Patterns of host-plant choice in bees of the genus Chelostoma: the constraint hypothesis of host-range evolution in bees

To trace the evolution of host-plant choice in bees of the genus Chelostoma (Megachilidae), we assessed the host plants of 35 Palearctic, North American and Indomalayan species by microscopically analyzing the pollen loads of 634 females and reconstructed their phylogenetic history based on four genes and a morphological dataset, applying both parsimony and Bayesian methods. All species except two were found to be strict pollen specialists at the level of plant family or genus. These oligolectic species together exploit the flowers of eight different plant orders that are distributed among all major angiosperm lineages. Based on ancestral state reconstruction, we found that oligolecty is the ancestral state in Chelostoma and that the two pollen generalists evolved from oligolectic ancestors. The distinct pattern of host broadening in these two polylectic species, the highly conserved floral specializations within the different clades, the exploitation of unrelated hosts with a striking floral similarity as well as a recent report on larval performance on nonhost pollen in two Chelostoma species clearly suggest that floral host choice is physiologically or neurologically constrained in bees of the genus Chelostoma. Based on this finding, we propose a new hypothesis on the evolution of host range in bees.     

Phylogeny and floral hosts of a predominantly pollen generalist group of mason bees (Megachilidae: Osmiini)

Within the genus Osmia, the three subgenera Osmia, Monosmia, and Orientosmia form a closely‐related group of predominantly pollen generalist (‘polylectic’) mason bees. Despite the great scientific and economic interest in several species of this clade, which are promoted commercially for orchard pollination, their phylogenetic relationships remain poorly understood. We inferred the phylogeny of 21 Osmia species belonging to this clade by applying Bayesian and maximum likelihood methods based on five genes and morphology. Because our results revealed paraphyly of the largest subgenus (Osmia s.s.), we synonymized Monosmia and Orientosmia under Osmia s.s. Microscopical analysis of female pollen loads revealed that five species are specialized (‘oligolectic’) on Fabaceae or Boraginaceae, whereas the remaining species are polylectic, harvesting pollen from up to 19 plant families. Polylecty appears to be the ancestral state, with oligolectic lineages having evolved twice independently. Among the polylectic species, several intriguing patterns of host plant use were found, suggesting that host plant choice of these bees is constrained to different degrees and governed by flower morphology, pollen chemistry or nectar availability, thus supporting previous findings on predominantly oligolectic clades of bees. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111 , 78–91.     

Phylogenetic relationships within Diadasia, a group of specialist bees

We estimated phylogenetic relationships among species of the bee genus Diadasia, a group of new world, specialist bees. We sequenced approximately 2 kb of the mitochondrial genes cytochrome oxidase subunit I and II and tRNA leucine and approximately 1 kb of the nuclear gene elongation factor 1-alpha for 24 North American Diadasia species, 4 South American species, and five outgroup genera. Parsimony analyses of the two data sets were highly congruent. A combined analysis produced a well-resolved phylogenetic hypothesis that supported the monophyly of Diadasia, but not that of traditional subgenera: Diadasia s. str. was paraphyletic in all analyses. With one exception, the North and South American species formed separate clades, supporting previous hypotheses that two lineages of Diadasia have dispersed from South to North America: a more recent dispersal of D. ochracea and an older dispersal of the ancestor to all other North American species. Different species of Diadasia specialize on pollen from at least five different plant families; the phylogeny presented here, along with known host affinities, indicates that host-switching has been rare.     

Invasive Africanized honey bee impact on native solitary bees: a pollen resource and trap nest analysis

Little is known of the potential coevolution of flowers and bees in changing, biodiverse environments. Female solitary bees, megachilids and Centris , and their nest pollen provisions were monitored with trap nests over a 17-year period in a tropical Mexican biosphere reserve. Invasion by feral Apis (i.e. Africanized honey bees) occurred after the study began, and major droughts and hurricanes occurred throughout. Honey bee competition, and ostensibly pollination of native plants, caused changes in local pollination ecology. Shifts in floral hosts by native bees were common and driven by plant phylogenetics, whereby plants of the same families or higher taxa were substituted for those dominated by honey bees or lost as a result of natural processes. Two important plant families, Anacardiaceae and Euphorbiaceae, were lost to competing honey bees, but compensated for by greater use of Fabaceae, Rubiaceae, and Sapotaceae among native bees. Natural disasters made a large negative impact on native bee populations, but the sustained presence of Africanized honey bees did not. Over 171 plant species comprised the pollen diets of the honey bees, including those most important to Centris and megachilids (72 and 28 species, respectively). Honey bee pollination of Pouteria (Sapotaceae) plausibly augmented the native bees' primary pollen resource and prevented their decline. Invasive generalist pollinators may, however, cause specialized competitors to fail, especially in less biodiverse environments.  No claim to original US government works. Journal compilation © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 98 , 152–160.     

Pollen hosts of western palaearctic bees of the genus Colletes (Hymenoptera: Colletidae): the Asteraceae paradox

To assess the pollen hosts of 60 western palaearctic bee species of the genus Colletes (Colletidae), we microscopically analysed 1336 pollen loads of collected females. Twenty‐six species (43.3%) were found to be specialized at the level of plant family, subfamily or genus. Thirty‐four species (56.7%) proved to be pollen generalists to varying degrees, visiting the flowers of up to 15 different plant families. Flowers of the subfamily Asteroideae (Asteraceae) are by far the most important pollen source, contributing 23.6% to the pollen‐plant spectrum of the whole bee genus. The high significance of Asteroideae pollen is due to the large number of specialists: 14 Colletes species belonging to four different taxonomic groups harvest pollen exclusively or predominantly on flowers of the Asteroideae. By striking contrast, Asteroideae pollen plays only a marginal role in the diets of the pollen generalists: it was recorded in only 2.7% of the pollen loads and in seven out of the 34 pollen generalists. Among the few generalists exploiting Asteroideae for pollen, three closely related species have ancestors which were possibly specialized on Asteraceae. The pattern of use of Asteroideae pollen by the Colletes bees supports recent findings that this pollen possesses unfavourable or protective properties, which render its digestion difficult, and suggests that bees need physiological adaptations to successfully utilize it. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 719–733.     

Host range evolution in a selected group of osmiine bees (Hymenoptera: Megachilidae): the Boraginaceae‐Fabaceae paradox

Bees are extraordinarily diverse with respect to host plant choice and adaptation. Recent findings suggest that bee host range might be largely governed by evolutionary constraints related to pollen digestion or flower recognition and handling. In the present study, we applied phylogenetic inference to investigate whether such constraints underlie host plant choice in bees of the Annosmia‐Hoplitis group (Megachilidae) and to what extent these bees have evolved specialized adaptations for pollen collection. We demonstrate that most pollen specialist species exclusively exploit either Boraginaceae or Fabaceae, whereas all pollen generalists harvest pollen from both Boraginaceae and Fabaceae. The counterintuitive affinity towards these two plant families, which are neither closely related nor share similar flower morphologies, demonstrates that pollen host choice is considerably constrained in this group of bees. We hypothesize that this Boraginaceae‐Fabaceae paradox might be the result of (1) similar secondary metabolites in the pollen of both families; (2) metabolites that can be detoxified by the same physiological tools; or (3) similar pollen nutrient composition. Contrary to the widely held belief that specialized adaptations for pollen collection are rare among bees, such adaptations are common in the Annosmia‐Hoplitis bees, where they have evolved several times independently to exploit flowers of widely different morphologies. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ●● , ●●–●●.     

The diversity of hemi-epiphytic figs (Ficus ; Moraceae) in a Bornean lowland rain forest

The diversity and niche specificity of hemi-epiphytic figs in a lowland dipterocarp forest in Sarawak were investigated in 1998. Twenty-seven fig species (264 individuals, c. 120 ha) colonized a diversity of host taxa (35 families), but densities were very low and only 1.77% of trees> 30 cm d.b.h. were occupied. There were no significant associations with host taxa or host-bark roughness but among 11 common species (≥9 individuals) the distributions of all other parameters (host-d.b.h., height and position of colonization, crown illumination, soil-texture and slope-angle) were significantly different, and we identified five fig guilds. The guilds corresponded to canopy strata, and appeared to reflect the establishment microsite requirements of different species. A fundamental trade-off within the hemi-epiphytic habit was revealed: Species colonizing larger hosts were rarer, because of lower host densities and more specific microsite requirements, but had better light environments and attained a larger maximum size. The single strangler species appeared to escape many of these constraints, and an important source of mortality caused by host-toppling, indicating the advantages of this strategy. Thus, the hemi-epiphytic figs in this community have come to fill a remarkable diversity of niches, despite low levels of competition, through the exigencies of a complex environment.  © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78 , 439–455     

Pirate ants (Polyergus breviceps) and sympatric hosts (Formica occulta and Formica sp. cf. argentea): host specificity and coevolutionary dynamics

The pace and trajectory of coevolutionary arms races between parasites and their hosts are strongly influenced by the number of interacting species. In environments where a parasite has access to more than one host species, the parasite population may become divided in preference for a particular host. In the present study, we show that individual colonies of the pirate ant Polyergus breviceps differ in host preference during raiding, with each colony specializing on only one of two available Formica host species. Moreover, through genetic analyses, we show that the two hosts differ in their colony genetic structure. Formica occulta colonies were monogynous, whereas Formica  sp. cf. argentea colonies were polygynous and polydomous (colonies occupy multiple nest sites). This difference has important implications for coevolutionary dynamics in this system because raids against individual nests of polydomous colonies have less impact on overall host colony fitness than do attacks on intact colonies. We also used primers that we designed for four microsatellite loci isolated from P. breviceps to verify that colonies of this species, like other pirate ants, are comprised of simple families headed by one singly mated queen.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 565–572.     

Differences in environmental predictability underlie divergent competitive abilities in three congeneric hydroids

Species in which individuals experience predictable and uniform environments should be most finely adapted to their environment. Many hydrozoan species in the genus Hydractinia simultaneously occupy similar microhabitats (gastropod shells inhabited by hermit crabs) but experience considerable differences in their immediate environment (size and species of shells and hosts). In the present study, hydroid species experience differences in environmental predictability and traits that mediate competitive ability (growth form and growth rate). The inferred competitive ability was directly proportional to the extent to which the gastropod environment promotes interactions between small, juvenile colonies, which always end in competitive elimination. Extensive intraspecific variation in competitive ability was explained primarily by crab host species or site. Dense host populations impose more severe disturbance regimes that favour competitively inferior, but disturbance-resistant, phenotypes. Interplay between different types of variation (gastropods and hermit crabs) provides a possible mechanism for the maintenance of intraspecific growth form variation.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 96 , 322–338.     

Flies on thistles: support for synchronous speciation?

Synchronous speciation of hosts and herbivorous insects predicts a congruent topology of host and insect phylogenies and similar evolutionary ages of host and insect taxa. To test these predictions for the specialized herbivorous fly genus Urophora (Diptera: Tephritidae), we used three different approaches. (i) We generated a phylogenetic tree of 11 European Urophora species from allozyme data and constructed a phylogeny of their hosts from published sources. Superimposing the Urophora tree on the host-plant tree we found no evidence for general congruence. (ii) We correlated genetic distances (Nei distances) of the host plants vs. the genetic distances of associated Urophora species. Overall, the relationship was not positive. Nevertheless, for some pairs of Urophora species and host plants genetic distances were in the same order of magnitude. (iii) We collected allozyme data for pairs of thistle taxa and pairs of herbivores on thistles together with independent time estimates. With these data we calibrated a molecular clock. There was a non-linear relationship between phylogenetic age and genetic distance, rendering the dating of deep events in thistle–insect evolution difficult. Nevertheless the derived molecular clock showed that the split of insect taxa lagged behind the split of hosts.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 775–783.     

Temporally persistent patterns of incidence and abundance in a pollinator guild at annual and decadal scales: the bees of Larrea tridentata

A recent alternative model to conventional coevolution, the geographic mosaic theory of coevolution, posits that reciprocal selection is episodic and local, rather than persistent and spatially extensive. However, little empirical evidence addresses this model's tenets, in particular the temporal stability of local plant–pollinator interactions. We evaluated this tenet using the richly diverse guild of bees at creosote bush ( Larrea tridentata ), a generalist plant that hosts many specialist bees. We systematically resampled over 2–5 years at 11 sites across the south-western USA. Incidence and abundance also were compared for survey sites sampled 20 ± 2 years earlier. Average Morisita-Horn faunal similarities of local bee guilds was 87% for sequential years and 36% after 20 years. Similarities in taxonomic composition of resampled local bee guilds could be statistically represented as a random assemblage drawn from the regional source pool of 54–68 bee species that could be expected at Larrea , weighted by regional abundance. At every site, only the minority of abundant bee species was typically persistent in local guilds, even after > 20 years. Most bee species in the Larrea guild were chronically uncommon, geographically sporadic and temporally unpredictable, attributes that render them numerically inconsequential as pollinators in their local guild. Persistence among abundant bee species in local pollinator assemblages satisfies one condition by which reciprocal selection could act locally. The well-being of these more abundant core species, and not bee diversity per se , may better characterize the health of such plant–pollinator associations.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 319–329.     

Phylogenetic analyses of DNA and allozyme data suggest that Gonioctena leaf beetles (Coleoptera; Chrysomelidae) experienced convergent evolution in their history of host-plant family shifts

A phylogenetic analysis of the genus Gonioctena (Coleoptera, Chrysomelidae) based on allozyme data (17 loci) and mitochondrial DNA sequence data (three gene fragments, 1,391 sites) was performed to study the evolutionary history of host-plant shifts among these leaf beetles. This chrysomelid genus is characteristically associated with a high number of different plant families. The diverse molecular data gathered in this study are to a large extent congruent, and the analyses provide a well-supported phylogenetic hypothesis to address questions about the evolution of host-plant shifts in the genus Gonioctena. The most-parsimonious reconstruction of the ancestral host-plant associations, based on the estimated phylogeny, suggests that the Fabaceae was the ancestral host-plant family of the genus. Although most of the host-plant shifts (between different host species) in Gonioctena have occurred within the same plant family or within the same plant genus, at least eight shifts have occurred between hosts belonging to distantly related and chemically dissimilar plant families. In these cases, host shifts may have been simply directed toward plant species available in the environment. Yet, given that two Gonioctena lineages have independently colonized the same three new plant families (Salicaceae, Betulaceae, Rosaceae), including four of the same new genera (Salix, Alnus, Prunus, Sorbus), some constraints are likely to have limited the different possibilities of interfamilial host-plant shifts.     

Biology of Rhoophilus loewi (Hymenoptera: Cynipoidea: Cynipidae), with implications for the evolution of inquilinism in gall wasps

The sole described indigenous afrotropical cynipid, the South African endemic Rhoophilus loewi , was originally described as a gall inducer. As predicted by phylogenetic relationships, we confirm that R. loewi is instead an inquiline wasp. Although all other cynipid inquilines develop in galls induced by cynipid wasps, or very rarely cecidomyiid midges, R. loewi is the only cynipid inquiline of a lepidopteran gall, inducing secondary inquiline cells in galls induced by a cecidosid moth genus Scyrotis on Rhus species (Anacardiaceae). Rhoophilus is a lethal inquiline; its larval cells expand into the hollow interior of the host gall resulting in death of the gall inducer. Rhoophilus loewi is redescribed and the final-instar larva described for the first time. We describe host-plant associations, the morphology and phenology of gall formation, and the suite of parasitoid Hymenoptera associated with these galls. The community centred on Scyrotis galls is compared with those observed in other hosts of inquiline cynipids. