Settling Decisions and Heterospecific Social Information Use in Shrikes

Animals often settle near competitors, a behavior known as social attraction, which belies standard habitat selection theory. Two hypotheses account for these observations: individuals obtain Allee benefits mediated by the physical presence of a competitor, or they use successfully settled individual as a source of information indicating the location of high quality habitat. We evaluated these hypotheses experimentally in two species of shrikes. These passerine birds with a raptor-like mode of life impale prey to create larders that serve as an indicator of male/habitat quality. Thus, two forms of indirect information are available in our system: a successfully settled shrike and its larder. Typically these two cues are associated with each other, however, our experimental treatment created an unnatural situation by disassociating them. We manipulated the presence of larders of great grey shrikes and examined the settling decisions of red-backed shrikes within and outside the great grey shrike territories. Male red-backed shrikes did not settle sooner on plots with great grey shrikes compared to plots that only contained artificial larders indicating that red-backed shrikes do not use the physical presence of a great grey shrike when making settling decisions which is inconsistent with the Allee effect hypothesis. In contrast, for all plots without great grey shrikes, red-backed shrikes settled, paired and laid clutches sooner on plots with larders compared to plots without larders. We conclude that red-backed shrikes use larders of great grey shrikes as a cue to rapidly assess habitat quality.     

Foraging behaviour and diet in two sympatric shrike species during autumn migration across the Arabian Peninsula

In situations of restricted food supply, the trophic niches of closely-related species of animals should be separate. For sit-and-wait hunters, such as shrikes, this is associated with competition for food and hunting sites. In the present study, the foraging behaviour of two shrike species – Red-tailed Shrike Lanius phoenicuroides and Red-backed Shrike Lanius collurio – was studied in a desert habitat in Oman. The fieldwork was carried out in September 2019, during the peak migration of these birds. Their behaviour was recorded in detail during 30-minute observation bouts. A General Linear Mixed Model with logit link function and binomial error variance was used to compare their behaviour. The type of perch and its height did not differ between them, but there were significant differences in their use of look-out posts only in the mean duration of a single perching event, which was more than twice as long in Red-backed Shrike. No differences in prey size were found between the species and hunting success (the ratio of successful attacks to all attacks) was similar in both (RtS-RbS: 46 vs. 61%). Dietary diversity was twice as great in Red-tailed Shrike as in Red-backed Shrike, but in general, their diets did not differ very much. Dietary overlap between the species at this level of prey identification was 92%. This absence of differences in some aspects of behaviour and diet may be due to the similarity of the two species, above all their same body size, and even the possibility of hybridization. If the species compared are so similar due to body size, behaviour and evolutionary relationship their food niches may overlap.     

Maternal Condition Affects the Offspring Testosterone Concentration in Early Nestling Period: An Experimental Test on Tawny Owls

This study addressed the question of whether maternal condition during egg laying or the rearing environment has a greater effect on offspring testosterone levels. We tested this in field experiments on a population of tawny owls Strix aluco in Duna‐Ipoly National Park, Hungary. In the experiments with females of poor condition, when broods were supplied with extra food none of the nestlings died, whereas in control broods, which were not supplied with extra food, some nestlings did die. Large differences in testosterone levels were correlated positively with hatching order both in experimental and control broods. However, it was only in control broods that the later‐hatched nestlings with low testosterone concentrations, died. In the experiments with females of good condition, the males were removed and females and their broods were supplied with restricted amounts of food. In these broods starving nestlings, whose growth had stopped, were considered as having died and were removed from the nest and hand‐reared. In control broods all nestlings fledged, and both in experimental and control broods testosterone concentrations were more even between siblings. Both types of trials confirmed a maternal influence on offspring testosterone concentration: large between‐sibling differences in concentrations in the broods of females of poor condition, where some nestlings died, could not be reduced with increased food supply, and the more even concentrations in the broods of females in good condition, where all nestlings survived, could also not be increased by restricting the food supply.     

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.     

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.     

