Rapid increase in cuckoo egg matching in a recently parasitized reed warbler population

Parasitic cuckoos lay eggs that mimic those of their hosts, and such close phenotypic matching may arise from coevolutionary interactions between parasite and host. However, cuckoos may also explicitly choose hosts in a way that increases degree of matching between eggs of cuckoos and parasites, with female preference for specific host phenotypes increasing the degree of matching. We tested for temporal change in degree of matching between eggs of the parasitic European cuckoo (Cuculus canorus) and its reed warbler (Acrocephalus scirpaceus) host during 24 consecutive years in a recently parasitized reed warbler population. Cuckoo-host egg matching in an ultraviolet-brownness component yielding most of the chromatic variance of eggs improved during the study period. Improved matching was not due to changes in cuckoo egg phenotype. Cuckoo eggs matched host eggs for ultraviolet-brownness within nests irrespective of duration of sympatry. Ultraviolet-brownness of cuckoo eggs was similar to that of reed warbler eggs at parasitized nests, but differed from that of reed warbler eggs at unparasitized nests. These findings provide tentative support for the cuckoo preference hypothesis suggesting that cuckoo-host egg matching could partially be due to cuckoo females selecting host nests based on the appearance of their eggs.     

Attractive blue‐green egg coloration and cuckoo−host coevolution

Recent evidence suggests that blue‐green coloration of bird eggshells may be related to female and/or egg phenotypic quality, and that such colour may affect parental effort and therefore the nutritional environment of developing nestlings. Here we suggest that these relationships and the signal function of eggshell coloration would affect the outcome of coevolution between avian brood parasites and their hosts in at least three different non‐exclusive evolutionary pathways. First, by laying blue‐green coloured eggs, cuckoo females may exploit possible sensory biases of their hosts, constraining the evolution of parasitic egg recognition, and thus avoid rejection. Second, because of the relatively high costs of laying blue eggs, cuckoo females may be limited in their ability to mimic costly blue‐green eggs of their hosts because cuckoo females lay many more eggs than their hosts. Furthermore, costs associated with foreign egg recognition errors would be relatively higher for hosts laying blue eggs. Third, cuckoos may use coloration of host eggs for selecting individuals or specific hosts of appropriate phenotypic quality (i.e. parental abilities). We here explored some predictions emerging from the above scenarios and found partial support for two of them by studying egg coloration of European cuckoos (Cuculus canorus) and that of their 25 main hosts, as well as parasitism and rejection rate of hosts. Cuckoo hosts parasitized with more blue, green, and ultraviolet cuckoo eggs, or those laying more blue‐green eggs, were more prone to accept experimental parasitism with artificial cuckoo eggs. In addition, coloration of cuckoo eggs is more variable when parasitizing hosts laying bluer‐greener eggs, even after controlling for the effect of host egg coloration (i.e. degree of egg matching). Globally, our results are consistent with the proposed hypothesis that host egg traits that are related to phenotypic quality of hosts, such as egg coloration, may have important implications for the coevolutionary interaction between hosts and brood parasites. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 154–168.     

Egg characteristics are unreliable in determining maternity in communal clutches of guira cuckoos Guira guira

Egg characteristics have been used to determine egg maternity for birds in situations where two or more females lay eggs in a single nest (as in communal breeders and intraspecific parasites). We assessed the applicability of egg morphometry and eggshell appearance in ascribing egg ownership in communal clutches of guira cuckoos Guira guira , a species where up to seven females may lay eggs in a joint nest. We used both combined variables (including egg mass, length, width, shape and two eggshell variables), and a shape index to test whether eggs laid by each female were similar but different from eggs laid by other females. Also, we conducted discriminant function analyses to verify if eggs could be correctly classified to their mothers based on their characteristics. The correct maternity was determined by yolk protein electrophoresis of freshly-laid eggs. Individual female chutches were separated through egg characteristics or shape alone in 29% and 41% of the groups tested, respectively. Differences were mostly due to a single female that differed from her nest-mates in a unique egg variable. On average, 55% of the eggs analyzed were not assigned to the correct mother using egg dimensions and eggshell speckling pattern. In conclusion, the egg characteristics used do not reliably indicate maternity in guira cuckoo communal clutches. We therefore recommend a protein-based verification of egg appearance characteristics for assigning maternity in other species in which multiple female laying occurs.     

