Intraseasonal effects of clutch manipulation on parental provisioning and residual reproductive value of eastern phoebes (Sayornis phoebe)

Summary First clutches of double-brooded eastern phoebes Sayornis phoebe were manipulated (up two eggs, down 2 eggs or no change) to test for intraseasonal reproductive tradeoffs and to test whether size of first brood influenced food delivery rates to nestlings and nestling quality in second broods.Considering all nests from both broods, rate of feeding nestlings increased linearly with brood size but nestling mass per nest decreased with increasing brood size. High nestling weights in small broods may have resulted from parents delivering better quality food, but we did not test this.Among treatment groups in first broods, nestlings from decreased broods weighed more than those in control or increased broods. Treatment did not influence the likelihood that second nests would be attempted after successful first nests nor did it alter the interval between nests. Nestlings of parents that renested weighed more than those of parents that did not, regardless of treatment, suggesting that post-fledging care may preclude renesting. Mass of individual females did not change between broods, regardless of brood size. Clutch sizes of second attempts were not affected by manipulations of first broods but increasing first broods reduced the number of nestlings parents were able to raise to day 11 in their second broods. However, manipulation of first broods did not affect mean nestling mass per nest of nestlings that survived to day 11.In phoebes, parents of small first broods are able to raise nestlings in better condition. We predict that in harsh years, parents of small first broods would be more likely to renest. Parents of enlarged first broods sacrificed quality of offspring in second broods, which seems a reasonable strategy if nestlings from second broods have lower reproductive value.     

Supplemental food increases ornamentation of male nestling Eastern Bluebirds

Although the condition‐dependence and signaling function of ornamental plumage coloration among adult males is well studied, less research has focused on the information content of ornamental coloration among juvenile birds. Eastern Bluebird (Sialia sialis) nestlings grow their nuptial plumage while in the nest and dependent on parents for food, making them an ideal species for studying the development and function of elaborate plumage. Previous research suggests that plumage brightness of Eastern Bluebirds functions, in the juvenile stage, in parent–offspring interactions as a sexually selected trait in adults. Using an experimental approach, we tested the effects of supplemental food on the structural plumage coloration (i.e., tips of primary feathers) of Eastern Bluebird nestlings in Watauga County, North Carolina, during the 2011 breeding season. We provided supplemental mealworms daily to breeding pairs from the onset of incubation through the nestling period, and measured plumage brightness, UV chroma, and mass of nestlings (N = 89 males and 71 females). Male nestlings of supplementally fed parents exhibited brighter plumage. The mass and UV chroma of young bluebirds were not significantly affected by food supplementation. However, the relationship between mass and brightness differed between male nestlings in the control and supplementally fed treatments. Males reared in food‐supplemented territories exhibited a positive relationship between color and mass. Nestlings in control territories, however, exhibited a negative relationship between size and brightness, suggesting that reduced food availability results in a tradeoff between allocating resources toward somatic growth and development of bright plumage. Our results suggest that UV‐blue structural plumage in male juvenile Eastern Bluebirds is at least partially condition‐dependent and may help to explain why plumage color can influence social interactions in Eastern Bluebirds.     

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.     

Diets of adult and nestling starlings (Sturnus vulgaris) in Hawke’s Bay,New Zealand

Abstract

Gizzards were examined from 334 adult and 62 nestling starlings collected in mixed farmland during 1971–72. The birds ate insects, spiders, earthworms, snails, millipedes, centipedes, seeds, and fruits. Starlings ate fewer subterranean animals (7%) than those usually living partially hidden (45%), on the ground (31%), or on vegetation (18%). About half the invertebrates eaten were 2–5 mm long and about a quarter 6–10 mm long. Nestlings tend to be fed significantly larger items than were eaten by adults. The commonest items in adult starlings were Coleoptera adults, Lepidoptera larvae, Hemiptera, and fruits; in nestlings, Coleoptera adults and Diptera adults and larvae were important. Earthworms were found in all the nestlings. More than 50% of adult gizzards contained earthworm chaetae in wetter months, but fewer in drier months. The diet of starlings, despite considerable overlap with the foods eaten by mynas, rooks, and magpies, included somewhat different components and proportions of the food supply. Fruits were found in adults only; most were probably eaten after harvest. Although predation on two major pasture pests—Costelytra zealandica and Wiseana cervinata larvae—was insubstantial, nearly 40% of the total invertebrates eaten by adult starlings were insect pest species.     