Cost of reproduction, T-lymphocyte mediated immunocompetence and health status in female and nestling barn swallows Hirundo rustica

We investigate the trade-off between reproductive effort, health status and T-lymphocyte acquired immunity in female and nestling barn swallows Hirundo rustica using a brood size manipulation experiment. Maternal and total feeding effort increased with experimental brood size. Parents did not fully compensate for the increased food demand of the enlarged broods and as a consequence the per capita feeding rate of nestlings decreased with increasing experimental brood size. Body mass and a measure of T-cell mediated immunity in 12 days old nestlings also decreased with increasing experimental brood size. Different leucocyte concentrations and the heterophile/lymphocyte ratio – an index of stress – of nestlings did not change in relation to experimental brood size, suggesting that within brood competition did not affect stress to nestlings. The brood size manipulation had a significant effect on maternal T-cell mediated immunity, measured by the phytohemagglutinin skin test, but not on maternal body mass, haematocrit or differential or total white blood cell counts. Our results seem to support the prediction that under mild work stress females respond first by reducing the energetically expensive acquired immunity. Different leucocyte types and the heterophile/lymphocyte ratio appear less sensitive to parental workload.     

Experimental evidence for parental,but not parentally biased,favouritism in relation to offspring size in Blue Tits Cyanistes caeruleus

In species with biparental care, males and females share the benefits of investing in offspring but pay the costs individually. As a result of these evolutionary conflicts of interest between the sexes, it is expected that the two parents should follow different behavioural rules when providing food to the young. Such a discrepancy may be accentuated when parents have to choose between different subsets of offspring (e.g. large and small nestlings). We manipulated the degree of hatching asynchrony in Blue Tits Cyanistes caeruleus and quantified male and female feeding behaviour when nestlings were 7 and 10 days old. First, we tested for a difference in the role of the sexes during the nestling rearing period between experimentally asynchronous and synchronous control broods. We then used experimentally asynchronous broods to assess differences between the sexes in the pattern of food distribution in terms of number of feedings and prey types, between junior and senior siblings. When nestlings in experimental nests were 7 days old, females fed young more often than did males despite facing a trade‐off between brooding the smallest nestlings and bringing food to the nest. At this age, there was also a skew in food delivery in favour of senior siblings, whereas food was more evenly distributed across the brood when nestlings were 10 days old. We found no difference in how male and female Blue Tits distributed feeding visits among junior and senior nestlings. However, females fed the smallest nestlings with more spiders in comparison with their senior siblings. This could be related to their more suitable size relative to other prey types, their high content of essential nutrients, or both, and may represent a more cryptic form of parentally biased favouritism. We compare these findings with previous work on other species and discuss why parents did not feed junior siblings more frequently.     

Parental allocation of food to nestling tree swallows: the influence of nestling behaviour, sex and paternity

Parents are expected to invest more in young that provide the greatest fitness returns. The cues that parents use to allocate resources between their offspring have received much recent attention. In birds, parents may use begging intensity, position in the nest or nestling size as cues to provision the most competitive young or those most likely to survive. It may also benefit parents to invest in young differentially by sex or relatedness if the fitness returns of sons and daughters differ or broods are sired by multiple males. We examined the allocation of food to tree swallow, Tachycineta bicolor, nestlings in relation to their begging behaviour, size, sex and paternity. Provisioning by parents was not related to nestling size, sex or paternity. The begging behaviour of nestlings did not differ with respect to sex or paternity. Both parents were more likely to feed nestlings that begged first or were closer to the nest entrance, suggesting that parents allocate food resources in response to cues that nestlings control. As a consequence, brood reduction was facilitated by biased provisioning within the brood in addition to the nestling size hierarchies created by hatching asynchrony. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

The significance of seed food in chick development re‐evaluated by tracking day‐to‐day dietary variation in the nestlings of a granivorous passerine

