ABSTRACT Some host species of avian obligate brood parasites reject parasitic eggs from their nest whereas others accept them, even though they recognize them as foreign. One hypothesis to explain this seemingly maladaptive behavior is that acceptors are unable to pierce and remove the parasitic eggshell. Previous studies reporting on the force and energy required to break brood parasites' eggshells were typically static tests performed against hard substrate surfaces. Here, we considered host nest as a substrate to simulate this potentially critical aspect of the natural context for egg puncture while testing the energy required to break avian eggshells. Specifically, as a proof of concept, we punctured domestic chicken eggs under a series of conditions: varying tool shape (sharp versus blunt), tool dynamics (static versus dynamic) and the presence of natural bird nests (of three host species). The results show a complex set of statistically significant interactions between tool shapes, puncture dynamics and nest substrates. Specifically, the energy required to break eggs was greater for the static tests than for the dynamic tests, but only when using a nest substrate and a blunt tool. In turn, in the static tests, the addition of a nest significantly increased energy requirements for both tool types, whereas during dynamic tests, the increase in energy associated with the nest presence was significant only when using the sharp tool. Characterizing the process of eggshell puncture in increasingly naturalistic contexts will help in understanding whether and how hosts of brood parasites evolve to reject foreign eggs.

ABSTRACT Color and spatial vision is critical for recognition and discrimination tasks affecting fitness, including finding food and mates, and recognizing offspring. For example, as a counter defense to avoid the cost of raising the unrelated offspring of obligate interspecific avian brood parasites, many host species routinely view, recognize and remove the foreign egg(s) from their nests. Recent research has shown that host species visually attend to both chromatic and spatial pattern features of eggs; yet how hosts simultaneously integrate these features together when recognizing eggs remains an open question. Here, we tested egg rejection responses of American robins ( Turdus migratorius ) using a range of 3D-printed model eggs covered with blue and yellow checkered patterns differing in relative square sizes. We predicted that robins would reject a model egg if they could visually resolve the blue and yellow squares as separate features, or accept it if the squares blended together and appeared similar in color to the natural blue–green color of robin eggs as perceived by the avian visual system. As predicted, the probability of robins rejecting a model egg increased with greater sizes of its blue and yellow squares. Our results suggest that chromatic visual acuity and viewing distance have the potential to limit the ability of a bird to recognize a foreign egg in its nest, thus providing a limitation to host egg recognition that obligate interspecific avian brood parasites may exploit.

ABSTRACT Very little is known about how morphology affects the motion, spatial stability and resulting viability of avian eggs. The limited existing research focuses on the uniquely pyriform egg shapes found in the Alcidae bird family. This unusual shell shape was originally thought to suppress displacement and prevent egg loss on the cliffside nesting habitat of the Uria genus. Unfortunately, these early studies never isolated or quantified the specific morphological features (elongation, asymmetry and conicality) of these pyriform eggs, which limits their applicability to other taxa and has hampered a robust proof of concept. We isolated each feature as an enumerated variable, produced model 3D printed eggs with incremental expressions of a single variable and then with all three features co-varying simultaneously. Recorded motion (egg-rolling) trials were conducted to test the individual and combined effects of each morphological characteristic on displacement over a range of inclines representative of the conditions found in natural habitats. Increasing elongation and asymmetry significantly increased displacement, whereas increased conicality decreased displacement in the single-variable egg models. In the multivariable egg models, only conicality consistently suppressed displacement, while lower levels of asymmetry significantly increased displacement. Our findings broadly support previous studies' assertions of the adaptive value of the pyriform eggs while also providing methodology and comparative data for future analyses of the interactions between nesting habitat, behavior and egg shape, beyond the confines of a handful of focal species.

