Egg patterning is not a reliable indicator of intraspecific brood parasitism in the blue tit Cyanistes caeruleus

A recent study reported the incidence of intraspecific brood parasitism (IBP) in the blue tit, based on differences in pigmentation between eggs within clutches and the appearance of multiple eggs within a nest in a single day. Here we present data from another population of blue tits Cyanistes caeruleus in which we show that such evidence is not necessarily indicative of IBP. As in many other studies of birds, we found that the patterning and degree of pigmentation on eggs is individually highly characteristic of a female. As in the previous study of blue tits, a minority of clutches (about 3% of 904 clutches) contained one, two or three eggs that were distinctly different from the other eggs in the clutch, either in size or patterning, but not both. Molecular genetic analyses of parentage in a sample of these clutches with suspected IBP demonstrated unequivocally that all eggs within each clutch had in fact been laid by a single female, in all cases the social parent. We also found no evidence of IBP from paternity analyses of a sample of 3,529 offspring from 427 broods in this population, adding to previous evidence of the absence of IBP in this species. These findings cast doubt on the utility of purely observational approaches to provide evidence of intraspecific brood parasitism; our study suggests that egg morphology and records of laying patterns are inadequate to enable the identification of IBP.     

Effectiveness of nest defence in the Acadian Flycatcher Empidonax virescens

We used presentations of models to determine the effectiveness of nest defence in the Acadian Flycatcher Empidonax virescens against a nest predator (Blue Jay Cyanocitta cristata ) and a brood parasite (Brown-headed Cowbird Molothrus ater ). Principal components analysis (PCA) of four component variables of nest defence (call rate, swoop rate, closest approach and number of adults) generated a measure of overall nest defence (aggression). We determined effectiveness of defence by looking for correlations between measures of defence and measures of nest success (nest predation and brood parasitism). We also determined whether nest defence increased with clutch size, nestling age and time in the breeding season. Defence against model Brown-headed Cowbirds did not correlate with levels of parasitism, clutch size, age of young or time of breeding. There was, however, a strong, but insignificant, trend for nests with high levels of all measures of defence to suffer less from brood parasitism. Aggression, vocalization rate, closest approach and number of adults defending against models of predatory Blue Jays correlated positively with nesting success during the egg stage but not the nestling stage of the nesting cycle. Aggression, vocalization rate, closest approach correlated with clutch size and age of the brood. These results suggest that nest defence can effectively deter nest predators, but may be less effective against brood parasites. Different behavioural components of nest defence may work at different stages of the nest cycle and against different nest predators. The components of nest defence that correlated with nest success also correlated with clutch value, a result consistent with hypotheses on the evolution of nest defence.     

Egg rejection and clutch phenotype variation in the plain prinia Prinia inornata

Avian hosts of brood parasites can evolve anti‐parasitic defenses to recognize and reject foreign eggs from their nests. Theory predicts that higher inter‐clutch and lower intra‐clutch variation in egg appearance facilitates hosts to detect parasitic eggs as egg‐rejection mainly depends on the appearance of the egg. Therefore, we predict that egg patterns and rejection rates will differ when hosts face different intensity of cuckoo parasitism. We tested this prediction in two populations of the plain prinia Prinia inornata: Guangxi in mainland China with high diversity and density of cuckoo species, and Taiwan where there is only one breeding cuckoo species, the oriental cuckoo Cuculus optatus. As expected, egg patterns were similar within clutches but different among clutches (polymorphic eggs) in the mainland population, while the island population produced more uniform egg morphs. Furthermore, the mainland population showed a high rate of egg rejection, while the island population exhibited dramatically reduced egg grasp‐rejection ability in the absence of parasitism by the common cuckoo Cuculus canorus. Our study suggests that prinias show lower intra‐clutch consistency in egg colour and lose egg‐rejecting ability under relaxed selection pressure from brood parasitism.     