The dietary adjustment of nestlings of granivorous birds to a seed diet and the different morphological characteristics of ingested food have rarely been examined in natural conditions. It has been suggested that the provision of cereal grains to nestlings of some seed‐eating bird species in modern agroecosytems is the result of poor food conditions after agricultural intensification. We analysed the abundance of invertebrate prey in the main foraging habitat of parent birds, daily changes (from hatching to fledging) in the efficiency of cereal seed digestion, and the dietary characteristics, diet composition and prey type delivered to nestlings of the Yellowhammer Emberiza citrinella. Analysis of faecal sacs from nests located in breeding habitat with an abundant invertebrate fauna revealed no relationship between the proportion of cereal seeds in the diet of nestlings and the food supply in the main foraging sites of the parents. Neonate nestlings (1 day old) exclusively received weakly chitinized invertebrate prey (arachnids and flies), but from the second day of life the nestlings were fed a variety of highly chitinized invertebrate prey, the percentage biomass of which did not change for the remainder of the nesting period. Cereal grains started to be delivered to 3‐day‐old nestlings and were already efficiently digested, and the percentage biomass of this food type increased progressively with nestling age. We suggest that the provisioning of cereal grains to nestlings is not forced by external factors, such as modern agricultural intensification; rather, it is an intentional behaviour of parent birds aimed at achieving physiological adjustment to seed food in the early stages of ontogeny.     

Sex‐Specific Parental Care in Response to Predation Risk in the European Roller,Coracias garrulus

Sublethal effects of predation constitute an important part of predation effects, which may modulate prey population and community dynamics. In birds, the risk of nest predation may cause a reduction in parental activity in the care of offspring to reduce the chance of being detected by predators. In addition, parents may modify their parental food allocation preferences within the brood in response to predation risk. Our aim in this study was to evaluate the effects of risk of nest predation on parental care and within‐nest food allocation in the European Roller (Coracias garrulus), an asynchronously hatching bird. We manipulated brood predation risk by placing a snake model near the nests that simulates the most common nest predator in the Mediterranean region. Our results show that males but not females increased their provisioning rate when they were exposed to the model and that despite this, nestlings’ body mass decreased in response to this temporary increase in predation risk. We did not find evidence that parents changed their food allocation strategy towards senior or junior nestlings in their nests in response to predation risk. These results show that the European roller modifies parental care in response to their perception of predation risk in the nest and a sex‐specific sensitivity to the threat, which suggests a different perception of offspring reproductive value by parents. Finally, our results show that changes in parental behaviour in response to nest predation risk might have consequences for nestling fitness prospects.     

Complex feeding behaviour by magpies in nests with great spotted cuckoo nestlings

Parent decisions about food allocation are usually based on simple time‐saving rules that optimize their own fitness; however, they can sometimes vary depending on the prevailing ecological conditions both outside and inside the nest. Parent–offspring interactions also become more complex when parents suffer from brood parasitism, which implies that they care for the parasite's eggs and unrelated young. The great spotted cuckoo Clamator glandarius is a specialist brood parasite that uses the magpie Pica pica as its primary host. Here, by filming food allocation by magpie parents in natural non‐parasitized and experimentally parasitized and non‐parasitized magpie nests, we have found that magpie provisioning behaviour is highly complex including two types of feedings apart from normal ones. First, false feedings, when the parent touched the chick's beak but did not leave any food, occurred more frequently when feeding a cuckoo than when feeding magpie nestlings. Second, two types of what we have called coax feedings: 2a) when magpie parents induce a nestling to beg by waking it up by touching it softly with the beak, and 2b) when parents disregard begging signals (always from brood parasitic great spotted cuckoos) while coaxing one non‐begging nestling (always one of their own) to feed it. We suggest that brood parasitism, involving selfish excessively begging nestlings, could have acted as a selective pressure for both false and coax feedings to evolve, as both imply ignoring nestlings that beg too much. We also discuss that these parental responses could have evolved either by a discrimination without recognition mechanism, or, more probably, by a recognition‐based discrimination mechanism.     

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.     

Begging in Common Redstart Nestlings: Scramble Competition or Signalling of Need?