Provisioning with cereal grain depresses the body condition of insectivorous Yellowhammer Emberiza citrinella nestlings

Capsule The body condition of nestling Yellowhammers was negatively correlated with the amount of cereal grain in the diet. This supports the hypothesis that cereal grain is an inferior nestling food relative to invertebrates. Cereal grain is frequently provisioned to nestlings in a range of bunting species breeding on farmland, suggesting that invertebrates may be limiting in these habitats.     

Reduced nestling growth of East African Stonechats Saxicola torquata axillaris in the presence of a predator

We investigated nestling growth of tropical East African Stonechats Saxicola torquata axillaris to evaluate the effects of nest predation, predator presence and food availability. We provided some Stonechat pairs with supplemental food, while others in a similar habitat served as a control. Concomitantly, we assessed the presence of Fiscal Shrikes Lanius collaris in supplemental fed and unsupplemented territories. Fiscal Shrikes prey on adult Stonechats and nestlings. We found that nestling growth was considerably reduced in Stonechat pairs that shared their territory with a Shrike. This effect was greater in nestlings of pairs that did not receive supplemental food. The reduction in nestling growth rates was significantly correlated with a reduced rate of visiting by the parents. Behavioural observations further suggested that parents reduced their feeding visits to the nest presumably to minimize their own predation risk, rather than predation risk of their brood. Our experiments show that the lower reproductive investment in tropical Stonechats can be attributed to risk-sensitive behaviour of the parents, especially when food is in limited supply.     

Heritability of Nestling Growth in Cross-Fostered European Starlings Sturnus Vulgaris

In altricial birds, growth rates and nestling morphology vary between broods. For natural selection to produce evolutionary change in these variables, there must exist heritable variation. Since nestling traits are not any longer present in parents, traditional offspring-parent regressions cannot estimate heritabilities of these. In this study, a partial cross-fostering experiment was performed, where nestlings of the European Starling (Sturnus vulgaris) were reciprocally exchanged between nests. The experiment demonstrated a significant heritability of nestling tarsus length and body mass, but not of the growth trajectories followed by individual nestlings. The heritability estimate for tarsus length obtained in the cross-fostering experiment using full-sib analysis was lower than those obtained by offspring-parent regressions. This is likely due to a genotype-by-environment effect on tarsus length, with nestlings destined to become large but in poor condition having a low probability of appearing as parents. The main reason for the low heritability of growth was probably the large within-brood variation in growth pattern due to the initial size hierarchy of nestlings. Nestlings demonstrated targeted growth, where small-sized nestlings that initially grew slower than their siblings, managed to catch up.     

Mouth colour components of begging are dynamic signals of quality in European starling nestlings

Offspring solicit food from their parents through begging signals. Nestling skin and flange coloration are begging signals that appear to convey information about nestling need or condition, and several experiments have shown that modifications of nestling coloration affect parental allocation decisions. However, it is important to examine the short‐term changes in these signalling components in response to food constraints since such dynamic changes are required for signals to indicate condition or need. Using a food deprivation experiment, we tested whether flange and skin reflectance in European starling Sturnus vulgaris nestlings change after a three‐hour interval. We investigated whether flange and skin reflectance changed according to the predictions arising from the ‘signal of quality’ or ‘signal of need’ hypotheses on the function of begging signals. We found that flange carotenoid and UV reflectance changed according to the signal of quality hypothesis with nestlings in good condition increasing their signal expression in response to the food deprivation, whereas those in poor condition decreased their signal expression. With the use of vision modelling, we show that changes in flange reflectance are detectable by starling parents. In contrast, we found a correlation going in the opposite direction for changes in skin UV reflectance. Nestlings with low lipid reserves increased their reflectance compared to nestlings with high reserves. However, vision modelling showed that short‐term changes in skin UV reflectance are not large enough to be detectable by the parents. Our study shows that flange carotenoid and UV reflectance are dynamic components of begging with short‐term variations that can be used by parents as signals of nestling quality.     