Relatedness and spatial proximity as determinants of host–parasite interactions in the brood parasitic Barrow's goldeneye (Bucephala islandica)

Recent studies, which have found evidence for kin-biased egg donation, have sparked interest in re-assessing the parasitic nature of conspecific brood parasitism (CBP). Since host–parasite kinship is essential for mutual benefits to arise from CBP, we explored the role of relatedness in determining the behaviour of conspecific nest parasites and their hosts in nesting female Barrow's goldeneyes ( Bucephala islandica ), a duck in which CBP is common. The results revealed that the amount of parasitism increased with host–parasite relatedness, the effect of which was independent of geographical proximity of host and parasite nests. Proximity per se was also positively associated with the amount of parasitism. Furthermore, while hosts appeared to reduce their clutch size as a response to the presence of parasitic eggs, the magnitude of host clutch reduction also tended to increase with increasing relatedness to the parasite. Hence, our results indicate that both relatedness and spatial proximity are important determinants of CBP, and that host clutch reduction may be an adaptation to nest parasitism, modulated by host–parasite relatedness. Taken together, the results provide a demonstration that relatedness influences host and parasite behaviour in Barrow's goldeneyes, resulting in kin-biased egg donation.     

Nest Sanitation as the Evolutionary Background for Egg Ejection Behaviour and the Role of Motivation for Object Removal

Higher interclutch colour variation can evolve under the pressure of brood parasitism to increase the detection of parasitic eggs. Nest sanitation could be a prerequisite for the evolution of anti-parasite defence in terms of egg ejection. In this respect, we used nest sanitation behaviour as a tool to identify: i) motivation and its underlying function and, ii) which features provoke ejection behaviour. Therefore, we experimentally tested whether size, colour or shape may influence ejection behaviour using artificial flat objects. We found a high interclutch variation in egg colouration and egg size in our tree sparrow (Passer montanus) population. Using colour and size we were in fact able to predict clutch affiliation for each egg. Our experiments further revealed the existence of direct anti-parasite behaviours and birds are able to recognise conspecific eggs, since only experimentally-deposited eggs have been removed. Moreover, experiments with different objects revealed that the motivation of tree sparrows to remove experimental objects from their nests was highest during egg laying for objects of varying size, most likely because of parasitism risk at this breeding stage. In contrary, motivation to remove white objects and objects with edges was higher during incubation stage as behavioural patterns connected to hatching started to emerge. The fact that rejection rate of our flat objects was higher than real egg ejection, suggests that egg ejection in tree sparrows and probably more general in small passerines, to be limited by elevated costs to eject eggs with their beaks. The presence of anti-parasite behaviour supports our suggestion that brood parasitism causes variation in egg features, as we have found that tree sparrows can recognise and reject conspecific eggs in their clutch. In conclusion, in tree sparrows it seems that nest sanitation plays a key role in the evolution of the removal of parasitic eggs.     

Egg discrimination in the Australian reed warbler (Acrocephalus australis): rejection response toward model and conspecific eggs depending on timing and mode of artificial parasitism

In a coevolutionary arms race between an interspecific broodparasite and its host species, both are expected to evolveadaptations and counteradaptations. We studied egg discriminationin the Australian warbler (Acrocephalus australis). This speciesis currently not significantly parasitized by the seven speciesof cuckoo for which it is a suitable host. However, experimentalbrood parasitism in the warbler revealed a fine tuned egg discriminationresponse towards non-mimetic and conspecific eggs, the firstsuch evidence in an Australian passerine: (1) non-mimetic eggswere significantly more often rejected than conspecific eggs;(2) only non-mimetic dummy eggs were rejected selectively,whereas rejection of conspecific eggs entailed a rejectioncost; (3) replacement of a host's egg with a conspecific eggduring egg laying resulted in a significantly higher rejectionrate than after the day of clutch completion; (4) by contrast,rejection rate after addition of a conspecific egg was independentof nest stage; (5) conspecific eggs introduced into a clutchduring the egg laying period led to a significantly highernest desertion rate and a lower egg ejection rate than afterthe day of clutch completion; and (6) addition of a conspecificegg led to egg ejection while egg replacement with a conspecificegg led to nest desertion. The fact that this species respondsdifferentially toward different modes of artificial parasitismsuggests that its egg discrimination has evolved to minimizethe costs of rejection and parasitism. The ability to rejecthighly mimetic conspecific eggs may explain the current paucityof brood parasitism in this species. The significance of thisfor brood parasite-host coevolution is discussed.     