Begging behaviour by the young affects parental food distribution among nestlings of altricial birds. We present an analysis of two types of begging behaviour (assuming the front nest positions and gaping) based on videotaped natural nestling feeding in European common redstart (Phoenicurus phoenicurus). We test whether these types of begging support the predictions of two mathematical models: scramble competition with competitive asymmetries between nestlings [Anim. Behav. 27 (1979) 1210] or honest signalling model [Nature 352 (1991) 328]. None of the measured variables of nestling or parental behaviour were affected by body weight differences between siblings. In contrast, both gaping and nest positioning were affected by individual differences in nestling hunger. In agreement with the honest signalling model, hungrier nestlings gaped with higher probability and started to gape sooner after the arrival of the parent than did their less hungry nestmates. Those nestlings with the shortest latency to gape also received food more often. Nest positioning was related to nestling hunger in a way unforeseen by the existing models. The intervals between nestling position changes were several times longer than the intervals between parental feeding visits, and parents preferred to feed nestlings in front positions, so nestlings in front positions were always less hungry than nestlings in back. Hence the pattern of movements influenced the feeding decision in favour of the more satiated nestlings and acted against the effect of gaping. Nestling movement seemed to be caused by the less hungry nestlings moving actively from front to rear positions. Low mortality of individual nestlings within broods that survived to fledging and small within‐brood variation in fledging weights indicated low competition among nestmates. We suggest that there are two behavioural mechanisms that contribute to the equalization of fledging weights in common redstart nestlings: the signalling of need through gaping and the regular turnover of nestlings at front positions.     

The influence of territory characteristics and food supply on the breeding performance of the Red-backed Shrike (<Emphasis Type="Italic">Lanius collurio</Emphasis>) in an extensively farmed region of eastern Poland

The breeding performance of the Red-backed Shrike (Lanius collurio) in relation to the territory characteristics was investigated. The study was conducted in an area of high agricultural land use, characterised by small field size, a more mosaic habitat and low use of mineral fertilisers in comparison to Western European countries. The influence of habitat structure and composition in the territory on the date of clutch initiation, nest predation and clutch size in the Red-backed Shrike was not found, but such an influence on the number of nestlings was shown. The minimum adequate model explained 18.3% of the variation between territories in nestling’s number. The number of fledglings was positively correlated with the area of pastures, meadows, orchards and fallows within territories, and negatively correlated with length of overhead transmission lines. The number of fledglings was correlated with food abundance within territories, based on to the results from pitfall traps. There was no correlation between territory food abundance and the size of clutch. The presence of pastures, meadows and fallows, which are foraging places, thus seems to be crucial for the protection of the Red-backed Shrike.     

Signalling of Nutritional Need by Magpie Nestlings

The relationship between begging behaviour, chick nutritional state, and parental distribution of food within broods was studied in 4- and 5-chick magpie Pica pica broods under natural conditions. Three components of the begging display (duration, latency, and posture) were highly correlated with each other and also with the emission and duration of begging calls. Begging performance was strongly influenced by the food intake of nestlings during the preceding 1-h interval, indicating that begging may reliably reflect the nutritional need of nestlings. Daily growth during the preceding day, as well as average cumulative food intake by the brood during the preceding 24 h, seemed not to affect begging in a similar way. Begging signals employed by hungrier nestlings involved a higher degree of muscular activity, thus supporting the prediction that nestlings in greater need should employ more costly signals. Overall, those nestlings who begged more tended to obtain more food, but the relationship between feeding success and begging behaviour was weak due to a high variation between broods in the way that parents seemed to respond to variations in begging behaviour. Possible causes for this variation, and its implications for the evolution of reliable begging displays, are discussed.     

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.     

