Migration allows animals to live in resource-rich but seasonally variable environments. Because of the costs of migration, there is selective pressure to capitalize on variation in weather to optimize migratory performance. To test the degree to which migratory performance (defined as speed of migration) of Golden Eagles (Aquila chrysaetos) was determined by age- and season-specific responses to variation in weather, we analyzed 1,863 daily tracks (n = 83 migrant eagles) and 8,047 hourly tracks (n = 83) based on 15 min GPS telemetry data from Golden Eagles and 277 hourly tracks based on 30 s data (n = 37). Spring migrant eagles traveled 139.75 ± 82.19 km day⁻¹ (mean ± SE; n = 57) and 25.59 ± 11.75 km hr⁻¹ (n = 55). Autumn migrant eagles traveled 99.14 ± 59.98 km day⁻¹ (n = 26) and 22.18 ± 9.18 km hr⁻¹ (n = 28). Weather during migration varied by season and by age class. During spring, best-supported daily and hourly models of 15 min data suggested that migratory performance was influenced most strongly by downward solar radiation and that older birds benefited less from flow assistance (tailwinds). During autumn, best-supported daily and hourly models of 15 min data suggested that migratory performance was influenced most strongly by south–north winds and by flow assistance, again less strongly for older birds. In contrast, models for hourly performance based on data collected at 30 s intervals were not well described by a single model, likely reflecting eagles' rapid responses to the many weather conditions they experienced. Although daily speed of travel was similar for all age classes, younger birds traveled at faster hourly speeds than did adults. Our analyses uncovered strong, sometimes counterintuitive, relationships among weather, experience, and migratory flight, and they illustrate the significance of factors other than age in determining migratory performance.

Knemidokoptes is a genus of subcutaneous mites found in the skin of multiple avian hosts, although few cases have been reported in wild raptors. Population monitoring of Black Sparrowhawks (Accipiter melanoleucus) on the Cape Peninsula, South Africa, between 2001 and 2012 revealed multiple birds with infection symptoms in some years. Examination of 3 dead birds displaying symptoms such as baldness and skin lesions confirmed infection by Knemidokoptes spp., whereas we found no cases of subclinical infection in birds without symptoms (n = 16). Up to 5% of birds in the population were infected in some years, which represents the first record of multiple birds displaying an infection by Knemidokoptes in a wild population of raptors. A male bias in infection prevalence was detected. Prevalence of infection symptoms was generally low in other populations elsewhere in South Africa, although possibly higher in urban areas. Breeding performance (both productivity and nesting success) was significantly lower for individuals following infection and also in comparison with noninfected birds throughout the study period. This is the first study to demonstrate the negative effect that these mites may have on breeding performance in a wild bird species, and our results suggest that this parasite could potentially influence population dynamics over time.

Research from the intensively studied northern temperate and boreal regions dominates avian reproductive phenology studies. However, in most other areas, long-term, high-quality phenological datasets are not available, limiting our ability to predict how reproductive timing may respond to rapid climate change. Here, we provide novel methods for combining conventional and nonconventional observations to understand phenological patterns in birds across a southern continent. Observations from egg collections, bird banding, nest record schemes, and citizen science were combined to determine egg-laying phenology for ∼50% of Australia's mainland breeding species. We investigated start, peak, and length of avian egg-laying periods (1) derived from different data sources, (2) across tropical, subtropical, desert, grassland, and temperate biomes, and (3) comparing 2 representative temperate regions of the northern and southern hemispheres. We found that start and peak egg-laying dates calculated from single-visit observations of young or eggs resulted in similar dates as those from more accurate multi-visit nest observations. This demonstration suggests that future studies aimed at assessing changes in the timing of breeding in response to climate change can utilize such observational data. This will significantly increase sample sizes, rather than restricting such analyses to just intensively tracked nests, for which accurate laying dates are available. We found that egg-laying phenology varies between biomes (tropical, subtropical, desert, grassland, temperate), with birds in the desert biome having the earliest peaks of egg-laying. Finally, the length of the egg-laying period differs significantly between hemispheres. The southern temperate zone species have extensive egg-laying periods and many species breed year-round in marked contrast to the highly predictable, springtime breeding in the north. Therefore, avian phenological patterns and documented responses to climate change from the well-sampled, but highly seasonal, northern hemisphere may not be transferrable across the globe.

