Prudent investment in rearing nestlings in bull-headed shrikes

I investigated seasonal changes in the relationships between brood size, body mass of nestlings and body mass of parents of the bull-headed shrike, Lanius bucephalus, in Ishikari, northern Japan. When the broods were 12days old, the body mass of the heaviest nestling in a brood did not differ among brood sizes, or throughout the season. However, the body mass of the lightest nestlings in a brood was different among brood sizes. The body mass of the lightest nestling in five- and six-nestling broods decreased throughout the season. The lightest nestling in four-nestling broods, and the lightest and the second lightest nestlings in five-nestling broods, were significantly lighter than the heaviest nestling in broods of this size. It is likely that pairs with six nestlings at 12days old can feed at least five of these nestlings enough to ensure their survival . The standardized body mass of parents (SBM), which was defined as the body mass divided by the length of the tarsus, did not differ among brood sizes, or throughout the season. It is possible that the relationship between the constancy of the SBM and the seasonal decline in the body mass of nestlings indicates that bull-headed shrikes have a limit to their parental efforts.     

Change in body mass in relation to breeding phase in bull-headed shrikes

Changes in body mass of both sexes of the bull-headed shrike, Lanius bucephalus, were investigated over the six breeding phases of this species. Standardized body mass (SBM), which was defined as body mass divided by the length of the tarsus, of the males was relatively constant over the phrases, while the SBM of the females changed drastically. In the females, the SBM during egg-laying was higher than the SBM in all the other breeding phases. The SBM of the females did not differ between the incubation and early-nestling phases, and was lowest during the fledgling phase. Mass loss in bull-headed shrikes did not correspond to an adaptive adjustment of body mass to permit a reduction in the power required for flight in the nestling period. It is likely that incubating females constantly maintain their body mass to invest in parental efforts from the incubating to nestling periods.     

Heavy and persistent rainfall leads to brood reduction and nest failure in a passerine bird

The process of reproduction is a sensitive period for bird offspring, since they are exposed to environmental conditions for several weeks. Within the long reproductive period, adult birds do not only have to maintain their own body condition, they also have to ensure nutritional needs of their nestlings. In the worst case, if weather conditions are very harsh, breeding is not successful and fails. This suggests that weather conditions might be an important driver of breeding success. Here, we studied the effect of weather conditions (temperature and precipitation) on the survival of 350 nests of great tits Parus major during nestling period in six years (2010–2015). We used traditional generalized linear mixed model (binomial response variable) and a dedicated survival-analysis program (MARK) to analyze the data. The two approaches allowed us to highlight and compare the effect of weather conditions on a fine (nestling survival) and coarse (nest fate) scale. Both methods showed that precipitation explained most of the variation in individual nestling and overall nest survival during the 15-day nestling period. Heavy and persistent rainfall did not only lead to brood reduction, it ultimately also led to losses of the entire brood. Likely causes were the negative effects of precipitation on thermoregulation, food availability and predation risk. However, while reduced food availability most likely might have led to brood reduction through selective individual nestling death, predation might have resulted in total nest failures.     

Reduced immunocompetence of nestlings in replacement clutches of the European magpie (Pica pica)

Laying date is one of the most important determinants of reproductive success and recruitment probability in birds. Late breeders usually fledge fewer chicks than individuals with earlier breeding dates, and fledglings produced late in the season have high mortality rates. Food availability and nestling mass have been evoked as the principal mechanistic links between laying date and offspring survival. Here we suggest that another factor may actually account for the difference in survival rate between early and late offspring: immunocompetence. We predicted that nestlings produced later in the season or in replacement clutches should have lower immune responses when challenged with an antigen, than early nestlings or nestlings produced in first clutches. This hypothesis was tested in a population of magpies (Pica pica), in which we experimentally induced breeding failure in a group of nests and compared the immune response of nestlings in replacement clutches with the immune response of first clutch nestlings. Cellular immune response, as measured by wing web swelling (a correlate of T-lymphocyte production after injection of phytohaemagglutinin-P), significantly decreased with hatching date and was significantly lower in nestlings of replacement clutches. Furthermore, coefficients of intraclutch variation in immune response were higher in nestlings of replacement clutches. This experiment demonstrates an inverse relationship between immune responsiveness and breeding date, and reduced recruitment probability of late nestlings may be a direct consequence of their inability to cope with parasites.     