Egg rejection by Iberian azure-winged magpies Cyanopica cyanus in the absence of brood parasitism

The Iberian azure-winged magpie Cyanopica cyanus shows a remarkable ability to discriminate against great spotted cuckoo Clamator glandarius eggs. Here, I studied whether egg recognition in this species could be a derived feature resulting from intra-specific brood parasitism. Azure-winged magpies showed a very high level of discrimination and rejection of great spotted cuckoo models (73.7%), and of conspecific eggs (42.8%), even when no evidence of great spotted cuckoo or conspecific brood parasitism has been found in the population. Azure-winged magpie discriminated more readily than magpies, the current favourite host of the great spotted cuckoo. The high rejection rate of conspecific eggs by the azure-winged magpie suggests that it is quite possible that egg discrimination in this species evolved in response to conspecific brood parasitism rather than to cuckoo parasitism.     

Can intraspecific brood parasitism be detected using egg morphology only?

Intraspecific brood parasites lay their eggs in the nests of conspecifics. There are a number of methods for detecting intraspecific brood parasitism (IBP) in birds based on egg morphology. Here we test Eadie's (1989) method, which calculates the Euclidean distances between eggs in a given clutch in a three-dimensional space (weight, length and width). A parasitised clutch is predicted to contain an egg (or eggs) that is significantly different from the clutch's other eggs. Data from three species were analysed. Our captive zebra finch Taeniopygia guttata clutches did not include any instances of IBP, the wild jackdaw Corvus monedula data were unlikely to contain any, and for the goldeneye Bucephala clangula data set we had an observational estimate of IBP. We simulated IBP in the zebra finch, jackdaw and goldeneye data to test whether the method reliably detects an experimentally 'parasitised' clutch. We show that the distributions of the test statistics greatly overlap in 'parasitised' and unmodified clutches, and are dependent on the clutch size. We therefore conclude that the method can only be used with caution, after calibrating it for a given population.     

Can birds count eggs in their nests?

Sensory mechanisms controlling avian clutch size have diversified into distinct types, according to the nature of the input that is used to disrupt the growth of ovarian follicles and hence halt egg‐laying. In an article on brood parasitism, Lyon (2003) claimed that female American Coots Fulica americana can reduce their clutch size on the basis of visual cues in response to eggs laid in their nests by other females; in this species, therefore, egg counting would be used to control clutch size. After a close examination of the physiological determination of clutch size in American Coots, I show that seven of 17 parasitized clutches were smaller than the range controlled through the mechanism using an input to disrupt follicular growth (7–10 eggs per clutch). My reanalysis suggests that American Coots are incapable of adjusting clutch size via counting and re‐asserts that a species that can count eggs has yet to be found among birds that rely upon their own body heat for incubation.     

Brood parasite eggs enhance egg survivorship in a multiply parasitized host

Despite the costs to avian parents of rearing brood parasitic offspring, many species do not reject foreign eggs from their nests. We show that where multiple parasitism occurs, rejection itself can be costly, by increasing the risk of host egg loss during subsequent parasite attacks. Chalk-browed mockingbirds (Mimus saturninus) are heavily parasitized by shiny cowbirds (Molothrus bonariensis), which also puncture eggs in host nests. Mockingbirds struggle to prevent cowbirds puncturing and laying, but seldom remove cowbird eggs once laid. We filmed cowbird visits to nests with manipulated clutch compositions and found that mockingbird eggs were more likely to escape puncture the more cowbird eggs accompanied them in the clutch. A Monte Carlo simulation of this 'dilution effect', comparing virtual hosts that systematically either reject or accept parasite eggs, shows that acceptors enjoy higher egg survivorship than rejecters in host populations where multiple parasitism occurs. For mockingbirds or other hosts in which host nestlings fare well in parasitized broods, this benefit might be sufficient to offset the fitness cost of rearing parasite chicks, making egg acceptance evolutionarily stable. Thus, counterintuitively, high intensities of parasitism might decrease or even reverse selection pressure for host defence via egg rejection.     