Experimental food supplementation affects the physical development,behaviour and survival of Little Owl Athene noctua nestlings

In birds, energy supply during growth is a major predictor of the fledglings' physical condition and survival prospects. Differential quantity and quality of fledglings produced under varying nestling food supplies are likely to affect the number of offspring that recruit into the breeding population. However, the underlying mechanisms and associated consequences are still poorly known. Using a partial cross‐fostering and food supplementation experiment, we estimated the effect of variation in food supply during growth on nestling survival and fledgling phenotypic traits of Little Owls Athene noctua. Survival to fledging was much higher in food‐supplemented nestlings (98.6%) than in control nestlings (82.4%). Furthermore, supplemented nestlings were on average 8.9 g heavier and were more likely to develop subcutaneous fat deposits (99.4 vs. 73.7% of treatment and control nestlings, respectively). Supplemented nestlings also had on average longer wings than control nestlings, but tarsi and culmen did not differ significantly. Furthermore, experimentally supplemented fledglings struggled more when handled and emerged sooner from tonic immobility than control fledglings. The irises of supplemented fledglings were less intensely coloured. The experimentally induced changes in nestling development probably affect individual performance beyond fledging. Nestlings from orchard‐dominated habitats were larger than those from habitats dominated by arable land. As nestling food supply is largely determined by natural food availability, we conclude that habitat quality affects Little Owl productivity and offspring quality, and ultimately, population dynamics.     

Dark nests and conspicuousness in color patterns of nestlings of altricial birds

Nests of altricial birds exhibit variable spectral properties that may affect the efficacy (conspicuousness) of the colored begging traits that a nestling displays to its parents. Here we explored whether selection for efficient perception has favored the evolution of nestling color designs that maximizes nestling detectability in variable light environments. Visual models were used to estimate how parents perceive the coloration of mouths, flanges, heads, and breasts of nestlings within their nest in 21 species of European birds. We show that the largest chromatic and achromatic contrasts against the nest background appeared for nestling mouths and flanges, respectively. Nestlings of open-nesting species showed a larger general achromatic contrast with the nest than did nestlings of hole-nesting species. However, nestlings of hole nesters showed a more evident achromatic contrast between flanges and other traits than did nestlings of open nesters. In addition, species with larger clutch sizes showed larger general achromatic contrasts with the nest. Gaping traits of open-nesting species contrasting with the nest background were better perceived under rich light regimes than under poor ones. These findings are consistent with a scenario in which selection for nestling detectability in dark environments has favored the evolution of particular achromatic components of gape coloration but also nestling traits that enhance signal efficacy by maximizing color contrasts within a nestling.     

Incubation temperature affects growth and energy metabolism in blue tit nestlings

Because the maintenance of proper developmental temperatures during avian incubation is costly to parents, embryos of many species experience pronounced variation in incubation temperature. However, the effects of such temperature variation on nestling development remain relatively unexplored. To investigate this, we artificially incubated wild blue tit (Cyanistes caeruleus L.) clutches at 35.0°, 36.5°, or 38.0°C for two-thirds of the incubation period. We returned clutches to their original nests before hatching and subsequently recorded nestling growth and resting metabolic rate. The length of the incubation period decreased with temperature, whereas hatching success increased. Nestlings from the lowest incubation temperature group had shorter tarsus lengths at 2 weeks of age, but body mass and wing length were not affected by temperature. In addition, nestlings from the lowest temperature group had a significantly higher resting metabolic rate compared with mid- and high-temperature nestlings, which may partly explain observed size differences between the groups. These findings suggest that nest microclimate can influence nestling phenotype, but whether observed differences carry over to later life-history stages remains unknown.     

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.     

Carotenoids in nestling Montagu’s harriers: variations according to age, sex, body condition and evidence for diet-related limitations

Carotenoids are colored pigments forming the basis of many avian social traits. Before their utilization carotenoids must be acquired through diet and mobilized for specific uses. The relationships between carotenoid-based coloration, circulating carotenoids and body condition have been well studied in adult birds, but little is known in nestlings. Here, we investigated variations in carotenoid-based coloration in a raptor nestling, the Montagu’s harrier (Circus pygargus), both in captivity and in natural conditions, and within a vole (poor-carotenoid source and cyclic prey) specialist population. We studied these variations according to nestling age and sex, and possible limitations in carotenoid availability by comparing years of contrasted prey abundance and using carotenoid supplementation experiments. Captive nestlings, fed only with mice, were strongly carotenoid limited. Wild nestlings were also carotenoid limited, especially in a year of high vole abundance. Nestlings were in better condition but less colored during a peak vole abundance year than during a low vole abundance year, when harriers targeted more alternative preys (birds, insects). Thus, variation in vole abundance resulted in a de-coupling of body condition and carotenoid-based coloration in this population. This suggested that the positive relation between the body condition and carotenoid-based traits, typically found in adult birds, could be restricted to adults or nestlings of species that feed on carotenoid-rich food. Our results should stimulate more work on the functions and mechanisms of carotenoid-based traits in nestlings, which deserve more attention and most likely differ from those of adult birds.     