When monitoring species with extensive ranges in harsh climates, comprehensive studies across a species' range are both logistically and technically challenging and therefore rare. Such scarcity in data collection is particularly true in the polar regions where sea ice and weather constraints prevent widespread access to sites for much of the year, specifically during winter. Penguins (Spheniscidae) show large variations in winter strategies with many species migrating long distances while others are distinctly sedentary, remaining at the colony year-round. However, in some species, their attendance at the breeding site during the nonbreeding winter period depends greatly on the colony location and environmental factors. Here we aim to examine fluctuations in winter attendance at the breeding site in Gentoo Penguins (Pygoscelis papua) along a latitudinal gradient of 7 sites, ranging from Martillo Island, a colony in Argentina, to Petermann Island, a colony located toward the southern edge of the species' range on the Western Antarctic Peninsula. We use an established network of cameras to observe winter populations of penguins across a large latitudinal gradient. This study provides the first evidence of across-year variation in Gentoo Penguin nonbreeding attendance at the breeding site. We found that both temporal and spatial factors drive winter attendance in this species with distinct patterns between years and colony locations, particularly at the edges of its range. Additionally, environmental and temporal factors, including sea ice extent directly offshore and photoperiod, appear to dictate Gentoo Penguin winter attendance across their range. As Gentoo Penguins are neither sedentary nor migratory during the nonbreeding period, understanding patterns in winter site occupation across time and space provides insight into half of the species' annual cycle and has important implications in the face of changes in climate, direct human disturbance, and fisheries activities.

In species with biparental care, pairs share a cooperative interest in offspring survival but may be in conflict over their relative investments, as reported in recent turn-taking studies of chick-provisioning birds. Turn-taking in Common Murres (Uria aalge) involves the foraging bird returning to the colony to provision the chick and the brooding parent departing. We examined whether Common Murres in poor condition had slower or more irregular turn-taking behavior, as has been documented in Common Murres equipped with geologgers. Irregularities include the brooding bird not trading parental roles with its returning mate or a bird returning to the colony without a fish. Irregular turn-taking sequences generally took longer than normal turn-taking sequences and differed in the rate and synchrony of allopreening, the main interactive behavior between mates. There was a delayed onset of allopreening when nest reliefs were protracted, whereas returners that did not bring a fish started allopreening sooner than either their brooding partners or other returners that brought a fish. Common Murres in better condition (higher body mass and lower lipid metabolite levels) left the colony sooner after their returning mates fed the chick compared to Common Murres in worse condition. Birds with higher chick-feeding rates brought fish back in more visits, which suggests that these were higher-quality birds. When birds vary in their turn-taking ceremonial behaviors, they may be negotiating by providing their partners with cues about their condition. Since Common Murres have long-term pair bonds and both parents are necessary to raise offspring, mates should respond to information from their partners if they can do so without compromising their own condition beyond a critical threshold.

In spite of strong selection by time-dependent mortality on the length of the embryo development (incubation) period, time to hatching varies substantially among species, independently of body size. One view, strongly supported by the work of Thomas Martin and his colleagues, maintains that this variation reflects parental strategies to minimize their own mortality risk at the nest—strategies that influence egg temperature and embryo growth rate. A second, not incompatible, view maintains that variation in the incubation period reflects a trade-off between the growth rate of the embryo and its subsequent quality as a free-living individual. We evaluate several lines of evidence relating nest attendance by adults and the quality of the immune system to the length of the incubation period. Particularly important is the role of sibling competition in selecting for rapid embryo growth and early hatching, and the fact that many species with prolonged incubation periods are raised either as single chicks or in broods with staggered hatching, which predetermines the outcome of sibling competition.