ABSTRACT Obligate avian brood parasites are raised by heterospecific hosts and, therefore, lack crucial early exposure to relatives and other conspecifics. Yet, young brood parasites readily recognize and affiliate with others of their own species upon independence. One solution to this social recognition paradox is the ontogenetic ‘password’ mechanism used by obligate parasitic brown-headed cowbirds ( Molothrus ater ), whereby conspecific identification is initially mediated through the cowbird chatter: a non-learned vocal cue. We explored the neural basis of such password-based species recognition in juvenile and adult male cowbirds. We found that cowbird auditory forebrain regions express greater densities of the protein product of the immediate-early gene ZENK in response to the password chatter call relative to control sounds of mourning dove ( Zenaida macroura ) coos. The chatter-selective induction of ZENK expression occurs in both the caudal medial nidopallium (NCM) and the caudal medial mesopallium (CMM) in adults, but only within the NCM in juveniles. In contrast, we discovered that juvenile cowbirds exhibit neural selectivity to presentations of either conspecific or heterospecific songs, but only in CMM and only after recent experience. Juvenile cowbirds that did not have previous experience with the song type they were exposed to during the test period exhibited significantly lower activity-dependent gene expression. Thus, in juvenile male cowbirds, there is early onset of species-specific selective neural representation of non-learned calls in NCM and recently experienced song in CMM. These results suggest that NCM is evolutionarily co-opted in parasitic cowbirds to selectively recognize the password chatter, allowing juvenile cowbirds to identify adult conspecifics and avoid mis-imprinting upon unrelated host species. These ontogenetic comparisons reveal novel insights into the neural basis of species recognition in brood parasitic species.

ABSTRACT Avian eggshells are variable in appearance, including coloration. Here, we demonstrate that Raman spectroscopy can provide accurate diagnostic information about major eggshell constituents, including the pigments biliverdin and protoporphyrin IX. Eggshells pigmented with biliverdin showed a series of pigment-diagnostic Raman peaks under 785 nm excitation. Eggshells pigmented with protoporphyrin IX showed strong emission under 1064 nm and 785 nm excitation, whereas resonance Raman spectra (351 nm excitation) showed a set of protoporphyrin IX informative peaks characteristic of protoporphyrin IX. As representative examples, we identified biliverdin in the olive green eggshells of elegant crested tinamous ( Eudromia elegans ) and in the blue eggshells of extinct upland moa ( Megalapteryx didinus ). This study encourages the wider use of Raman spectroscopy in pigment and coloration research and highlights the value of this technique for non-destructive analyses of museum eggshell specimens.

ABSTRACT Hosts of avian brood parasites can avoid the reproductive costs of raising genetically unrelated offspring by rejecting parasitic eggs. The perceptual cues and controls mediating parasitic egg discrimination and ejection are well studied: hosts are thought to use differences in egg color, brightness, maculation, size and shape to discriminate between their own and foreign eggs. Most theories of brood parasitism implicitly assume that the primary criteria to which hosts attend when discriminating eggs are differences between the eggs themselves. However, this assumption is confounded by the degree to which chromatic and achromatic characteristics of the nest lining co-vary with egg coloration, so that egg–nest contrast per se might be the recognition cue driving parasitic egg detection. Here, we systematically tested whether and how egg–nest contrast itself contributes to foreign egg discrimination. In an artificial parasitism experiment, we independently manipulated egg color and nest lining color of the egg-ejector American robin ( Turdus migratorius ), a host of the obligate brood parasitic brown-headed cowbird ( Molothrus ater ). We hypothesized that the degree of contrast between foreign eggs and the nest background would affect host egg rejection behavior. We predicted that experimentally decreasing egg–nest chromatic and achromatic contrast (i.e. rendering parasitic eggs more cryptic against the nest lining) would decrease rejection rates, while increasing egg–nest contrast would increase rejection rates. In contrast to our predictions, egg–nest contrast was not a significant predictor of egg ejection patterns. Instead, egg color significantly predicted responses to parasitism. We conclude that egg–egg differences are the primary drivers of egg rejection in this system. Future studies should test for the effects of egg–nest contrast per se in predicting parasitic egg recognition in other host–parasite systems, including those hosts building enclosed nests and those parasites laying cryptic eggs, as an alternative to hypothesized effects of egg–egg contrast.