The effects of diet quality and quantity on plumage colour and growth of great tit Parus major nestlings: a food manipulation experiment along a pollution gradient

The yellow carotenoid-based plumage coloration of great tit Parus major nestlings is found to be paler in polluted and urban environments. Because carotenoid pigmentation is often considered to be a condition dependent trait in birds we wanted to find out whether food-limitation and poor nestling condition could explain the pale plumage colour in a polluted area. P. major nestlings were supplemented with variable diets along a well known heavy metal pollution gradient around a copper smelter: two food treatments with carotenoids, one food treatment with little carotenoid and one unsupplemented control. Our field experiment showed that nestlings in the polluted area grew better with carotenoid rich diets, while such effect was not found in the unpolluted area. Nestlings showed higher plasma carotenoid (lutein) levels and higher plumage carotenoid chroma values in the unpolluted area than in the polluted area. However, plasma lutein levels or plumage colour were not associated with heavy metal levels in nestling faeces (a proxy for dietary exposure). Our results provide only weak evidence for carotenoid-based colouration to be condition-dependent in great tit nestlings as we found a positive relationship between body mass and carotenoid chroma in the non-supplemented control group only. The positive relationship between body mass and plumage colour intensity is more likely to be produced by the fact that good availability of caterpillars, an important food source for P. major, also means a good availability of carotenoids to nestlings. Our results suggest that main reason for pale nestling plumage in the polluted area is lower quality invertebrate food, and not nutrition-related oxidative stress.     

Intensity of mouth coloration in Blackcap Sylvia atricapilla nestlings affects food distribution among siblings but not provisioning of the whole brood

Among various begging stimuli, mouth coloration has received increasing attention in recent years, and previous research has demonstrated that mouths of nestling Canaries Serinus canaria get redder with the extent of food deprivation and that parents preferentially feed nestlings of redder gapes. This study assesses whether the intensity of red mouth colour in nestling Blackcaps Sylvia atricapilla is a signal in parent–offspring communication. This is one of the few species with a naturally red gape in which the function of mouth redness has been tested. Three predictions were experimentally tested: (1) reddening the gape of a single nestling within a brood increases its provisioning in relation to other siblings; (2) reddening the gapes of all nestlings within a brood increases parental feeding rate; and (3) food deprivation increases nestling mouth redness. The effect of nestling quality on mouth redness was also assessed. The intensity of gape coloration affected food distribution, but in a way opposite to that expected: an increase in mouth redness of the nestling caused reduced feeding by parents. However, reddening the gapes of all nestlings had no effect on provisioning of the whole brood, suggesting that Blackcap parents use different cues for provisioning particular nestlings and the whole brood. Intensity of mouth redness in Blackcap nestlings was not affected either by food deprivation or by nestling quality in terms of mass and rank in the nest.     

Seasonal changes in cell‐mediated immunocompetence and mass gain in nestling barn swallows: a parasite‐mediated effect?

Reproduction in seasonal environments is usually timed so peak demand for food by offspring coincides with peak availability. Hence, late breeders will encounter a scarcity of food. Since parasite populations grow during the reproductive season of their hosts, late reproducing animals will also face an increasing challenge by parasites. We hypothesised that seasonal decrease in food availability and seasonal increase in parasite abundance will cause a trade-off between growth and immune function. This prediction was tested in nestling barn swallows ( Hirundo rustica ) from first and second broods. Nestlings from second broods mounted stronger T cell mediated immune responses to a challenge with a novel antigen, but had lower rates of mass gain, than nestlings from first broods, consistent with the prediction. Broods in which at least one nestling died had lower levels of T cell mediated immune response, but not lower rates of mass gain, than broods without mortality, suggesting that brood reduction is mediated through an inability of offspring to defend themselves against parasites rather than an inability to grow. Possible mechanisms include scarcity of specific nutrients needed for immune responses, and/or parasites being concentrated on a single or few nestlings.     

Resource allocation between growth and endothermy allows rapid nestling development at low feeding rates in a species under high nest predation

Nestling development is among the most energy‐demanding periods of a bird’s lifetime and altricial species require extensive parental energy investment in the form of feeding and heating. In the present study I analyze the relation and trade‐offs between nestling growth, development of thermoregulation and feeding rate in blackcap Sylvia atricapilla, a species suffering from high nest predation. Nestlings were characterized by rapid growth but they achieved only 80% of adult mass prior to fledging. Body mass showed highest relative growth rate before nestlings achieved homeothermy. The onset of endothermy, indicated in day 7, coincided with 90% of nestling fledgling weight, indicating that the two processes are separated in time. A strong negative correlation between feeding rate and growth rate demonstrates that blackcap nestlings develop their bodies under relatively low feeding rates and more feeding is needed for maintenance of body temperature than for body growth. The study indicates high cost of endothermy for parents – endothermic nestlings received over 100% more feedings than ectothermic ones. The findings are discussed in the light of adaptation of the species to high predation risk.     