Rejection of parasitic eggs in relation to egg appearance in magpies

The coevolutionary process between avian brood parasites and their hosts predicts that low intraclutch variation in egg colour appearance favours egg discrimination of parasite eggs by hosts. Low intraclutch variation would also result in high interclutch variation, which would increase the difficulty of evolution of mimicry by the cuckoo, because many host colour patterns might coexist in the same host population. We explored this possibility using an experimental approach in the common magpie, Pica pica, and great spotted cuckoo, Clamator glandarius, system. We artificially parasitized magpie nests with great spotted cuckoo model eggs to assess host response in two populations in Spain (Guadix and Doñana) in relation to intraclutch variation in egg appearance, measured by ultraviolet-visible reflectance spectrophotometry. Individuals that rejected model cuckoo eggs had higher intraclutch variation than accepters, suggesting that an increase, rather than a decrease, in intraclutch variation in magpie egg appearance was advantageous for cuckoo egg discrimination.     

A shared chemical basis of avian host-parasite egg colour mimicry

Avian brood parasites lay their eggs in other birds' nests and impose considerable fitness costs on their hosts. Historically and scientifically, the best studied example of circumventing host defences is the mimicry of host eggshell colour by the common cuckoo (Cuculus canorus). Yet the chemical basis of eggshell colour similarity, which impacts hosts' tolerance towards parasitic eggs, remains unknown. We tested the alternative scenarios that (i) cuckoos replicate host egg pigment chemistry, or (ii) cuckoos use alternative mechanisms to produce a similar perceptual effect to mimic host egg appearance. In parallel with patterns of similarity in avian-perceived colour mimicry, the concentrations of the two key eggshell pigments, biliverdin and protoporphyrin, were most similar between the cuckoo host-races and their respective hosts. Thus, the chemical basis of avian host-parasite egg colour mimicry is evolutionarily conserved, but also intraspecifically flexible. These analyses of pigment composition reveal a novel proximate dimension of coevolutionary interactions between avian brood parasites and hosts, and imply that alternative phenotypes may arise by the modifications of already existing biochemical and physiological mechanisms and pathways.     

Are blackcaps current winners in the evolutionary struggle against the common cuckoo?

Blackcaps Sylvia atricapilla reject artificial cuckoo eggs, and their eggs vary little in appearance within clutches, whereas among clutches eggs vary considerably. Low variation within clutches facilitates discrimination of parasitic eggs, whereas high variation among clutches makes it harder for the cuckoo to mimic the eggs of a certain host species. These traits have most probably evolved as counteradaptations against brood parasitism by the common cuckoo Cuculus canorus, even though blackcaps are not regularly parasitised today. In this study, we investigated how fine-tuned the rejection of parasitic eggs is in this species by introducing three types of eggs into their nests: a real non-mimetic egg the approximate size of a cuckoo egg, an artificial mimetic egg the size of a cuckoo egg and a real conspecific egg. As the rejection frequency of both mimetic and non-mimetic artificial cuckoo eggs has been shown to be high in previous studies, the variation in rejection behaviour between individuals is low, indicating that most individuals within the population are able to reject parasitic eggs. Thus, we predict that (1) the intraclutch variation in egg appearance should be generally low in all individuals, and that (2) regarding conspecific eggs, rejection decisions should be highly dependent on the degree of mimicry between parasitic and host eggs. We found support for these predictions, which indicates that due to their highly sophisticated countermeasures against brood parasitism, blackcaps can probably be regarded as current winners of the arms race with the common cuckoo. Furthermore, the high and consistent rejection frequency of cuckoo eggs found throughout Europe for this species supports the spatial habitat structure hypothesis, which claims that woodland-nesting species breeding near trees, like blackcaps, presumably experienced a high level of parasitism throughout their range in the past and, therefore, their rejection behaviour, once evolved, spread rapidly to all populations.     

Avian vision and the evolution of egg color mimicry in the common cuckoo

Coevolutionary arms races are a potent force in evolution, and brood parasite-host dynamics provide classical examples. Different host-races of the common cuckoo, Cuculus canorus, lay eggs in the nests of other species, leaving all parental care to hosts. Cuckoo eggs often (but not always) appear to match remarkably the color and pattern of host eggs, thus reducing detection by hosts. However, most studies of egg mimicry focus on human assessments or reflectance spectra, which fail to account for avian vision. Here, we use discrimination and tetrachromatic color space modeling of bird vision to quantify egg background and spot color mimicry in the common cuckoo and 11 of its principal hosts, and we relate this to egg rejection by different hosts. Egg background color and luminance are strongly mimicked by most cuckoo host-races, and mimicry is better when hosts show strong rejection. We introduce a novel measure of color mimicry-"color overlap"-and show that cuckoo and host background colors increasingly overlap in avian color space as hosts exhibit stronger rejection. Finally, cuckoos with better background color mimicry also have better pattern mimicry. Our findings reveal new information about egg mimicry that would be impossible to derive by the human eye.     