Behavioural responses to ectoparasites: time-budget adjustments and what matters to Blue Tits Parus caeruleus infested by fleas

Blue Tit nests are often heavily infested by fleas, which feed on the incubating female and the nestlings. Depending on habitat quality, the drawing of blood by fleas reduces offspring quality, or it is compensated by an increase in food provisioning by the adults and may reduce their future reproduction. Given these fitness costs, tits are expected to have evolved behavioural responses enabling them to remove, destroy or minimize the contact with fleas. To identify these traits, we video-recorded the changes in frequency and duration of the hosts' potential anti-flea behavioural defences in nests experimentally infested with low and high flea densities. We also investigated whether flea load affected the number of male feeds delivered to incubating females, and whether the parents increased their rate of food provisioning to the nestlings equally at high flea density. Flea density significantly affected the nest sanitation and sleeping behaviour of Blue Tit females but had no significant effect on grooming. Female Blue Tits increased the frequency but decreased the duration of bouts of these behavioural traits, and hence their time-budgets, based on per hour duration of behaviour, were not significantly affected by flea density. High flea density reduced nestling weight at the early nestling stage but these costs were fully compensated by an increase in female feeding effort. Males did not increase their frequency of food provisioning to incubating females nor to nestlings in heavily infested nests. The results are discussed in the light of parasite-mediated selection on host behaviour and the reciprocal host selection on flea life-history and behavioural traits.     

IBERIAN AZURE-WINGED MAGPIE CYANOPICA (CYANA) COOKI NESTLINGS BEGGING CALLS: CALL CHARACTERIZATION AND HUNGER SIGNALLING

ABSTRACT

Nestling begging behaviour has long been seen as a signal by which nestlings solicit care from parents and most of the existing evidence provides some support for it being an honest signal. Begging is a multicomponent signal in which both sound and vision components are usually important. Although it is known that begging encodes information about nestling hunger the present knowledge about the specific behavioural features that convey the information is still scarce. The aim of this study was to describe begging calls of Iberian Azure-winged Magpie Cyanopica (cyana) cooki nestlings and examine how information on nestling hunger might be encoded in the begging calls. Nestlings were experimentally submitted to different periods of food deprivation and the call variation within individuals was studied. The young were individually tested and stimulated to beg by simulating parental visits. When subject to increasing food deprivation periods, nestlings increased the response level to simulated parental visits. The study also found that for the studied size differences, nestlings did not differ in their response level. Results confirmed that information on nestlings' hunger might be encoded in parameters of the calling behaviour. When the food deprivation periods increased, nestlings tended to start begging earlier, begged more often, extended their calling bout and increased the call duration, changing both at the level of the call and vocal begging bout. Overall the results support the view of begging as an honest signal, namely that begging should reflect nestling hunger and that only some call features might encode information about hunger.     

Do parents play a role in the timing and process of fledging by nestling Mountain Bluebirds?

What causes young birds to leave nests remains unclear for almost all altricial species. For many years, the assumption was that parents often controlled the time of fledging by coaxing young from nests, e.g., by holding food within view, but out of reach, of nestlings. This assumption, though, was based solely on scattered anecdotal reports of such behavior. We used continuous video‐recording of nests to assess the role of parents, if any, in the timing and process of fledging of cavity‐nesting Mountain Bluebirds (Sialis currucoides). We placed perches ~50 cm in front of nest‐box entrances to give parents ample opportunity to display food to nestlings. We found no evidence that parents routinely initiated the fledging process. On the day of fledging, parents did not perch on supplemental perches with food more often, or for longer periods of time, than on the day before fledging. Also, after going to nest‐box entrances, parents never held food away from a nestling reaching for the food. Parents were usually absent (16 of 19 cases) when the first nestling fledged. In the remaining three cases, a parent perched with food in view of a nestling for 8, 15 and 65 s, respectively, just before that nestling fledged. Although these might have appeared to be attempts at coaxing, in each case, the parent was encountering, for the first time, a nestling partially emerging from the nest entrance. Parents may simply have hesitated to approach nests because the nestling's position prevented parents from delivering food in the normal manner. Finally, the rate at which parents fed nestlings on the day of fledging did not differ from the rate the day before, suggesting that parents do not try to use hunger to induce fledging. Our results are consistent with previous research suggesting that, in Mountain Bluebirds, it is a nestling that initiates fledging, typically when it reaches some threshold state of development.     

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.