies have shown that in great tits Parus major hatching date has a stronger effect on the fitness of juvenile males than on that of juvenile females. We tested whether this difference was reflected in a seasonal decline in the proportion of sons per breeding attempt. Although offspring sex ratio was more variable than would be expected from a binomial distribution, there was no significant relationship between the proportion of sons and the laying date of the clutch. Moreover, individual females did not adjust the sex ratio of their offspring following an experimental delay of breeding. This study therefore fails to demonstrate adaptive seasonal variation in great tit offspring sex ratios.     

Is parental competitive ability in winter negatively affected by previous springs’ family size?

Reproductive behavior cannot be understood without taking the local level of competition into account. Experimental work in great tits (Parus major) showed that (1) a survival cost of reproduction was paid in environments with high levels of competition during the winter period and (2) experimentally manipulated family size negatively affected the ability of parents to compete for preferred breeding boxes in the next spring. The fact that survival was affected in winter suggests that the competitive ability of parents in winter may also be affected by previous reproductive effort. In this study, we aim to investigate whether (1) such carryover effects of family size on the ability of parents to compete for resources in the winter period occurred and (2) this could explain the occurrence of a survival cost of reproduction under increased competition. During two study years, we manipulated the size of in total 168 great tit broods. Next, in winter, we induced competition among the parents by drastically reducing the availability of roosting boxes in their local environment for one week. Contrary to our expectation, we found no negative effect of family size manipulation on the probability of parents to obtain a roosting box. In line with previous work, we did find that a survival cost of reproduction was paid only in plots in which competition for roosting boxes was shortly increased. Our findings thus add to the scarce experimental evidence that survival cost of reproduction are paid under higher levels of local competition but this could not be linked to a reduced competitive ability of parents in winter.     

Dispersal and recruitment during population growth in a colonial bird, the great cormorant Phalacrocorax carbo sinensis

While the factors influencing reproduction and survival in colonial populations are relatively well studied, factors involved in dispersal and settlement decisions are not well understood. The present study investigated exchanges of great cormorants Phalacrocorax carbo sinensis among six breeding colonies over a 13‐year period when the breeding population in Denmark increased from 2800 to 36 400 nests. We used a multistate capture‐recapture model that combined multisite resightings and recoveries to examine simultaneously recruitment, natal dispersal, breeding dispersal and annual survival of first‐year, immature and breeding great cormorants. Mean survival of first‐year birds (0.50±0.09, range=0.42–0.66 among colonies) was lower than survival of breeders (0.90±0.06, range=0.81–0.97). Mean survival of immature birds over the study period was 0.87±0.08. Dispersal from a colony increased with decreasing mean brood size in the colony in both first‐time and experienced breeders. The choice of the settlement colony in first‐time breeders was affected by conditions in the natal colony and in the colonies prospected during the pre‐breeding years. In particular, first‐time breeders recruited to colonies where they could expect better breeding success. Experienced breeders relied mainly on cues present early in the season and on their own breeding experience to choose a new breeding colony. Newly established colonies resulted mainly from the immigration of first‐time breeders originating from denser colonies. Dispersal was distance‐dependent and first‐time breeders dispersed longer distances than breeders. We suggest that the prospecting behaviour allows first‐time breeders to recruit in nearby as well as more distant potential breeding colonies. Dispersing breeders preferred to settle in neighbouring colonies likely to benefit from their experience with foraging areas. We discuss the importance of these movements for growth and expansion of the breeding population.     

Numerical and behavioural responses of migrant passerines to experimental manipulation of resident tits (Parus spp.): heterospecific attraction in northern breeding bird communites?