How to evade a coevolving brood parasite: egg discrimination versus egg variability as host defences

Arms races between avian brood parasites and their hosts often result in parasitic mimicry of host eggs, to evade rejection. Once egg mimicry has evolved, host defences could escalate in two ways: (i) hosts could improve their level of egg discrimination; and (ii) negative frequency-dependent selection could generate increased variation in egg appearance (polymorphism) among individuals. Proficiency in one defence might reduce selection on the other, while a combination of the two should enable successful rejection of parasitic eggs. We compared three highly variable host species of the Afrotropical cuckoo finch Anomalospiza imberbis, using egg rejection experiments and modelling of avian colour and pattern vision. We show that each differed in their level of polymorphism, in the visual cues they used to reject foreign eggs, and in their degree of discrimination. The most polymorphic host had the crudest discrimination, whereas the least polymorphic was most discriminating. The third species, not currently parasitized, was intermediate for both defences. A model simulating parasitic laying and host rejection behaviour based on the field experiments showed that the two host strategies result in approximately the same fitness advantage to hosts. Thus, neither strategy is superior, but rather they reflect alternative potential evolutionary trajectories.     

The evolution of egg size in the brood parasitic cuckoos

We compared genera of nonparasitic cuckoos and two groups ofparasitic cuckoos: those raised together with host young ("nonejectors")and those in which the newly hatched cuckoo either ejects thehost eggs or chicks, or kills the host young ("ejectors"). Nonejectorsare similar to their hosts in body size and parasitize largerhosts than do ejectors, which parasitize hosts much smallerthan themselves. In both types of parasite, the cuckoo's eggtends to match the host eggs in size. To achieve this, nonejectorshave evolved a smaller egg for their body size than have nonparasiticcuckoos, and ejectors have evolved an even smaller egg. Amongejector cuckoo genera, larger cuckoos have larger eggs relativeto the eggs of their hosts, and the relationship between cuckooegg volume (mass of the newly-hatched cuckoo) and host egg volume(mass to be ejected) did not differ from that predicted by weight-liftingallometry. However, comparing among Cuculus cuckoo species,the allometric slope differed from the predicted, so it is notclear that egg size is related to the need to give the cuckoochick sufficient strength for ejection. Comparing the two mostspeciose ejector genera, Chrysococcyx cuckoos (smaller and parasitizedome-nesting hosts) lay eggs more similar in size to their host'seggs than do Cuculus cuckoos (larger and parasitize open cup–nestinghosts). Closer size-matching of host eggs in Chrysococcyx mayreflect the following: (1) selection to reduce adult body massto facilitate entry through small domed nest holes to lay, and(2) less need for a large egg, because longer incubation periodsin dome-nesting hosts allow the young cuckoo more time to growbefore it need eject host eggs.     

Fecundity and egg viability in relation to female body size in Neodiprion sertifer (Hymenoptera: Diprionidae)

Relationships between female size and fecundity, egg viability and embryonal development were studied in Neoriprion sertifer. Sawfly females, reared from the 1st instar larvae on Scots pine needles heavily affected by industrial pollutants or on relatively unpolluted needles, were induced to lay eggs parthenogenetically in the field in Finland. Females reared on polluted needles were smaller and oviposited an average of 68.8 eggs, and those reared on unpolluted needles 79.4 eggs per female. However, the higher egg viability in the former group masked the effect of reduced female size. This compensation was primarily related to the size of the females, larger individuals producing relatively fewer viable eggs in both groups.     

