Importance of feeding ecology to the reproductive success of Blackbirds Turdus merula nesting in rural habitats

The feeding ecology of Blackbirds Turdus merula breeding in contiguous woodland and farmland habitats was studied over three years. The aim of the study was to investigate how reproductive success was influenced by nestling diet and the provisioning rates of parents feeding nestlings. Parental provisioning rates increased with brood size, and consequently individual nestlings were no lighter in larger broods. None of the environmental factors measured had strong effects on parental provisioning rate. The nestling diet was dominated by caterpillars and earthworms, the former occurring in a short period in the middle of the breeding season. The availability of earthworms was higher in woodland and was dependent on rainfall in farmland. Nestling mass and provisioning rates were marginally higher under predominantly earthworm diets. Nestling mass increased with rainfall in farmland only, and was higher in farmland than in woodland or woodland-edge, although it is doubtful whether this result is of any significance for fledgling survival. Overall, Blackbirds were able to provision their nestlings adequately throughout the breeding season across a range of conditions. There was no evidence to suggest that reproductive success was constrained by aspects of feeding ecology within the natural range of brood size.     

Brood division and transition to independence in Blackbirds Turdus merula

Blackbirds Turdus merula rearing young in the Botanic Garden, Oxford, were observed over three summers (1979–81). Between 0 and 10 days after fledging the young were divided between the parents so that each fed only certain fledglings and refused to feed others. This division was temporally stable. In broods where another nesting attempt followed, the male usually took responsibility for all the young, but in final broods they were divided fairly evenly. The probability of a fledgling surviving to independence declined as the number of young a parent fed increased; thus division should be favoured. However, if the female fed some young the inter-nest interval increased and the potential number of young that could be raised in that season was curtailed. This trade-off is examined by means of a model. After fledging the young improved in their ability to capture prey successfully and increased the size of the prey taken. At 15–24 days after fledging, self-feeding became more profitable than parental feeding and the young became fully independent. The timing of this transition is influenced by the parents, and it is suggested that by dividing the brood, parental feeding can be more carefully regulated so that finer control over the timing of independence is achieved.     

Changes in migration behaviour of Blackbirds Turdus merula from the Netherlands

Capsule Dutch Blackbirds from all regions had almost ceased migrating in the period between 1955 and 1990, probably as a result of climate change.     

The haematozoan parasites of Common Blackbirds Turdus merula: associations with host condition

A population of Common Blackbirds Turdus merula was studied to investigate the relationships between the presence of blood parasites and host morphometrics, a putative sexually selected trait (bill colour), and reproductive parameters. Haematozoa of four genera were detected and their prevalence was high. Infection was negatively associated with adult morphometrics: adults infected with Leucocytozoon were in relatively poor body condition and had shorter wings than uninfected birds. The bill colour of males infected with Plasmodium tended to be duller than that of uninfected males, and in females Haemoproteus infection was significantly positively associated with bill coloration. Haematozoan infection of females was unrelated to measured reproductive parameters, and there was no relationship between blood parasite infection and the provision of parental care.     

Investigator activities reduce nest predation in blackbirds Turdus merula

The effect of investigator perturbance has been traditionally considered detrimental for avian nesting success in terms of enhanced nest predation. This conclusion was however based on a taxonomically biased group of species and, thus, the study of that effect in additional species was essential for reaching a more firm conclusion. Furthermore, although it has been suggested that the effect of nest visiting could also depend on nest predator community, no study has so far tested this hypothesis yet. Trying to detect possible influence of nest‐visiting rates and predator community on nesting success we visited European blackbird Turdus merula nests at two different experimental rates in two populations that considerably differ in the composition of their nest predator communities and natural nest predation rates. Contrary to the traditional ideas, our results not only show that investigator disturbance significantly reduces nest predation, but also that this reduction is maintained in both populations despite the difference in the community of nest predators. We discuss these findings and suggest that predators, especially mammals, might avoid places disturbed by investigators.     