Summary We studied experimentally interspecific competition among foliage-gleaning passerine birds by manipulating the density of resident tits. In 1988 tit density was experimentally increased on three small islands in a central Finnish lake, and decreased on three other islands by tit removal. In order to avoid the effects of between-island differences in habitat quality, the role of the islands was reversed when the experiment was repeated in the following year. Censuses and observations on foraging and feeding behaviour were conducted to assess the numerical and behavioural responses of migrant conguilders (mainly chaffinches and willow warblers) with respect to the manipulated abundance of the tits. We also measured whether variation in food consumption of tits affected the frequency with which the migrants found food by calculating average intervals between successful prey captures, time lags to prey-capture and giving-up times. Our results indicate that interspecific competition is of minor importance in structuring breeding bird assemblages and species feeding ecologies on the study islands. No consistent difference in foraging or feeding niches of chaffinches and willow warblers was found between low and high tit density conditions. Niche overlap analysis showed no avoidance by chaffinches and willow warblers of the microhabitats which tits used. Tit abundance had no significant effect on feeding success or behaviour. Experimentally increased abundance of resident birds was associated with increased abundance of breeding migrants, however. This pattern was found not only in the foliage gleaning guild but also with all passerine birds, indicating that food was not an important contributor to this pattern. We elaborate a hypothesis suggesting heterospecific attraction in northern breeding bird assemblages. Habitat generalist migrants may use the presence of residents as an indicator of safe and/or productive breeding sites in northern unpredictable circumstances.     

Density-dependent immigration promotes population stability in a long-distance migratory bird

The spatial structure of populations determines the relative importance of reproduction, survival and movement on population dynamics. However, the mechanisms by which local individuals and immigrants interact and the subsequent effects of immigrants on productivity are poorly known. We developed an integrated population model (IPM) to study the extent and consequences of immigration on the dynamics of a neotropical migrant (American redstart, Setophaga ruticilla) over an 11-year period in Ontario, Canada. New immigrants represented the majority of the study population each year with higher immigration rates for males than females and for first-year breeders than breeders in their second year or older. Immigration was negatively density dependent, with immigrants replacing previously established breeders in a compensatory manner following their death or emigration. Because of the tradeoff between immigration and apparent survival, neither had a strong influence on population growth and reproductive output was most strongly correlated with a change in abundance between years. However, if immigration ceased, the study population would become locally extinct within 7 years and thus immigrants were essential for local population persistence. We found no evidence for reduced breeding success when immigrants represented a higher proportion of the study population. Our research highlights the importance of movement in the stability of open populations and the strong correlation between the fates of local breeders and the number of immigrants entering the population. We recommend the use of IPMs to address the spatial scale over which immigration occurs and how different scales influence its contribution to population dynamics.     

Caterpillar biomass depends on temperature and precipitation,but does not affect bird reproduction

Complex changes in phenological events appear as temperatures are increasing: In deciduous forests bud burst, hatching of herbivorous caterpillars, egg laying and nestling time of birds when feeding chicks on caterpillars, may differentially shift into early season and alter synchronization. If timing of bird reproduction has to match with short periods of food availability, phenological mismatch could negatively affect reproductive success. Using a unique empirical approach along an altitudinal temperature gradient, we firstly asked whether besides temperature, also precipitation and leaf phenology interplay and affect caterpillar biomass, since impacts of rainfall on caterpillars have been largely neglected so far. Secondly, we asked whether abundance of caterpillars and thereby body mass of great tit nestlings, which are mainly fed with caterpillars, vary along the altitudinal temperature gradient. We demonstrated that next to temperature also precipitation and leaf phenology affected caterpillar biomass. In our beech forest, even along altitudes, caterpillars were available throughout the great tit breeding season but in highly variable amounts. Our findings revealed that although timing of leaf phenology and great tit breeding season were delayed with decreasing temperature, caterpillars occurred synchronously and were not delayed according to altitude. However, altitude negatively affected caterpillar biomass, but body mass of fledglings at high altitude sites was not affected by lower amounts of caterpillar biomass. This might be partially outweighed by larger territory sizes in great tits.     

Does foraging behaviour explain the poor breeding success of great tits Parus major in northern Europe?