Effects of experimental calcium availability,egg parameters and laying order on Great Tit Parus major eggshell pigmentation patterns

Many bird species lay eggs speckled with protoporphyrin‐based spots, however, for most of them the function of eggshell spotting is unknown. A plausible hypothesis is that protoporphyrin might have a structural function in strengthening the eggshell and is therefore deposited when calcium is scarce. In this study, we experimentally provided Great Tit Parus major females with supplemental calcium to examine its effect on the protoporphyrin‐based maculation of their eggs. In addition, we studied variation in eggshell pigmentation patterns in relation to other egg parameters and laying order. Calcium‐supplemented females laid larger eggs but shell thickness was not significantly affected by the treatment. Calcium supplementation may reduce the time and energy females devote to searching for calcium‐rich material, so that they can collect more nutrients and so lay larger eggs. Furthermore, pigment darkness was associated with egg volume and shape, which suggests that female quality and environmental food availability may also influence the shell pigmentation pattern. Within clutches, later‐laid eggs had larger and darker spots that were distributed more unevenly on the shell surface. This within‐clutch pattern could be explained by the increase in egg volume and egg shape and a decline in shell thickness with egg‐laying order, which characteristics were all related to shell‐spotting pattern. Eggs with a coronal ring had thinner shells, but pigment intensity and spot size were not related to shell thickness. Thus, our results suggest that concentrated spotting distribution may have a mechanical function, supporting the structural‐function hypothesis.     

Rejection of artificial cuckoo (Cuculus canorus) eggs in relation to variation in egg appearance among reed warblers (Acrocephalus scirpaceus)

Passerines that are exposed to brood parasitism can evolve reduced intraclutch variation in egg appearance to facilitate recognition and rejection of the parasitic egg. This has been shown to be true for European passerine species that are assumed to have participated in an evolutionary arms race with the cuckoo (Cuculus canorus). However, few investigations have been carried out with the aim of finding out whether there is a relationship between these two traits within a species. In this study, we compare the level of intraclutch variation in egg appearance and the rejection of an unlike parasitic egg within a population of reed warblers (Acrocephalus scirpaceus) in the south-eastern part of the Czech Republic. We parasitized reed warbler nests with an artificial non-mimetic cuckoo egg, and then monitored the reaction of the hosts. In 27 out of 48 nests (56.3%) the parasitic egg was rejected. The rejecter pairs had a statistically significantly lower intraclutch variation in egg appearance than the acceptor pairs. We discuss possible explanations for the observed relationship between rejection of unlike eggs and intraclutch variation in egg appearance within this population of reed warblers. The results are consistent with the evolutionary arms race hypothesis, but the intermediate rejection rate found in this population could also be maintained by an equilibrium between acceptors and rejecters due to rejection costs.     

Ant predation and the cost of egg carrying in the golden egg bug: experiments in the field

Golden egg bug ( Phyllomorpha laciniata ) females lay eggs on the backs of both female and male conspecifics. Bugs receive eggs voluntarily and involuntarily, and even when males carry eggs, many eggs are not fertilised by the carrier. Carrying the eggs of another individual is unexpected, particularly if egg carrying bears a cost in survival. We examined the predation risk associated with egg carrying experimentally in the field. P . laciniata individuals were enclosed with workers of one of two ant species, Pheidole pallidula or Cataglyphis piliscapus, which co-occur with the bug in the wild. Pheidole pallidula workers preyed on golden egg bugs and their eggs, but Cataglyphis piliscapus workers did not . In P. pallidula enclosures, golden egg bugs carrying larger egg loads were eaten first. These results suggest that golden egg bugs experience high predation pressure and that egg carrying increases the risk of predation. Due to the direct survival costs associated with egg carrying and the lack of relatedness between the eggs and the carrier, we suggest the golden egg bug as the first known intraspecific parasite in which parasitism is not related to active parental care.     

Responses of great reed warblers Acrocephalus arundinaceus to experimental brood parasitism: the effects of a cuckoo Cuculus canorus dummy and egg mimicry