In 1921, The Auk published an unresolved mystery submitted by Joseph Grinnell, founding director of the Museum of Vertebrate Zoology at the University of California, Berkeley. He had collected two Oak Titmice (Baeolophus inornatus) whose breast feathers appeared to be yellow; the feathers were covered in microscopic particles. He invited readers to further investigate the nature of these particles. Nearly a century later, we have determined that Grinnell's mysterious grains are willow (Salix) pollen, a diagnosis he rejected because the size of the particles had been mismeasured, and because he lacked a plausible explanation for how the birds would become coated in pollen. We investigated this case using Grinnell's own specimens, field notes, and correspondence, a testament to Grinnell's dedication to scientific rigor through carefully documented and curated natural history collections.

Brood parasitism incurs severe fitness costs for hosts. Diverse host adaptations to avoid parasitism exist at various stages of the host breeding cycle, but the literature suggests that egg-stage defenses are the most evolved. Fitness costs of parasitism would be minimized if hosts avoided parasitism prior to egg laying, but it remains unclear whether hosts are able to reduce parasitism risk via informed habitat choices. We conducted a playback experiment to examine the capability of forest passerine birds to perceive vocal cues of the Common Cuckoo (Cuculus canorus) to estimate local parasitism risk and adjust their breeding-habitat choices accordingly. The density of open-nesting host species was considerably lower in sites of high perceived parasitism risk than in control sites. Hosts thus seem able to reduce parasitism risk through informed breeding-habitat choices, but this behavior is restricted to open-nesting hosts. Cavity-nesting hosts did no t respond to the simulated increase in parasitism risk, nor did bird species that are not known as regular Common Cuckoo hosts. Informed habitat selection as an adaptation against parasitism may have important implications for parasite–host coevolutionary interactions, via reduced selection for later-stage host adaptations, and for bird community structure in general.

Vocal communication in songbirds is important for aggressive signaling, such that an honest signal allows receivers to assess a competitor's qualities. One aspect of song that conspecifics may assess is vocal performance. An example of vocal performance is how well an individual performs the trade-off between trill rate and bandwidth in production of repeated notes. This type of vocal performance (vocal deviation) is thought to be an honest signal because a male's ability to maximize both bandwidth and trill rate is limited by motor constraints on sound modification. Further, how well a male can repeat this trade-off may provide receivers with information about the signaler, and a male's own level of vocal performance can affect the strength of response to high-performance songs. We tested whether males assess each other based on vocal performance in an important model species, the White-crowned Sparrow (Zonotrichia leucophrys). We show that male White-crowned Sparrows respond more strongly to high-performance songs than to lower-performance songs in 2 different locations, supporting the hypothesis that males utilize vocal performance to assess competitors. We also provide initial evidence that vocal performance varies among males and is repeatable within individuals.

Males of many animal species display to attract and stimulate potential mates. In socially monogamous species, males will court females both to establish a pair bond and to solicit extrapair copulations. We investigated whether paired and unpaired male Dark-eyed Juncos (Junco hyemalis) differ in their courtship of a novel female and whether their courtship behavior is related to their morphology or circulating hormones. We conducted simulated courtship interactions (SCI) by presenting free-living paired and unpaired males with a live, caged conspecific female accompanied by playback of a precopulatory trill, a signal of female receptivity, and tested predictions of multiple hypotheses. We quantified courtship behaviors for 20 min and then captured the males, measured them, and collected blood samples to quantify circulating post-SCI testosterone and corticosterone levels as well as restraint-induced corticosterone. Paired males approached the female more rapidly and spent more time in close proximity to the female than unpaired males. Paired males were also more active and spent more time with body feathers fully erect, but sang fewer songs, compared to unpaired males. Unpaired males were smaller in mass than paired males and had higher post-SCI corticosterone and restraint-induced corticosterone than paired males, but the groups did not differ in post-SCI testosterone. We discuss whether these findings should be interpreted as differences in how males court when seeking a social mate vs. an extrapair mate or as inherent differences between successful and unsuccessful males.