The sight of an adult brood parasite near the nest is an insufficient cue for a honeyguide host to reject foreign eggs

Hosts of brood‐parasitic birds typically evolve anti‐parasitism defences, including mobbing of parasitic intruders at the nest and the ability to recognize and reject foreign eggs from their clutches. The Greater Honeyguide Indicator indicator is a virulent brood parasite that punctures host eggs and kills host young, and accordingly, a common host, the Little Bee‐eater Merops pusillus frequently rejects entire clutches that have been parasitized. We predicted that given the high costs of accidentally rejecting an entire clutch, and that the experimental addition of a foreign egg is insufficient to induce this defence, Bee‐eaters require the sight of an adult parasite near the nest as an additional cue for parasitism before they reject a clutch. We found that many Little Bee‐eater parents mobbed Greater Honeyguide dummies while ignoring barbet control dummies, showing that they recognized them as a threat. Surprisingly, however, neither a dummy Honeyguide nor the presence of a foreign egg, either separately or in combination, was sufficient to stimulate egg rejection.     

To eject or to abandon? Life history traits of hosts and parasites interact to influence the fitness payoffs of alternative anti‐parasite strategies

Hosts either tolerate avian brood parasitism or reject it by ejecting parasitic eggs, as seen in most rejecter hosts of common cuckoos, Cuculus canorus, or by abandoning parasitized clutches, as seen in most rejecter hosts of brown‐headed cowbirds, Molothrus ater. What explains consistent variation between alternative rejection behaviours of hosts within the same species and across species when exposed to different types of parasites? Life history theory predicts that when parasites decrease the fitness of host offspring, but not the future reproductive success of host adults, optimal clutch size should decrease. Consistent with this prediction, evolutionarily old cowbird hosts, but not cuckoo hosts, have lower clutch sizes than related rarely‐ or newly parasitized species. We constructed a mathematical model to calculate the fitness payoffs of egg ejector vs. nest abandoner hosts to determine if various aspects of host life history traits and brood parasites’ virulence on adult and young host fitness differentially influence the payoffs of alternative host defences. These calculations showed that in general egg ejection was a superior anti‐parasite strategy to nest abandonment. Yet, increasing parasitism rates and increasing fitness values of hosts’ eggs in both currently parasitized and future replacement nests led to switch points in fitness payoffs in favour of nest abandonment. Nonetheless, nest abandonment became selectively more favourable only at lower clutch sizes and only when hosts faced parasitism by a cowbird‐ rather than a cuckoo‐type brood parasite. We suggest that, in addition to evolutionary lag and gape‐size limitation, our estimated fitness differences based on life history trait variation provide new insights for the consistent differences observed in the anti‐parasite rejection strategies between many cuckoo‐ and cowbird‐hosts.     

The evolution of cuckoo parasitism: a comparative analysis

Cuckoos (family Cuculidae) show the highest diversity of breeding strategies within one bird family (parental care, facultative and obligate brood parasites). We used independent contrasts from two phylogenies to examine how this variation was related to 13 ecological and life-history variables. The ancestral state was probably tropical, resident, forest cuckoos with parental care. The evolution of brood parasitism was correlated with a shift to more open habitats, a change in diet, increases in species breeding-range size and migration, and a decrease in egg size. Once parasitism had evolved, more elaborate parasitic strategies (more harmful to host fitness) were correlated with decreased egg size, a change in diet, increased breeding-range size and migration, a shortened breeding season and a decrease in local abundance. Establishing the most probable evolutionary pathways, using the method of Pagel, shows that changes in ecological variables (such as migration, range size and diet type) preceded the evolution of brood parasitism, which is likely to be a later adaptation to reduce the cost of reproduction. By contrast, brood parasitism evolved before changes in egg size occurred, indicating that egg size is an adaptive trait in host--parasite coevolution. Our results suggest that the evolution of cuckoo brood parasitism reflects selection from both ecological pressures and host defences.     

Sex-specific defence behaviour against brood parasitism in a host with female-only incubation

Nest protection against intruders is an indispensable component of avian parental care. In species with biparental care, both mates should evolve nest defence behaviour to increase their reproductive success. In most host-parasite systems, host females are predicted to have more important roles in nest defence against brood parasites, because they typically are primarily responsible for clutch incubation. Male antiparasitic behaviour, on the other hand, is often underestimated or even not considered at all. Here we investigated sex-specific roles in four aspects of great reed warbler (Acrocephalus arundinaceus) nest defence against a brood parasite—the cuckoo (Cuculus canorus), namely (1) mobbing, (2) nest attendance/guarding, (3) nest checking and (4) egg ejection. Using dummy experiments, simulating brood parasitism and by video-monitoring of host nests we found that males took the key roles in cuckoo mobbing and nest guarding, while females were responsible for nest checking and egg ejection behaviours. Such partitioning of parental roles may provide a comprehensive clutch protection against brood parasitism.     