Egg rejection behaviour towards parasitic eggs was studied in a great reed warbler Acrocephalus arundinaceus population in central Hungary, which was heavily (about 65%) parasitised by the common cuckoo Cuculus canorus . Clutches were experimentally parasitised during the egg-laying period with artificial, moderately mimetic cuckoo eggs or with conspecific eggs that were good mimics of the hosts' eggs. Great reed warblers rejected 76.2% of the artificial cuckoo eggs, mainly by ejection, but accepted most of the conspecific eggs (87.5%). Cuckoo eggs in naturally parasitised clutches were rejected at a lower rate (32%). When, in addition to the egg mimicry experiments, a stuffed cuckoo was placed near the nest, accompanied by the recording of a female cuckoo call, hosts' rejection rate of the artificial cuckoo egg increased from 76% to 96%. The sight of the cuckoo, on the other hand, did not influence host's rejection behaviour when a conspecific egg was used in the experiment. A stuffed collared dove Streptopelia decaocto , accompanied by its call, was used as a control, and did not cause any increased rejection. Great reed warblers were more aggressive towards the cuckoo than to the dove dummy. When the cuckoo eggs in naturally parasitised clutches were exchanged with artificial cuckoo eggs, we observed no increase in the rejection rate. We conclude that great reed warblers in our heavily parasitised population are capable of detecting brood parasitism in their clutch by identifying the parasitic egg. The efficiency of this identification depends mainly on the mimicry of the foreign egg. The sight of the cuckoo at the nest may increase rejection rate by stimulus summation, and this conditional effect is mainly affected by the degree of mimicry of the parasitic egg.     

How is host egg mimicry maintained in the cuckoo (Cuculus canorus)?

To investigate the evolutionary mechanism (host specificity vs. random searching) maintaining mimicry between cuckoo egg appearance and that of different European cuckoo Cuculus canorus hosts, we studied the level of mimicry between the appearance of C. canorus eggs and that of their hosts' eggs in different habitats in southern Finland by using ultraviolet-visible reflectance spectrophotometry. In the main habitat used by C. canorus for reproduction, eggs laid in nests of different host species differed in appearance. Host use by C. canorus was not related to the abundance of hosts, and the level of mimicry was not related to host abundance in the habitat. Furthermore, a close match between C. canorus egg appearance and that of host eggs within habitats was detected after removing the potentially confounding effect of host abundance. In the only two suitable host species nesting in trees (namely chaffinch Fringilla coelebs and brambling Fringilla montifringilla ) we detected changes in C. canorus egg appearance that paralleled those of the two host species. Thus our findings suggest the existence of a correlation between the appearance of C. canorus eggs and that of their hosts' eggs within different habitat types, and suggest that mimicry is maintained by strict host preferences by each C. canorus female when laying.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 82 , 57–68.     

Conspecifics enhance egg production in an egg-carrying bug

Variation in host availability and quality is likely to affectfemale decision making on when to lay eggs in arthropods. Inthe present study, we show a case in which female reproductionis limited by the availability of conspecifics in a speciesin which females preferably oviposit on conspecific bodies.Female golden egg bugs (Phyllomorpha laciniata; Heteroptera,Coreidae) deposit eggs mainly on the bodies of both male andfemale conspecifics. Bugs other than parents carry most eggs.Egg carrying is costly for individuals. A few eggs are laidon the bug's food plant, but in most habitats, those eggs donot survive owing to intensive ant predation. We explored ifconspecific presence affects female egg production (eggs laidand eggs in reproductive tract) in the field. In our experiment,conspecific presence (two females enclosed with food plant)increased female egg-laying rate and egg production comparedwith that of females that were alone with the food plant. Innature, gaining a conspecific host is difficult, and the encounterrate of conspecifics (i.e., operational host density) is likelyto be important for female reproduction. There are opposinginterests between an egg-laying female and the recipient (maleor female) because most eggs are dumped on individuals thatare not the parent of the eggs.     

Egg colour mimicry in the common cuckoo Cuculus canorus as revealed by modelling host retinal function

Some parasite cuckoo species lay eggs that, to the human eye, appear to mimic the appearance of the eggs of their favourite hosts, which hinders discrimination and removal of their eggs by host species. Hitherto, perception of cuckoo-host egg mimicry has been estimated based on human vision or spectrophotometry, which does not account for what the receivers' eye (i.e. hosts) actually discriminates. Using a discrimination model approach that reproduces host retinal functioning, and museum egg collections collected in the south of Finland, where at least six different races of the European cuckoo (Cuculus canorus) coexist, I first assess whether the colour design of cuckoo eggs of different races maximizes matching for two favourite avian hosts, viz. the redstart (Phoenicurus phoenicurus) and the pied wagtail (Motacilla alba). Second, I assess the role of nest luminosity on host perception of mimicry by the same two hosts. Phoenicurus-cuckoo eggs showed a better chromatic matching with the redstart-host eggs than other cuckoo races, and in most cases can not be discriminated. Sylvia-cuckoo eggs, however, showed better achromatic matching with redstart-host eggs than Phoenicurus-cuckoo eggs. Also, Motacilla-cuckoo eggs showed poorer chromatic and achromatic matching with pied wagtail-host eggs than Sylvia-cuckoo eggs. Nest luminosity affected chromatic and achromatic differences between cuckoo and host eggs, although only minimally affected the proportion of cuckoo eggs discriminated by chromatic signals. These results reveal that cuckoo races as assessed by humans do not entirely match with host perception of matching and that achromatic mechanisms could play a main role in the discrimination of cuckoo eggs at low-light levels.     