Avian plumage has captivated scientists and the public alike for generations and has been a fundamental study system for research into signal evolution. By contrast, relatively little attention has been paid to avian bare parts such as exposed skin, bills, feet, and combs, despite considerable variation in structure and coloration within and between species. To better understand the potential signaling role of bare parts, we conducted a comprehensive literature search that returned 321 published studies. In reviewing these studies, we found that (1) bare-part color is widely distributed taxonomically and is produced by diverse mechanisms; (2) many bare parts are likely to be dynamic, honest signals of current condition or status and can also reflect genetic makeup and early developmental conditions; and (3) bare parts can function as pluripotent social signals, mediating interactions between competitors, mates, and kin. Differences between bare parts and plumage in phenology and information content support a multiple-messages interpretation of their respective signaling roles, in that bare parts may contain information that is complementary to, but distinctive from, information conveyed by plumage-based signals. We consider it likely that a great deal of bare-part variation is “hidden in plain sight,” in that meaningful variation may not be recorded by many current studies. We urge more careful and extensive characterization of bare-part coloration in a wider range of species because of its potential to advance our understanding of signal function and constraints, with particular reference to the role of dynamic color signals and the evolution of multiple ornamentation.

Although the Hermit Thrush (Catharus guttatus) exhibits distinctive morphological and genetic differences across its breeding range (Alvarado et al. 2014), comparatively little is known about geographic variation in song structure and whether song variation corresponds with those morphological and genetic differences. To address this, we collected recordings from throughout the species' breeding range and assessed frequency and temporal characteristics of song structure. Eastern and western birds differed dramatically with respect to song structure, most notably with respect to introductory note frequencies: eastern birds exhibited a larger range of introductory note frequencies than western birds. They also differed with respect to temporal characteristics: western birds sang longer introductory notes, but shorter post-introductory portions, than eastern birds. Numerous differences in structure were also detected within each lineage. For example, within the western lineage, introductory note frequencies were much higher in the Western Lowland group that occupies the Pacific coast compared to the inland Western Mountain group. There were also some differences seen within those groups; for example, in the western Canadian subspecies C. g. euborius recordings from high altitudes (>1900 m in the Canadian Rockies) were structurally distinct from those recorded at lower altitudes. Highlighting the distinctive geographic differences in song structure, a discriminant function analysis using song structure characteristics classified recordings into their geographic groups with 97.0% accuracy. This array of geographic song differences, taking place at multiple levels, suggests that Hermit Thrush song has been shaped by a variety of forces, ranging from large-scale geological events to factors such as habitat differences and cultural drift that operate on a smaller geographic scale (i.e. within subspecies).

It is journal policy for The Auk: Ornithological Advances to use the Checklist of North American Birds for English-language bird names. Accordingly, Podiceps nigricollis californicus refers to the Eared Grebe throughout the article published on July 22, 2015, in The Auk 132(4):771–786, doi  10.1642/AUK-14-259.1.