Adaptive latitudinal variation in Common Blackbird Turdus merula nest characteristics

Nest construction is taxonomically widespread, yet our understanding of adaptive intraspecific variation in nest design remains poor. Nest characteristics are expected to vary adaptively in response to predictable variation in spring temperatures over large spatial scales, yet such variation in nest design remains largely overlooked, particularly amongst open‐cup‐nesting birds. Here, we systematically examined the effects of latitudinal variation in spring temperatures and precipitation on the morphology, volume, composition, and insulatory properties of open‐cup‐nesting Common Blackbirds’ Turdus merula nests to test the hypothesis that birds living in cooler environments at more northerly latitudes would build better insulated nests than conspecifics living in warmer environments at more southerly latitudes. As spring temperatures increased with decreasing latitude, the external diameter of nests decreased. However, as nest wall thickness also decreased, there was no variation in the diameter of the internal nest cups. Only the mass of dry grasses within nests decreased with warmer temperatures at lower latitudes. The insulatory properties of nests declined with warmer temperatures at lower latitudes and nests containing greater amounts of dry grasses had higher insulatory properties. The insulatory properties of nests decreased with warmer temperatures at lower latitudes, via changes in morphology (wall thickness) and composition (dry grasses). Meanwhile, spring precipitation did not vary with latitude, and none of the nest characteristics varied with spring precipitation. This suggests that Common Blackbirds nesting at higher latitudes were building nests with thicker walls in order to counteract the cooler temperatures. We have provided evidence that the nest construction behavior of open‐cup‐nesting birds systematically varies in response to large‐scale spatial variation in spring temperatures.     

Age‐specific reproduction and disposable soma in an urban population of Common Blackbirds Turdus merula

The mechanism of senescence is an important subject of current research, but our knowledge of the factors influencing the rate of ageing in naturally occurring populations remains rudimentary. Evolutionary theories of senescence predict that investment in reproduction in early life should come at the cost of reduced somatic maintenance and thus result in earlier or more rapid senescence. We use data on the complete reproductive histories of 431 Common Blackbirds (222 males and 209 females) collected during a 19‐year study of the ecology of an urban population of this species to test the main hypotheses addressing the issue of senescence. On average, the birds in this population survived for 3.7 (± 1.9 sd) years. Reproductive success in females peaked at the age of 4, but in males remained stable until the 5th year of life. We observed declines in reproductive success, indicative of senescence, after the peak years in both sexes. The mechanism of age‐related changes in the reproduction of females confirms the individual improvement and selective disappearance hypotheses. In the case of males, the increase in reproductive performance comes as a consequence of the disappearance of poor reproducers. The parental investment associated with early life fecundity (the first two breeding seasons in males and females) impairs the breeding success of females later on. Contrary to expectations, there was no negative impact of high early life fecundity on either mortality or lifespan. Individuals of both sexes with a high early life fecundity had a higher lifetime reproductive success than those in which early life fecundity was low. Hence, the most profitable strategy is to maximize reproductive effort in the early stages of life. This yields the highest lifetime reproductive success, despite the increased impact of senescence, especially in females. These results are consistent with the disposable soma hypothesis.     

Effects of sex and sampling site on the relative proportion of pesticides in uropygial gland secretions of European Blackbirds (Turdus merula)

Dichlorodiphenyltrichloroethane (DDT) is a pesticide that was commonly used for decades worldwide. The use of DDT was banned in the 1970s and 1980s in Europe because of its high toxicity and persistence in the environment, bioaccumulation in living organisms and biomagnification through food webs. However, monitoring using both invasive and non-invasive methods has routinely reported the occurrence of DDT metabolites such as dichlorodiphenyldichloroethylene (DDE) in wild birds, providing valuable information about the exposure to pesticides and potential differences between species and over time. Here, we analysed the relative proportion of DDE in the uropygial gland secretions of European Blackbirds Turdus merula from two localities in southern Spain. Given the negative effects of this pollutant on animal immunity, we also tested for associations between the prevalence of haemosporidians and the relative proportion of DDE in their secretions. Relative proportions of DDE varied between sampling sites and were higher in females than in males, regardless of their age. In spite of the potential immunosuppressive effect of DDE, haemosporidian infection was not associated with DDE presence.     

Breeding success and nest site stereotypy in a population of Brewer's Blackbirds (Euphagus cyanocephalus)

Summary Nest site stereotypy and nest-site related breeding success was studied in a population of Brewer's Blackbirds in eastern Washington, USA. Four types of nest sites could be distinguished. Based on individually colour-ringed nestlings, imprinting or genetic fixation of a particular type of nest site can be ruled out since there is no correspondence between the type of nest a bird grew up in and the type it used when breeding. Bush nests make up about two thirds of all nests, and they are relatively more frequent than the other types in the first half of the breeding season, whereas the other types become more frequent in the second half of the nesting season. Differential breeding success was studied and its dependence on colony composition was investigated.The hypothesis that the birds are able to utilize nest site flexibility as an anti-predator strategy within colonies is not supported. Some nest sites are predictably less suitable at a particular time of the season, and the birds have been at least partially able to incorporate this into their nest-site selection behaviour. There is no optimal type of nest site for all conditions. The selective pressures on birds using the different types of nest sites are discussed.     

Long-Term Effects of Chronic Light Pollution on Seasonal Functions of European Blackbirds (Turdus merula)

Light pollution is known to affect important biological functions of wild animals, including daily and annual cycles. However, knowledge about long-term effects of chronic exposure to artificial light at night is still very limited. Here we present data on reproductive physiology, molt and locomotor activity during two-year cycles of European blackbirds (Turdus merula) exposed to either dark nights or 0.3 lux at night. As expected, control birds kept under dark nights exhibited two regular testicular and testosterone cycles during the two-year experiment. Control urban birds developed testes faster than their control rural conspecifics. Conversely, while in the first year blackbirds exposed to light at night showed a normal but earlier gonadal cycle compared to control birds, during the second year the reproductive system did not develop at all: both testicular size and testosterone concentration were at baseline levels in all birds. In addition, molt sequence in light-treated birds was more irregular than in control birds in both years. Analysis of locomotor activity showed that birds were still synchronized to the underlying light-dark cycle. We suggest that the lack of reproductive activity and irregular molt progression were possibly the results of i) birds being stuck in a photorefractory state and/or ii) chronic stress. Our data show that chronic low intensities of light at night can dramatically affect the reproductive system. Future studies are needed in order to investigate if and how urban animals avoid such negative impact and to elucidate the physiological mechanisms behind these profound long-term effects of artificial light at night. Finally we call for collaboration between scientists and policy makers to limit the impact of light pollution on animals and ecosystems.     

No evidence of sex‐biased dispersal in an island population of Common Blackbirds Turdus merula

Natal dispersal has major consequences for the dynamics and genetic structure of populations. Female‐biased natal dispersal, otherwise the norm in birds, is overridden when the place to move is limited, as on isolated islands. This effect was confirmed for the fist time in a European study system, the Common Blackbird breeding on Heligoland Island. Spatially restricted and sexually uniform natal dispersal may be a prerequisite for successful establishment of populations on remote islands or isolated habitat fragments, and this could play a major role in speciation processes.     

The relationship between habitat choice and lifetime reproductive success in female red deer

In non-territorial species, individuals can move freely and should be distributed in an ideal free manner between habitats and areas with respect to resources that influence lifetime reproductive success (LRS). Consequently, no relationship between diet quality and LRS should be expected. However, there have been no attempts to test this prediction. The present paper investigates the relationship between forage habitat use and LRS in red deer (Cervus elaphus) hinds within three neighbouring areas on the Isle of Rum which differed in their amounts of high-quality-forage habitat. Within areas, hinds move widely and have access to the same resources. We found no correlation between LRS of individual hinds and their use of high-quality-forage habitat (i.e. short Agrostis/Festuca grassland). Our analysis suggests that high hind densities on short Agrostis/Festuca grassland offset any advantages of increased access to preferred forage. These results support the hypothesis that red deer hinds are distributed in an ideal free manner with respect to the use of high-quality-forage habitat. However, hinds rarely leave areas where they are born, and the analysis suggests that constraints in changing areas hindered an ideal free distribution on a larger spatial scale. Consequently, mean LRS was not the same within the three investigated areas: one area, with a low amount of short Agrostis/Festuca grassland and a low hind density, contributed more male offspring (and more total offspring) per hind to the population than the other two areas. Received: 11 September 1998 / Accepted: 15 March 1999     

Ectoparasites,nest site choice and breeding success in the pied flycatcher

It has recently been suggested that nest box studies might bias the measurement of behavioural and life-history traits, because the removal of old nests may reduce the load of ectoparasites. This experimental artefact may have notable effects on nest site choice and breeding success in cavity-breeding birds. We tested (i) if pied flycatchers Ficedula hypoleuca prefer clean nest boxes and (ii) if old nest material affects the number of parasites and the breeding success of pied flycatchers. In the first experiment we offered birds one cleaned nest box and one nest box with old nest material from the previous year. The two nest boxes were placed in very similar sites near each other. In this experiment all pied flycatchers clearly preferred dirty nest boxes. In the second part of the study we distributed clean nest boxes and dirty ones on the same study area. After breeding was over we counted the number of fleas Ceratophyllus gallinae in the nest material. This flea species was the most abundant and probably the most hazardous parasite in the nests. Surprisingly, we found that there were significantly more fleas in the nest boxes with nests of the current year only than in the boxes with nests of both current and previous year. This might explain the preference for the dirty boxes. However, our results do not indicate that the number of fleas affects breeding success in the pied flycatcher.     

Aggressive neighbors and dense nesting: nest site choice and success in high-Arctic common eiders

Minimizing the risk of nest predation has led some bird species to exploit the nest defense behavior of other species. At Nasaruvaalik Island, Nunavut, Canada, some common eiders (Somateria mollissima borealis) nest within the boundaries of Arctic tern (Sterna paradisaea) colonies, while others nest elsewhere on the island, away from the terns. We tested the effects of location (within vs. outside the tern colonies), density of common eider nests, and annual variation on the nesting parameters of common eiders. Our results suggest that nesting in association with Arctic terns does not confer an obvious benefit to eiders. Such associative nesting of eiders and terns may be the result of overlapping habitat preferences between the two species, or a general scarcity of suitable nesting habitat for ground-nesting species in the high Arctic. However, eiders nesting in higher densities with other eiders had greater nest success and lower total clutch predation, indicating a positive correlation between nest density and success.     

Effects of urbanisation on disease prevalence and age structure in blackbird Turdus merula populations

Despite increasing interest in urban ecology most attention has focussed on describing changes in assemblage composition and structure along urbanisation gradients, whilst relatively little research has focussed on the mechanisms behind these changes. Ecological theory predicts that alterations in biotic interactions are particularly likely to arise, especially with regard to disease risk. Here, we report on differences in prevalence of avian malaria and tick infection and intensity in 11 paired urban and rural blackbird Turdus merula populations from across the western Palearctic. We find large and consistent reductions in tick prevalence and intensity in urban areas. There are also large reductions in the prevalence of avian malaria in many, but not all, urban areas. The proportion of first year birds in urban populations is significantly lower than that in rural ones, and across the more natural rural sites southerly populations contain fewer first years than northern ones. These patterns are expected to arise if survival rates are higher in urban areas, and are negatively correlated with latitude.     

人工巢箱条件下杂色山雀的巢位选择及其对繁殖成功率的影响

杂色山雀(Parus varius)是一种分布区域极其狭窄的小型森林洞巢鸟类,种群数量稀少,在中国大陆仅见繁殖于辽宁省的东南部山区和毗邻辽宁的吉林省西南部山区,为当地留鸟.为了探讨巢址选择对杂色山雀繁殖的影响,找出影响杂色山雀繁殖成功率的巢址选择方面的主要因子,于2009-2011年3-7月,在辽宁省仙人洞国家级自然保护区通过悬挂人工巢箱,进行杂色山雀对人工巢箱的巢址选择以及不同的巢址对杂色山雀繁殖成功率的影响研究.研究表明:杂色山雀多在针阔混交林中活动和繁殖,对位于赤松与蒙古栎混交林中的巢箱有一定偏好.野外共发现24巢杂色山雀在人工巢箱中繁殖,其中15巢繁殖成功,9巢繁殖失败.对杂色山雀利用的巢址样方主成分分析表明,巢位因子( 19.826％)、乔木因子(17.571％)、灌木因子(13.11％)、光照因子(11.587％)、隐蔽因子(10.562％)和边缘效应因子(7.572％)是影响杂色山雀对人工巢箱选择的重要因子.利用巢箱与对照巢箱相比,两者在距水源距离、距路距离、乔木平均高度、灌木平均高度和植被类型这5个环境变量上存在显著差异.繁殖成功巢与繁殖失败巢相比,繁殖成功的巢箱所在位置距路稍远,坡度较高,乔木最大高度较高,灌木盖度略低于繁殖失败巢箱.人为干扰和天敌捕食是造成杂色山雀繁殖失败的主要原因.以上研究结果表明,巢向偏南、距地面2 m以上、周围乔木高大、灌木平均高度大于1.5m、盖度在45％-55％之间、距水源20 m左右、距路20 m以外的位于针阔混交林边缘的巢箱是杂色山雀繁殖的最优巢址.目前杂色山雀的种群数量还很稀少,希望本研究能对这一珍稀鸟类的保护提供重要参考.