Brood size manipulations reveal relationships among physiological performance,body condition and plumage colour in Northern Flicker Colaptes auratus nestlings

Visual signals of quality in offspring, such as plumage colour, should honestly advertise need and/or body condition, but links between nutritional status, physiological performance and the expression of colours are complex and poorly understood. We assess how food stress during rearing affected two physiological measures (T‐cell‐mediated immune function and corticosterone level in feathers: CORT_f) and how these two variables were related to carotenoid and melanin coloration in Northern Flicker Colaptes auratus nestlings. We were also interested in how these two physiological measures were influenced by the sex of the nestling. We experimentally manipulated brood size to alter levels of food availability to nestlings during development. We measured carotenoid‐based colour (chroma and brightness) in wing feathers and the size of melanin spots on breast feathers. In agreement with our prediction, nestlings in the reduced brood treatment had better body condition and stronger immune responses than those in the control and brood enlargement treatments. This supports the hypothesis that immune responses are energetically costly. In contrast, CORT_f was not related to nestling body condition or sex and was unaffected by brood size manipulation. Nestlings of both sexes with stronger T‐cell‐mediated immune responses had larger melanin spots but only males with higher immune responses also had brighter flight feathers. Feather brightness decreased with increasing CORT_f levels. Our study is one of the few to examine the relationship between multiple physiological and plumage measures in nestlings and shows that plumage colour and immune function signalled body condition of nestlings, but that feather corticosterone levels did not.     

Prothonotary warbler nestling growth and condition in response to variation in aquatic and terrestrial prey availability

Aquatic prey subsidies entering terrestrial habitats are well documented, but little is known about the degree to which these resources provide fitness benefits to riparian consumers. Riparian species take advantage of seasonal pulses of both terrestrial and aquatic prey, although aquatic resources are often overlooked in studies of how diet influences the reproductive ecology of these organisms. Ideally, the timing of resource pulses should occur at the time of highest reproductive demand. This study investigates the availability of aquatic (mayfly) and terrestrial (caterpillar) prey resources as well as the nestling diet of the prothonotary warbler (Protonotaria citrea) at two sites along the lower James River in Virginia during the 2014 breeding season. We found large differences in availability of prey items between the two sites, with one having significantly higher mayfly availability. Nestling diet was generally reflective of prey availability, and nestlings had faster mean growth rates at the site with higher aquatic prey availability. Terrestrial prey were fed more readily at the site with lower aquatic prey availability, and at this site, nestlings fed mayflies had higher mean growth rates than nestlings fed only terrestrial prey. Our results suggest that aquatic subsidies are an important resource for nestling birds and are crucial to understanding the breeding ecology of riparian species.     

Haemoglobin concentration and body condition of nestling Great Tits Parus major : a comparison of first and second broods in two contrasting seasons

The physiological condition of nestling altricial birds depends on the quantity and quality of food delivered to them by parents. One indicator of the condition of Great Tit Parus major nestlings is the haemoglobin concentration in their blood. The present study demonstrates the influence of weather conditions (temperature and rainfall) on nestling haemoglobin concentrations during two consecutive breeding seasons in two different habitat types (parkland vs. woodland) in the city of Łódź in Central Poland. This influence probably results from the effects of weather on the trophic base of the Tits. Dry, hot weather strongly affected bush and herbal foliage later in the breeding season (mid-June to mid-July) in 2006, presumably by interfering with the development of herbivorous arthropod populations. This in turn caused food shortages for second broods of Great Tits, which resulted in nestlings having low haemoglobin levels. In the following year, temperature was on average lower, and rainfall was regular but not very heavy. These conditions enabled the development of arthropod assemblages, and the trophic base for birds was much richer. Haemoglobin concentrations in the blood of nestlings from second broods were significantly higher than those of first broods and, unexpectedly, second-brood nestlings in 2007 were on average in better physiological state than first-brood nestlings in 2006 in both habitats. The relationship between haemoglobin concentration, brood category and year was very similar to that for nestling body mass. However, it was independent of both body mass and brood size. In some years and under certain conditions, second broods can be more successful than first broods.