Effects of Shiny Cowbird Molothrus bonariensis parasitism on different components of House Wren Troglodytes aedon reproductive success

Avian brood parasites, including cuckoos and cowbirds, have multiple negative effects on their hosts. We analysed the effects of Shiny Cowbird Molothrus bonariensis parasitism on different components (e.g. egg losses, hatching success, chick survival and nest abandonment) of House Wren Troglodytes aedon reproductive success. We also conducted an experiment to discriminate between two mechanisms that may reduce hatching success in parasitized clutches: lower efficiency of incubation due to the increase in clutch volume and disruption of host incubation by the early hatching of Cowbirds. Egg puncturing by Shiny Cowbirds reduced host clutch size at hatching by 10–20%, and parasitized nests had a decrease in hatching success of 40–80%. Egg losses and hatching failures were positively associated with the intensity of parasitism. Brood reduction was greater in parasitized nests, but the growth rate of the chicks that fledged was similar to that in unparasitized nests. The combined effects of egg losses, hatching failures and brood reduction decreased the number of fledged chicks by 80%. In addition, egg puncturing increased the likelihood of nest abandonment by Wrens. Experimental data showed that hatching failures occurred when there was a combination of: (1) an increase in the volume of the clutch by the addition of the Cowbird egg without removal of host eggs, and (2) the addition of the Cowbird egg before the onset of incubation. This was relatively common in House Wren nests, as Cowbirds generally parasitize before the onset of incubation. Our results indicate that Shiny Cowbird parasitism imposes a major impact on House Wrens, as it affects all components of the Wren's reproductive success.     

Hatching asynchrony, nestling competition, and the cost of interspecific brood parasitism

All parental hosts of heterospecific brood parasites must paythe cost of rearing non-kin. Previous research on nest parasitismby brown-headed cowbirds (Molothrus ater) concluded that competitivesuperiority of the typically more intensively begging and largercowbird chick leads to preferential feeding by foster parentsand causes a reduction in the hosts' own brood. The larger sizeof cowbird nestlings can be the result of at least two causes:(1) cowbirds preferentially parasitize species with smallernestlings and lower growth rates; and/or (2) cowbirds hatchearlier than hosts. I estimated the cost of cowbird parasitismfor each of 29 species by calculating the difference betweenhosts' published brood sizes in nonparasitized and parasitizednests and using clutch size to standardize values. In this analysis,greater incubation length and lower adult mass, surrogate measuresof the hatching asynchrony and size difference between parasiteand hosts, were both related to greater costs of cowbird parasitismwithout bias owing to phylogeny. To establish causality, I manipulatedclutch contents of eastern phoebes (Sayornis phoebe) and examinedwhether earlier hatching by a single cowbird or phoebe egg reducesthe size of the rest of the original host brood. As predicted,greater hatching asynchrony increased the proportion of theoriginal phoebe brood that was lost. This measure of the costof parasitism was partially owing to increased hatching failureof the original eggs in asynchronous broods but was not at allrelated to the size differences of older and younger conspecificnestmates. However, proportional brood loss owing to an earlierhatching conspecific was consistently smaller than brood lossowing to asynchronous cowbirds in both naturally and experimentallyparasitized phoebe nests. These results imply that althoughhatching asynchrony is an important cause of the reduction ofhost broods in parasitized clutches, competitive features ofcowbird nestlings remain necessary to explain the full extentof hosts' reproductive costs caused by interspecific brood parasitism.     