Migration is a behavioral strategy to access resources that change across a landscape. Animals must ultimately interpret cues to properly time movements that match changing resource quality. Many animals do this by responding primarily to indirect indicators of resource quality such as an internal biological clock or photoperiod. Others are heavily dependent on more direct indicators such as weather or plant phenology. Timing of movement also can be modified by factors specific to individuals. We used time-to-event models for Greater Sage-Grouse (Centrocercus urophasianus) to investigate whether an altitudinal avian migrant was influenced by direct indicators of resource quality when timing migration, and whether timing was influenced by individual characteristics, during the spring (winter to breeding range), summer (breeding to summer range), and fall (summer to winter range) transitional seasons. Greater Sage-Grouse interpreted direct indicators of resource quality, especially temperature, when timing movements between seasonal ranges. Timing was also dependent on individual characteristics including location, reproductive status, and habitat use. After we identified which migration cues were important, we evaluated if migratory and nonmigratory individuals were experiencing similar environmental conditions, which may partly explain why there are different behaviors in a partially migrant population. For the summer and fall transitions, migratory grouse experienced more stimulatory migration cues because of differences in elevation of seasonal ranges. Migratory birds were likely avoiding more rapid plant desiccation in warmer breeding ranges and avoiding higher snow accumulation in colder summer ranges with more precipitation. Altitudinal migrants are likely to use direct indicator cues because they have great utility when migration distances are relatively short. In addition, landscapes with altitudinal migrants have sharp environmental gradients creating conditions conducive for partially migratory behavior in a population.

Biparental care is found across taxa, but sex-specific contributions vary greatly both between and within species. Factors underlying intraspecific variation may include the phase of the reproductive cycle and sex-specific duties resulting in physiological or environmental constraints that differ between the sexes. For pair members to achieve high breeding success while maintaining good condition, parental duties should be well coordinated between mates. Avian breeding behavior is mediated by hormones such as prolactin and corticosterone, which link the internal and environmental parameters of individuals and regulate resource allocation. Using automated recording at the nest, we analyzed parental attendance by Common Terns (Sterna hirundo) in relation to sex, reproductive phase (incubation, chick rearing), time of day, breeding experience, and hormone levels, and related it to reproductive success. Nest attendance of females exceeded that of males, especially at night and during chick rearing. Greater breeding experience was linked to increased attendance at night by males and during incubation by females. Parental nest attendance during incubation was not related to duration of the incubation phase, nor to hatching success. However, fledging success increased with greater parental attendance during the early chick rearing period, reflecting a higher vulnerability of small chicks to mortality compared with eggs. Prolactin values of male parents were significantly lower during chick rearing than incubation and in comparison with values of female parents. Female breeding experience was positively correlated with increased hormone values during chick rearing. Male attendance during chick rearing covaried with hormone levels: moderately increased corticosterone could support males in their new role as feeders by facilitating high activity and foraging effort, whereas lowered prolactin might indicate less contact with chicks and reduced body weight. We show that Common Terns exhibit a flexible system of sex-specific parental care during incubation and chick rearing, reflected in hormone levels, which mediate breeding behavior and ultimately affect reproductive success.

Albatross fossils have been collected from the late Oligocene Lincoln Creek Formation and the early/middle Miocene Astoria Formation near the townsite of Knappton, Pacific County, Washington (USA). The albatross from the Lincoln Creek Formation, Diomedavus knapptonensis, n. gen. et sp., is smaller than all extant albatrosses and represents the oldest published fossil albatross from the North Pacific Basin. Diomedavus knapptonensis is clearly distinguished from extant albatrosses in several osteological features; some are likely plesiomorphic, supporting a phylogenetic position outside the crown group. The unusual shape of the deltopectoral crest of the humerus suggests that D. knapptonensis also differed from extant albatrosses in its flight performance. A partial skeleton from the Astoria Formation likely represents another new albatross species; however, it is not named because there is no overlap with the skeletal elements of other fossil Diomedeidae. This species, here informally termed the “Astoria Formation albatross,” also identified as a stem group representative of the Diomedeidae, differs from extant species in subtle features of the leg bones. Therefore, stem group albatrosses inhabited the North Pacific Basin before the emergence of the taxon Phoebastria, which includes extant North Pacific albatrosses. Cenozoic albatrosses were widely distributed in the Northern Hemisphere and the complex biogeographic history of the Diomedeidae may have been shaped by changes in global marine or atmospheric circulation systems and the loss of safe breeding grounds.