Carefully controlled gas exchange across the eggshell is essential for the development of the avian embryo. Water vapour conductance ( G H 2 O ) across the shell, typically measured as mass loss during incubation, has been demonstrated to optimally ensure the healthy development of the embryo while avoiding desiccation. Accordingly, eggs exposed to sub-optimal gas exchange have reduced hatching success. We tested the association between eggshell G H 2 O and putative life-history correlates of adult birds, ecological nest parameters and physical characteristics of the egg itself to investigate how variation in G H 2 O has evolved to maintain optimal water loss across a diverse set of nest environments. We measured gas exchange through eggshell fragments in 151 British breeding bird species and fitted phylogenetically controlled, general linear models to test the relationship between G H 2 O and potential predictor parameters of each species. Of our 17 life-history traits, only two were retained in the final model: wet-incubating parent and nest type. Eggs of species where the parent habitually returned to the nest with wet plumage had significantly higher G H 2 O than those of parents that returned to the nest with dry plumage. Eggs of species nesting in ground burrows, cliffs and arboreal cups had significantly higher G H 2 O than those of species nesting on the ground in open nests or cups, in tree cavities and in shallow arboreal nests. Phylogenetic signal (measured as Pagel's λ) was intermediate in magnitude, suggesting that differences observed in the G H 2 O are dependent upon a combination of shared ancestry and species-specific life history and ecological traits. Although these data are correlational by nature, they are consistent with the hypothesis that parents constrained to return to the nest with wet plumage will increase the humidity of the nest environment, and the eggs of these species have evolved a higher G H 2 O to overcome this constraint and still achieve optimal water loss during incubation. We also suggest that eggs laid in cup nests and burrows may require a higher G H 2 O to overcome the increased humidity as a result from the confined nest microclimate lacking air movements through the nest. Taken together, these comparative data imply that species-specific levels of gas exchange across avian eggshells are variable and evolve in response to ecological and physical variation resulting from parental and nesting behaviours.

SUMMARY Many avian hosts have evolved antiparasite defence mechanisms, including egg rejection, to reduce the costs of brood parasitism. The two main alternative cognitive mechanisms of egg discrimination are thought to be based on the perceived discordancy of eggs in a clutch or the use of recognition templates by hosts. Our experiments reveal that the great reed warbler ( Acrocephalus arundinaceus ), a host of the common cuckoo ( Cuculus canorus ), relies on both mechanisms. In support of the discordancy mechanism, hosts rejected their own eggs (13%) and manipulated (‘parasitic’) eggs (27%) above control levels in experiments when manipulated eggs were in the majority but when clutches also included a minority of own eggs. Hosts that had the chance to observe the manipulated eggs daily just after laying did not show stronger rejection of manipulated eggs than when the eggs were manipulated at clutch completion. When clutches contained only manipulated eggs, in 33% of the nests hosts showed rejection, also supporting a mechanism of template-based egg discrimination. Rejection using a recognition template might be more advantageous because discordancy-based egg discrimination is increasingly error prone with higher rates of multiple parasitism.

SUMMARY Avian plumage colors have emerged recently as model systems for investigating the types of information that can be signaled by showy sexual displays in animals. In many species, the brightness of carotenoid-based plumage reflects the health and condition of individuals and is used in mate selection. The information contained in melanin-based and structurally based ornamental colors in birds is less well resolved, however. We subjected male house sparrows Passer domesticus and brown-headed cowbirds Molothrus ater to stressful nutritional conditions during molt to test the hypothesis that melanin- and structurally based plumage colors are nutritionally condition-dependent. We restricted food access for treatment males during randomized 6 h periods on 4 days per week, while allowing control birds access to food ad libitum throughout the course of the molt. We found that the size and brightness of the melanin-based throat badges in male house sparrows were not affected by nutritional stress. Similarly, there were no differences between treatment and control male cowbirds in the size or brightness of the melanin-based brown hood. However, the structurally based iridescent plumage of cowbirds was indicative of the nutritional condition of males during molt. Nutritionally stressed cowbirds grew significantly less colorful plumage than did males with access to food ad libitum . These results are consistent with observations in other avian species that different types of plumage color communicate different sets of information. Melanin ornaments are less sensitive to nutritional conditions during molt and instead may reflect the hormonal status and/or competitive ability of males, whereas structural coloration appears to be an accurate signal of health and condition.