Hosts of avian brood parasites have evolved egg signatures with elevated information content

Hosts of brood-parasitic birds must distinguish their own eggs from parasitic mimics, or pay the cost of mistakenly raising a foreign chick. Egg discrimination is easier when different host females of the same species each lay visually distinctive eggs (egg ‘signatures’), which helps to foil mimicry by parasites. Here, we ask whether brood parasitism is associated with lower levels of correlation between different egg traits in hosts, making individual host signatures more distinctive and informative. We used entropy as an index of the potential information content encoded by nine aspects of colour, pattern and luminance of eggs of different species in two African bird families (Cisticolidae parasitized by cuckoo finches Anomalospiza imberbis, and Ploceidae by diederik cuckoos Chrysococcyx caprius). Parasitized species showed consistently higher entropy in egg traits than did related, unparasitized species. Decomposing entropy into two variation components revealed that this was mainly driven by parasitized species having lower levels of correlation between different egg traits, rather than higher overall levels of variation in each individual egg trait. This suggests that irrespective of the constraints that might operate on individual egg traits, hosts can further improve their defensive ‘signatures'' by arranging suites of egg traits into unpredictable combinations.     

Individual female clutch identification through yolk protein electrophoresis in the communally breeding guira cuckoo (Guira guira)

Avian communal breeding systems generate alternative behavioural strategies for females, resulting in differences in reproductive success. Identifying eggs of different females in such systems is problematic, however, due to egg destruction before incubation, difficulty of capturing adults, and/or inaccuracy of egg identification based on egg morphometry. Here, we describe a technique that uses electrophoresis of yolk proteins to determine egg ownership, which we applied to communally breeding guira cuckoos (Guira guira). Validation of the method included identical yolk protein banding patterns in all eggs of the same female, but different patterns in eggs of different females in budgerigars (Melopsittacus undulatus), and identical patterns in yolk follicles of the same females in guira cuckoos. We applied the protocol to 195 guira cuckoo eggs from 34 joint nests in 2 years. All multiple guira cuckoo eggs laid on the same day in single nests had distinct banding patterns of yolk proteins, practically eliminating the possibility of more than one female being represented by the same pattern. Some identical banding patterns were repeated in different days within a nesting bout, indicating that some females laid several eggs in shared nests. Identical patterns occasionally occurred in renestings of groups, indicating that some females lay eggs in consecutive nestings. Yolk protein electrophoresis is a useful tool to identify egg maternity in other circumstances, such as polygynous mating systems with joint nests and intraspecific parasitism. Additionally, it is an alternative method for species where electrophoresis of egg white proteins does not show sufficient polymorphism.     

Environmental cues or conspecific attraction as causes for egg mass aggregation in hydrobiosid caddisflies

The distribution of egg masses throughout the environment can strongly influence the population dynamics of aquatic insects. Many species lay eggs in aggregations and most eggs will subsequently hatch from only a few locations—knowing how and why these aggregations arise is therefore needed to understand the population dynamics of these species. The hydrobiosid caddisfly Ulmerochorema rubiconum lays eggs in large aggregations on the undersides of emergent rocks in streams. Our aim was to test whether females oviposit in response to conspecific egg masses or the environmental characteristics of rocks by manipulating the number and age of egg masses initially present on rocks and monitoring the accumulation of new masses. Our first experiment used rocks that had recently been used for oviposition and initial egg masses encompassed a range of ages; our second used rocks with no history of oviposition and initial masses of uniform age. Females did not respond to conspecific eggs of any age, as removing initial egg masses had no impact on the number of new masses laid in either experiment. There was a strong positive relationship between the number of initial and new masses, regardless of whether initial egg masses were removed, at the scale of whole rocks and for rock microhabitats. The results suggest that females select oviposition sites based primarily on physical characteristics of emergent rocks. We demonstrate for the first time that the spatial arrangement of egg masses may be consistent and predictable through time, with females showing a high degree of fidelity for particular rocks and microhabitats.