Flight ability has been lost many times in the family Anatidae (ducks, geese, swans, and allies), and this provides unique insights into the morphological and ecological evolution of the family. Although ∼15 fossil anatids have been reported to be flightless or possibly so, there has not been an established criterion that is widely applicable to assessing flight ability in fossil anatids. In this study, discriminant rules for the presence–absence of flight ability were constructed by linear discriminant analysis (LDA) based on 7 skeletal measurements in 93 modern anatids in order to set a basis for the inference of flight ability in fossil anatids. Model selection for LDA was conducted by a high-dimensional modification of Akaike's Information Criterion, and selected models discriminated the volant and flightless groups with only one misclassification (Tachyeres patachonicus). Flight abilities of fossil anatids were assessed by the constructed rules, supplemented by resampling experiments that were designed to assess the uncertainty in estimating skeletal proportions of fossil anatids in the absence of associated skeletons. The flightless condition was strongly supported for Cnemiornis spp., Branta rhuax, Hawaiian moa-nalos, Chenonetta finschi, Anas chathamica, Chendytes spp., Shiriyanetta hasegawai, Cayaoa bruneti, and the “Annaka Short-winged Swan,” whereas the volant condition was supported for Mergus milleneri and Bambolinetta lignitifila. Results were ambiguous for Branta hylobadistes and Anas marecula. The constructed rules can easily be applied to new observations in the future, although limitations in the inference of ecological traits in fossil species from morphological measurements, including the risk of extrapolations, should be appreciated.

We examined the impact of daily and severe multiday weather events on nest survival of Horned Larks (Eremophila alpestris) and Savannah Sparrows (Passerculus sandwichensis) breeding sympatrically in alpine habitat. The two species' thermal regimes varied. The breeding season of Horned Larks was ∼2°C colder and had more precipitation and more storms than that of Savannah Sparrows, which initiated laying 2 wk later. The breeding season of Savannah Sparrows was, on average, 27% shorter than that of Horned Larks. Overall daily nest survival (DNS) was similar for the two species, but Savannah Sparrows had more failure due to abandonment (33% of nests) than Horned Larks (10%). Using Program MARK and Akaike's Information Criterion model selection to evaluate effects of daily and cumulative temperature and precipitation on DNS, we found no direct effect of daily temperature on nest survival, but nest survival declined in colder years for both species. For Horned Larks, the top nest survival models included a decline in DNS with increasing nest age and number of storm events, and a temperature × storm interaction. Daily nest mortality (DNM) increased by 8–9× over background failure levels during cold storms (average = 5°C), but there was little change in DNM during warmer storms (8°C). For Savannah Sparrows, the top nest survival models included a negative influence of cumulative precipitation. The top model's predicted DNM was ∼4.6× higher after ≥2 days of precipitation than following days without rain. Both species coped well with the range of daily temperatures and single days of precipitation typical of alpine habitats, but the earlier-breeding Horned Larks were more susceptible to storm events, whereas extended precipitation events most strongly affected Savannah Sparrow nest survival. The ability of these songbirds to persist in alpine habitats may depend partly on the proportion of “cold” and “warm” storm events in future alpine climates.

The avian vocal organ, the syrinx, is located at the base of the trachea and is controlled by 4 intrinsic pairs of muscles, ventral and dorsal tracheobronchial and ventral and dorsal syringeal muscles. These muscles facilitate active regulation of airflow and sound features, and show exceptionally fast contraction kinetics, stemming from superfast and fast oxidative fiber type composition. Here we investigated to what degree fiber type composition varies across songbird species and whether or not singing ability is related to fiber type composition. In all 10 species studied, syringeal muscles are composed of fast and superfast muscle fibers in males and females. Syrinx size and muscle fiber diameter show some dependence on body size. Content of superfast fibers in the syrinx does not appear to be correlated with singing ability, because non-singing females do not consistently show lower superfast fiber content. Instead, the percentage of superfast fibers in syringeal muscles may be connected to the involvement of neuromuscular control in the generation of the acoustic structure and the entire vocal repertoire of a species.

Although it has been strongly implied that studies on structural function might resolve certain conflicting theories regarding the huge variability of coloration in avian eggs, including maculation, the fundamental physiological question about the functional role of eggshell speckling in the maternal deposition of micronutrients into maculated (pigment spots) and plain (unpigmented) eggshell regions remains largely unanswered. We measured (within the same egg) the concentrations of 2 micronutrients, the staple components of avian eggshells (calcium and magnesium), and 6 trace elements (copper, manganese, iron, cobalt, cadmium, and lead) in maculated (a superficial layer of brown pigment) and unpigmented areas of eggshells in 2 groups of Japanese Quail (Coturnix coturnix japonica) eggs representing 2 extremes of eggshell coloration: those with darker eggshells and more extensive pigment spots vs. those with bright eggshells and small, more clearly demarcated pigment spots. We found evidence that the concentrations of calcium and magnesium and of 2 trace elements (copper and cadmium) in both types of egg were significantly higher in speckled eggshell areas (where the shells were also significantly thicker) than in unpigmented ones. Conversely, lead (a toxic element) peaked markedly in the plain areas of eggshells compared with the speckled ones, whereas the concentrations of the 3 other trace elements (manganese, iron, and cobalt) measured in the speckled and unpigmented eggshell regions were variable and dependent on egg coloration. Our results give a clear indication that egg maculation can play a functional role in the chemistry of avian eggs, one that enables females to distribute micronutrients and trace elements into pigment spots and unpigmented regions of an eggshell to varying extents. This presumably represents a mechanism permitting prompt physiological adjustment to varying levels of maternal resources. It is possible that elements sequestered into protoporphyrin speckles are not further utilized by developing embryos; this might be a form of physiological chemoprotection through the disposal or deactivation of certain elements in pigment speckles, which do not (or only marginally) contribute to the biochemical processes of embryogenesis.

Avian brood parasitism provides a tractable system within which to study diverse aspects of animal ecology and evolution. Yet, parasite–host research has focused on specific adaptations and counter-adaptations, such as egg rejection by hosts and egg mimicry by parasites, leaving other aspects of these relationships poorly studied, including general life history traits of hosts that did not evolve as specific antiparasite defenses. In particular, the diet and fledging parameters (age, mass, success) of parasitic nestlings are poorly known, although they are central to our understanding of host selection and the potential for coevolution in parasite–host relationships. We focused on the diet composition and fledging parameters of parasitic Common Cuckoo (Cuculus canorus) nestlings raised by their only regular cavity-nesting host, the Common Redstart (Phoenicurus phoenicurus). In addition to invertebrates, both Common Cuckoo and Common Redstart nestlings were fed fruits and lizards by some host pairs. This is the first record of lizards being fed to Common Cuckoo nestlings by any host. Capitalizing on this unusual diet and between-host-pair variation (statistically confirmed by high repeatability of diet composition across host pairs), we tested for the first time whether nestling diet variation affected Common Cuckoo breeding success. Common Cuckoos, but not Common Redstarts, fed a combination of plants and vertebrates showed decreased fledging masses and delayed fledging. Common Cuckoos fed with plants (but not vertebrates) also had lower fledging masses. Vertebrates alone did not have any effects. Fledging success was not influenced. These patterns could not be explained by various potential confounding effects, including seasonal effects or inferior provisioning capacities of Common Redstarts feeding unusual diets. This suggests that plant and vertebrate material is digestible by Common Cuckoo nestlings but might cause ontogenetic stress. We show for the first time that diet composition may affect parasite success, not only at the level of host species, as documented by previous studies, but also intraspecifically (i.e. at the level of individual host pairs). Host diet selection may represent an important general life history trait that affects brood parasite fitness, even though it most likely did not evolve as a specific antiparasite defense.