Fitness-related consequences of egg mass in nestling house wrens

The fitness-related consequences of egg mass, independent of confounding influences associated with parental quality, remain poorly understood for wild birds in general and for passerines in particular. We performed cross-fostering experiments to test the hypothesis that egg mass, independent of parental quality, is the primary determinant of fitness-related traits in nestling house wrens (Troglodytes aedon), an insectivorous passerine. Nestling mass was significantly correlated with the mass of the eggs from which nestlings hatched early but not late in the nestling period in early-season broods. In contrast, in late-season broods, nestling mass was correlated with egg mass until nestlings achieved asymptotic mass. Neither nestling growth nor survival to nest leaving was related to egg mass in either early- or late-season broods; however, nestlings in late-season broods grew more slowly than did nestlings in early-season broods. We propose that nestling mass and egg mass remained correlated throughout the nestling period in late-season broods because decreased arthropod food resources late in the breeding season constrain parents'' ability to provision nestlings. We conclude that female house wrens in this population trade-off clutch size for greater egg mass to maximise reproductive success in late-season broods.     

A recipe for postfledging survival in great tits Parus major: be large and be early (but not too much)

Survival of juveniles during the postfledging period can be markedly low, which may have major consequences on avian population dynamics. Knowing which factors operating during the nesting phase affect postfledging survival is crucial to understand avian breeding strategies. We aimed to obtain a robust set of predictors of postfledging local survival using the great tit (Parus major) as a model species. We used mark–recapture models to analyze the effect of hatching date, temperatures experienced during the nestling period, fledging size and body mass on first‐year postfledging survival probability of great tit juveniles. We used data from 5192 nestlings of first clutches ringed between 1993 and 2010. Mean first‐year postfledging survival probability was 15.2%, and it was lower for smaller individuals, as well as for those born in either very early or late broods. Our results stress the importance of choosing an optimum hatching period, and raising large chicks to increase first‐year local survival probability in the studied population.     

High nest failure but better nestling quality for early breeders in an alpine population of Northern Wheatear (Oenanthe oenanthe)

Climate change is leading to the advancement of spring conditions, resulting in an earlier snowmelt and green-up, with highest rates of change in highly seasonal environments, including alpine habitats. Migratory birds breeding at high elevations need to time their arrival and lay dates accurately with this advancement, but also with the annually variable spring conditions at their breeding sites, to maximize nest survival probabilities and reproductive output. Nest survival probability and mean nestling mass were analysed in relation to lay date and habitat conditions in an alpine population of the migratory Northern Wheatear Oenanthe oenanthe collected over six consecutive breeding seasons in the Western Italian Alps. This open grassland species showed the lowest nest survival probability in years with an early onset of spring conditions. Within-season, nest survival was highest when breeding late, at lower elevations, and when grass cover and grass height were higher. Both across- and within-season, severe weather conditions may indirectly lead to higher early season nest failure rates by increasing predation risk. By contrast, mean nestling mass, and thus the quality of the fledglings, was lower when breeding late. This might be driven by a mismatch with the peak in food abundance. Breeding early is thus generally advantageous in terms of chick quality in our high-elevation population, but reproductive success is limited by the risk of nest failure that is higher in early springs and early in the season. This trade-off between breeding early and late may thus allow Northern Wheatears to maximize fitness under highly variable spring conditions. However, climate change may cause disruption to this trade-off, and shifts in phenology could become a threat for migratory alpine birds that might not be able to keep track of advancing spring conditions.     

Factors Affecting the Duration of Nestling Period and Fledging Order in Tengmalm’s Owl (Aegolius funereus): Effect of Wing Length and Hatching Sequence

In altricial birds, the nestling period is an important part of the breeding phase because the juveniles may spend quite a long time in the nest, with associated high energy costs for the parents. The length of the nestling period can be variable and its duration may be influenced by both biotic and abiotic factors; however, studies of this have mostly been undertaken on passerine birds. We studied individual duration of nestling period of 98 Tengmalm’s owl chicks (Aegolius funereus) at 27 nests during five breeding seasons using a camera and chip system and radio-telemetry. We found the nestlings stayed in the nest box for 27 – 38 days from hatching (mean ± SD, 32.4 ± 2.2 days). The individual duration of nestling period was negatively related to wing length, but no formally significant effect was found for body weight, sex, prey availability and/or weather conditions. The fledging sequence of individual nestlings was primarily related to hatching order; no relationship with wing length and/or other factors was found in this case. We suggest the length of wing is the most important measure of body condition and individual quality in Tengmalm’s owl young determining the duration of the nestling period. Other differences from passerines (e.g., the lack of effect of weather or prey availability on nestling period) are considered likely to be due to different life-history traits, in particular different food habits and nesting sites and greater risk of nest predation among passerines.     

Experimentally increased nest temperature affects body temperature,growth and apparent survival in blue tit nestlings

The thermal environment experienced by birds during early postembryonic development may be an important factor shaping growth and survival. However, few studies have directly manipulated nest temperature (T _n) during the nestling phase, and none have measured the consequences of experimental heat stress on nestlings’ body temperature (T _b). It is therefore not known to what extent any fitness consequences of development in a thermally challenging environment arise as a direct, or indirect, effect of heat stress. We, therefore, studied how experimentally increased T _n affected T _b in 8–12 d old blue tit Cyanistes caeruleus nestlings, to investigate if increased thermoregulatory demands to maintain normothermic T _b influenced nestling growth and apparent long‐term survival. Nestlings in heated nest‐boxes had significantly higher T _b compared to unheated nestlings during most of the experimental period. Yet, despite facing T _n  50°C (as measured in the bottom of the nest cup below the nestlings), the highest nestling T _b recorded was 43.8°C with nestlings showing evidence of controlled facultative hyperthermia without any increased nestling mortality in heated nests. However, body mass gain was lower in these nestlings compared to nestlings from control nest‐boxes. Contrary to our prediction, a larger proportion of nestlings from heated nest‐boxes were recaptured during their first winter, or subsequently recruited into the breeding population as first‐ or second‐year breeders. This result should, however, be treated with caution because of low recapture rates. This study highlights the importance of the thermal environment during nestling development, and its role in shaping both growth patterns and possibly also apparent survival.     

Storing prey in larders affects nestling haematological condition in the Red-backed Shrike (Lanius collurio)

Many animals, including shrikes (Laniidae), are known to store food. This behaviour ensures food availability during inclement weather, for rearing nestlings or signalling territory quality. Unlike most other shrike species, which accumulate food before breeding, Red-backed Shrikes Lanius collurio store food in larders, mainly during the nestling and fledgling periods. If this is an adaptive behaviour, variation in this behaviour should affect either or both the number and condition of nestlings, which may benefit from such larders. We assessed the haematological condition of Red-backed Shrike nestlings in east-central Poland in 2021 using a suite of blood parameters relating to sites where shrikes did or did not store food. Haemoglobin concentrations were positively correlated with larders, being significantly higher in nestlings of pairs that stored prey compared with those whose parents did not do so. White blood cell counts were lower in nestlings whose parents kept larders. Red blood cell counts and glucose concentrations in nestlings were not related to whether parents had larders. Food storage probably ensures a more constant access to food, which results in a superior haematological condition in nestlings.     

Breeding biology of the White-rumped Swallow Tachycineta leucorrhoa in Buenos Aires Province, Argentina

We conducted a study of the breeding biology of the White-rumped Swallow Tachycineta leucorrhoa nesting in nestboxes in a flat, farming landscape in Buenos Aires Province, Argentina. White-rumped Swallow nesting attempts were detected from the end of September to mid December, with most clutches laid during October. Birds laid clutches of 4–6 eggs with a mode of five eggs; most broods hatched synchronously (58%), but hatching spread could last up to 4 days. Nestling growth curves adjust well to logistic functions, and at day 15 nestlings attain the asymptotic weight of 21.6 g. Clutch size in White-rumped Swallows declined significantly as the season progressed. In addition, late-season eggs were smaller and late-season nestlings had a shorter nestling period and lower weight at day 15, probably leaving the nest lighter than early-season nestlings. These data suggest that the Swallows would benefit greatly from laying early in the season, which would provide nestlings with better survival prospects. However, both major sources of nest mortality, interspecific competition for nest-sites and nestling mortality during bad weather, decreased through the season. White-rumped Swallows follow the pattern found for other southern species, as it has smaller clutch size, lower growth rate and remains longer at the nest than its Northern Hemisphere congener the Tree Swallow Tachycineta bicolor .     

Age-related environmental sensitivity and weather mediated nestling mortality in white storks Ciconia ciconia

We studied environmental sensitivity and mortality related to weather inclemency in white stork nestlings Ciconia ciconia in their southern European boundary (Doñana, SW Spain). The study of homeothermy acquisition and fault bars (i.e. a measure of stress on feathers) revealed that stork nestlings were specially sensitive to environmental conditions occurring before 20 d of age. Accordingly, most of nestling mortality concentrated during this sensitive period: 91% of deaths corresponded to nestlings younger than 20 d, 73% concentrating on nestlings up to 10 d-old. Nestling mortality and total breeding failure were highly variable among years, being especially high when rainy periods coincided with the early live of nestlings (between 1 April and 15 May). Maximum temperatures had a positive correlation with breeding success and nestling survival but this effect disappeared when controlling for rainfall. Our results are in agreement with previous studies conducted in other white stork populations in other latitudes. We suggest that this could be the result of a low homeothermy capacity of young nestlings jointly with an early breeding phenology that expose white storks to rain, but not to high temperatures. In the context of global climate change we suggest that the current decrease on spring rainfall could increase nestling survival while punctual rainy springs could have a negative effect on the reproduction of white storks.     

Differential Begging and Locomotory Behaviour by Early and Late Hatched Nestlings Affecting the Distribution of Food in Asynchronously Hatched Broods of Altricial Birds

Distribution of food to early and late hatched nestlings was studied in asynchronously hatched broods of the great tit Parus major, the blackbird Turdus merula, and the fieldfare T. pilaris. Food distribution is related to the locomotory and begging behaviour and positions in the nest of these nestlings. Late hatched (small) nestlings were found to beg more often per feed than bigger nestlings and move more towards favoured positions in the nest to counteract selective feeding of bigger young. The functional significance of these differences in the behaviour of early and late hatched nestlings are discussed. It is argued that they are adaptive by 1) ensuring that each nestling survives when food supplies are ample, and 2) by mediating an optimal brood reduction when food is insufficient to raise the entire brood. The roles of asynchronous hatching, and selective feeding which follows from differential behaviour of early and late hatched young are discussed in relation to food conditions during the breeding season.     

The Social System of a Communal Breeder,the Bay-winged Cowbird Molothrus badius

Molothrus badius (bay-winged cowbird), an icterine blackbird with cooperative breeding, shares behavioural and ecological characteristics with other communal nesters: it is sedentary, has a high annual survival rate (76.2%) and a strong nest-site tenacity (mean breeding dispersal of 41.9 and 89.4 m for males and females). Behavioural data, including collective agonistic displays, suggest group territoriality. Before egg hatching most breeders occurred as single pairs showing territorial behaviour (82% of nests), and nesting was usually solitary (distances to nearest nests of 25–103 m). Most breeders were apparently monogamous, with a 2.5% incidence of extrapair copulations in the territory during clutch formation. During the nestling stage one to four helpers occurred at 95% of M. badius nests. Most helpers were 1–2 years old, but older breeding adults (mostly males) that failed to rear their own offspring helped at the end of the season. The number of helpers increased (up to 4) with nestling age. Helpers were also recruited during the postfledging period, and group size reached up to 10 adults at this stage. Helpers mobbed predators and brood parasites, and provided 35% of the nestling food. Provisioning rate was positively and significantly correlated with number of helpers, although age of nestlings was the best predictor of overall food delivery rate. The helping system was almost obligate and productivity comparisons between nests with/without helpers are not possible. Data suggest that helpers increased the breeding success per nest. The correlation between the provisioning rates of parents and helpers was negative but non-significant. In 18% of nests 3 to 4 individuals were present before the nestling period, including cases of apparently polyandrous trios and one case of joint nesting by two pairs. Within Brown 's (1987) categories of social organization M. badius is mainly group territorial with plural nesting. Habitat requirements of M. badius are wide and nest sites do not appear to limit breeding. Kinship plays a role in the social system, as 9 of 12 helpers marked as nestlings helped their parents.     

Optimal brood size and its limiting factors in the Rufous Turtle DoveStreptopelia orientalis

Optimal brood size and its limiting factors of the Rufous Turtle Dove,Streptopelia orientalis, were studied at the campus of the University of Tsukuba, Japan, during the breeding season in 1990–92. The dove usually lays two eggs in a nest. I made nests of a brood size of one and three by transferring a hatchling from one nest to the other, and compared their fledging success, factors of breeding failure, weight and tarsus length at fledging, growth rate and nestling period with those of a brood of two. The index of fitness (fledgling weight multiplied by average number of fledglings per nest) was almost the same in broods of two and three. However, the highest variation in fledging weight within the brood and the extension of nestling period were observed in broods of three, which caused the extension of inter-brood interval and consequently the smaller number of broods in the total breeding season. Therefore, broods of three would not have an advantage in producing more offspring than broods of two. Crop milk production had an effect on the growth of nestlings in the early phase of the nestling period, but the rapid growth in the granivorous phase compensated for the growth delay of the smallest nestling in broods of three. Small brood size and a large number of broods in a season would also be more effective under high predation pressure.     

Reduced reproductive performance associated with warmer ambient temperatures during incubation in a winter‐breeding,food‐storing passerine

Timing of reproduction can influence individual fitness whereby early breeders tend to have higher reproductive success than late breeders. However, the fitness consequences of timing of breeding may also be influenced by environmental conditions after the commencement of breeding. We tested whether ambient temperatures during the incubation and early nestling periods modulated the effect of laying date on brood size and dominant juvenile survival in gray jays (Perisoreus canadensis), a sedentary boreal species whose late winter nesting depends, in part, on caches of perishable food. Previous evidence has suggested that warmer temperatures degrade the quality of these food hoards, and we asked whether warmer ambient temperatures during the incubation and early nestling periods would be associated with smaller brood sizes and lower summer survival of dominant juveniles. We used 38 years of data from a range‐edge population of gray jays in Algonquin Provincial Park, Ontario, where the population has declined over 50% since the study began. Consistent with the “hoard‐rot” hypothesis, we found that cold temperatures during incubation were associated with larger brood sizes in later breeding attempts, but temperatures had little effect on brood size for females breeding early in the season. This is the first evidence that laying date and temperature during incubation interactively influence brood size in any bird species. We did not find evidence that ambient temperatures during the incubation period or early part of the nestling period influenced summer survival of dominant juveniles. Our findings provide evidence that warming temperatures are associated with some aspects of reduced reproductive performance in a species that is reliant on cold temperatures to store perishable food caches, some of which are later consumed during the reproductive period.     

Offspring pay sooner,parents pay later: experimental manipulation of body mass reveals trade-offs between immune function,reproduction and survival

Introduction

Life-history theory predicts that organisms trade off survival against reproduction. However, the time scales on which various consequences become evident and the physiology mediating the cost of reproduction remain poorly understood. Yet, explaining not only which mechanisms mediate this trade-off, but also how fast or slow the mechanisms act, is crucial for an improved understanding of life-history evolution. We investigated three time scales on which an experimental increase in body mass could affect this trade-off: within broods, within season and between years. We handicapped adult skylarks (Alauda arvensis) by attaching extra weight during first broods to both adults of a pair. We measured body mass, immune function and return rates in these birds. We also measured nest success, feeding rates, diet composition, nestling size, nestling immune function and recruitment rates.

Results

When nestlings of first broods fledged, parent body condition had not changed, but experimental birds experienced higher nest failure. Depending on the year, immune parameters of nestlings from experimental parents were either higher or lower than of control nestlings. Later, when parents were feeding their second brood, the balance between self-maintenance and nest success had shifted. Control and experimental adults differed in immune function, while mass and immune function of their nestlings did not differ. Although weights were removed after breeding, immune measurements during the second brood had the capacity to predict return rates to the next breeding season. Among birds that returned the next year, body condition and reproductive performance a year after the experiment did not differ between treatment groups.

Conclusions

We conclude that the balance between current reproduction and survival shifts from affecting nestlings to affecting parents as the reproductive season progresses. Furthermore, immune function is apparently one physiological mechanism involved in this trade-off. By unravelling a physiological mechanism underlying the trade-offs between current and future reproduction and by demonstrating the different time scales on which it acts, our study represents an important step in understanding a central theory of life-history evolution.

     

Influence of temperatures during the nestling period on post-fledging survival of great tit Parus major in a Mediterranean habitat

Survival during the first year is one of the most important factors determining fitness in birds. Birds have poor thermoregulatory abilities during the nestling period when the plumage is not fully developed, thus temperature during the nestling period is a serious candidate to affect post-fledging survival, although it has been usually ignored in previous studies. We analysed the relationship between temperatures and post-fledging survival in a great tit Parus major population in Sagunto (Spain) using capture-recapture data from 12 years. Hatching dates, mass at fledging and temperatures during the nestling period (maximum, minimum and mean ambient temperatures as an estimation of the mean and extreme weather conditions that chicks encountered at the nest) were used as individual covariates. Mean post-fledging survival was 0.13±0.01. Adult survival probability was 0.64±0.02. Multi-model inference suggested that post-fledging survival increased with fledging mass, and decreased as temperatures increased. Furthermore, the effect of mass on survival was less important as temperature was higher. We consider that high temperatures affect nestlings' health due to low thermoregulatory abilities of nestlings. Model selection did not support a relationship between hatching dates and survival once mass and temperatures were taken into account. The results suggest the possibility that the effect of date on post-fledging survival found in previous studies was, at least in part, a consequence of the seasonal pattern of temperature variation.     

Effects of temperature and nest heat exposure on nestling growth,dehydration and survival in a Mediterranean hole‐nesting passerine

Variable environments impose constraints on adaptation by modifying selection gradients unpredictably. Optimal bird development requires an adequate thermal range, outside which temperatures can alter nestling physiology, condition and survival. We studied the effect of temperature and nest heat exposure on the reproductive success of a population of double‐brooded Spotless Starlings Sturnus unicolor breeding in a nestbox colony in central Spain with a marked intra‐seasonal variation in temperature. We assessed whether the effect of temperature differed between first and second broods, thus constraining optimal nest‐site choice. Ambient temperature changed greatly during the chick‐rearing period and had a strong influence on nestling mass and all body size measures we recorded, although patterns of clutch size or nestling mortality were not influenced. This effect differed between first and second broods: nestlings were found to have longer wings and bills with increasing temperature in first broods, whereas the effect was the opposite in second broods. Ambient temperature was not related to nestling body mass or tarsus‐length in first broods, but in second broods, nestlings were lighter and had smaller tarsi with higher ambient temperatures. The exposure of nestboxes to heat influenced nestling morphology: heat exposure index was negatively related to nestling body mass and wing‐length in second broods, but not in first broods. Furthermore, there was a positive relationship between nest heat exposure and nestling dehydration. Our results suggest that optimal nest choice is constrained by varying environmental conditions in birds breeding over prolonged periods, and that there should be selection for parents to switch from sun‐exposed to sun‐protected nest‐sites as the season progresses. However, nest‐site availability and competition for sites are likely to impose constraints on this choice.     

Nestling growth and mortality of Pied Flycatchers Ficedula hypoleuca in relation to weather and breeding effort

The growth pattern and mortality of young Pied Flycatchers Ficedula hypoleuca were studied to focus on the mechanisms and constraints behind the widely studied optimization of clutch size. The clutch sizes were modified, and the growth and survival of chicks from different clutch sizes were monitored along with the prevailing weather during the nestling period to detect the effect of weather on reproductive success. The weather conditions during the feeding period of the nestlings varied within a season as well as between breeding seasons. The prevailing weather markedly affected both the growth rate and the survival of chicks, yet, the effects of weather on growth were not greater in enlarged clutches. The impact of adverse weather was more pronounced in the later phases of nesting, when the food demand of a brood was highest. Brood reduction and total nest losses were more likely during extensive rainfall during the nestling phase and also in the enlarged clutches. Thus, weather is a very important determinant of reproductive success in this species. Weather conditions during the breeding season are unpredictable, however, and therefore brood reduction through sibling competition is a mechanism whereby brood size can be adjusted to the level the parents can rear under the prevailing conditions.