CLUTCH SIZE AND EGG SIZE IN FREE-LIVING AND PARASITIC COPEPODS: A COMPARATIVE ANALYSIS

The evolution of reproductive strategies and the trade-off between number and size of eggs were investigated in a comparative analysis of free-living and parasitic copepods. Data from 1038 copepod species were used to obtain family averages for 105 families; the phylogenetic relationships among these families include 94 branching events or 94 independent contrasts on which the analysis was based. Transition from a free-living existence to parasitism on invertebrates resulted in small increases in body size. Transition from parasitism on invertebrates to parasitism on fish was associated with greater increases in body size. After controlling for body size, a switch to fish hosts resulted in an increase in the number of eggs produced and a reduction in egg size. Among all contrasts, there was a negative relationship between changes in relative clutch size and changes in relative egg size, suggesting the existence of a trade-off between egg size and numbers. However, opposite changes in these measures of clutch size and egg size were not quite more frequent than expected by chance, therefore indicating that investments into egg numbers are not necessarily made at the expense of egg size, and vice versa. Latitude affected copepod body size, clutch size, and egg size, whereas the effects of freshwater colonization or size of the fish host were not significant. Comparative analyses at either the genus or species levels within given taxa of copepods parasitic on fish provided limited support for a trade-off between clutch size and egg size, but were hampered by the small number of independent phylogenetic contrasts available. From the family-level comparative analysis, it appears that the evolutionary transition from a free life to parasitism on invertebrates, and the transition from parasitism on invertebrates to parasitism on fish, have led to changes in life-history traits in response to the different selective pressures associated with the different modes of life.     

Cold weather events provoke egg ejection behaviour in open‐nesting passerines

We analysed 25 conspecific egg ejection events from a large sample of nests (n = 762) of three passerine species. In two species, most of the events could not be explained by intraspecific brood parasitism or nest sanitation behaviour. At least some of these cases were behavioural responses to unfavourable weather conditions in the spring of 2017. We conclude that clutch size adjustment through egg ejection may be more widespread among passerines than previously thought.     

Reliability of egg morphology to detect conspecific brood parasitism in goldeneyes Bucephala clangula examined using protein fingerprinting

Eadie (1989) developed a method based on variation between females in egg length, width and weight to detect conspecific brood parasitism in the field: using these three egg measures, Euclidean distance between all pairs of eggs within a clutch is calculated, and if maximum Euclidean distance (MED) between any two eggs exceeds a threshold value the nest is considered parasitized. The MED method has been tested in Finnish and Scottish common goldeneye Bucephala clangula populations but the results have been contradicting. Here we use protein fingerprinting to assess the validity of the MED method. Data comprised 35 clutches of which we knew, based on protein fingerprinting, how many different females laid the clutch (range 1–5 females). The mean MED of non-parasitized clutches (laid by 1 female only) was 1.470 (95% CI: lower 1.169, upper 1.771; n=21) and that of parasitized clutches (laid by 2 or more females) was 3.654 (95% CL: lower 3.083, upper 4.225; n=14). Using a MED>3.0 as a criterion to identify parasitized clutches 89% of all clutches were classified correctly either parasitized or non-parasitized when compared to the identification based on protein fingerprinting. Clutch size and the number of females (beyond 2 females) did not affect the clutch MED, whereas the status of parasitism did. Repeatability of egg length, width and weight were: 0.63, 0.76 and 0.80, respectively, implying that, variation in these egg measures occurs among rather than within females. Our new results confirm that the MED method is reliable enough to detect parasitism in common goldeneye.     

Alloparental care in the sea: Brood parasitism and adoption within and between two species of coral reef Altrichthys damselfish?

The evolution of parental care opens the door for the evolution of brood parasitic strategies that allow individuals to gain the benefits of parental care without paying the costs. Here we provide the first documentation for alloparental care in coral reef fish and we discuss why these patterns may reflect conspecific and interspecific brood parasitism. Species‐specific barcodes revealed the existence of low levels (3.5% of all offspring) of mixed interspecific broods, mostly juvenile Amblyglyphidodon batunai and Pomacentrus smithi damselfish in Altrichthys broods. A separate analysis of conspecific parentage based on microsatellite markers revealed that mixed parentage broods are common in both species, and the genetic patterns are consistent with two different modes of conspecific brood parasitism, although further studies are required to determine the specific mechanisms responsible for these mixed parentage broods. While many broods had offspring from multiple parasites, in many cases a given brood contained only a single foreign offspring, perhaps a consequence of the movement of lone juveniles between nests. In other cases, broods contained large numbers of putative parasitic offspring from the same parents and we propose that these are more likely to be cases where parasitic adults laid a large number of eggs in the host nest than the result of movements of large numbers of offspring from a single brood after hatching. The evidence that these genetic patterns reflect adaptive brood parasitism, as well as possible costs and benefits of parasitism to hosts and parasites, are discussed.