Great tits Parus major have generally much poorer breeding success in northern Finland than in mid- and western Europe. The aim of this study is to find out whether the poor success is linked to foraging behaviour. This was studied by monitoring great tits' foraging behaviour and food abundance in different substrates during the breeding cycle in two populations, N Finland (65°N) and Latvia (56°N). It was shown that breeding success, from eggs to fledglings, was significantly poorer (56% vs. 93%) and caterpillar abundance considerably lower in the northern population. The general patterns in foraging behaviour were the same in N Finland and Latvia (especially the preference of birches Betula spp.) indicating that great tits used basically the same species-typical foraging strategy in both populations. However, thin branches of birch were preferred in the north but avoided in Latvia, which may suggest that northern great tits have changed their foraging niche towards the outer parts of canopy, a niche typically occupied by the blue tit. This shift is theoretically advantageous, since the outer parts of the canopy are the richest caterpillar source. In practice, however, the poorer success of great tits indicates that this is not a beneficial strategy. Primarily, great tits in the northern population seem to be food limited since they lay too large clutches in relation to accessible food resources. This may be because great tits are not adapted to lower caterpillar production in the northern margins of their distribution and cannot change their narrow diet (3/4 caterpillars), like e.g. willow tits can. In search for caterpillars, food accessibility is further limited because the great tits' normal foraging behaviour, with wide search radius, may not function properly in the denser, outer parts of the canopy. Great tits may also be too heavy to forage efficiently on leafed twigs.     

Predation by sparrowhawks decreases with increased breeding density in a songbird,the great tit

Predators may regulate prey populations if predation rate increases with prey density. Alternatively, if space-limited (e.g. territorial) predators become satiated when prey exceed a certain density, increased prey abundance may lead to reduced predation rate. These alternatives have been difficult to test experimentally for mobile prey in the wild. We present such a test, manipulating the density of great tits (Parus major) by adding nest boxes in territories of sparrowhawks (Accipiter nisus). Predation rate was measured for young tits after they left the nests. Although the great tit is an important prey, there was no evidence for regulation during the breeding season: the rate of hawk predation declined with increasing density of tits. This result was not confounded by changes in breeding density of alternative prey species (other songbirds). Hawk predation can therefore favour dense breeding in a territorial (solitary) bird, and conspecific attraction and aggregation reported in several territorial species may partly result from predation pressure. This result also has potential implications for conservation work.     

Density dependent effects between three competitive bird species

Summary Density and breeding success of the great tit Parus major, blue tit Parus caeruleus and collared flycatcher Ficedula albicollis were studied in nest box colony in oak forest over a period of 19 years.Intraspecific density dependent clutch size reduction was found with blue tit and great tit. In interspecific relation the high density of blue tits reduced the clutch size of great tits.In the hatching period neither intraspecific nor interspecific density dependence were showed between the tits when the third competitive species, collared flycatcher was present. The collared flycatcher significantly reduced the hatching success of both tit species and the number of fledglings of great tit.The effects of the great tits and combined density of the great and blue tits on the hatching failure and number of fledglings of collared flycatcher were found when the density of the tits was high. There were not significant relationships to the single density of blue tits.The temporal variability of the competition of the three bird species is discussed.     

The effect of climate change on partial migration – the blue tit paradox

Climate change has proven to affect various aspects of the migration of birds. In response to milder winters making the habitat more profitable and increasing the survival of residents, the migratory fraction of partially migratory populations has been predicted to decline. We studied the blue tit Parus caeruleus , a common partial migrant in southern Sweden. The numbers migrating at Falsterbo, a migratory passage site in SW Sweden, has increased during the last decades, in parallel with increasing winter and annual temperatures. Migration data from Falsterbo were compared with yearly indices of the size of the breeding population as estimated by the Swedish National Bird Monitoring Programme. Over the study period 1975–2004, also the breeding population has increased in size. The proportion of blue tits migrating each year did not change over the study period, or possibly even increased slightly, which is in contrast to how climate change has been predicted to influence populations containing both migratory and resident individuals. The most important factors determining the intensity of blue tit migration in a given year was the size of an important winter food source, the beech mast crop (more migrants at lower crops) and the size of the breeding population (more migrants at higher densities).     

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.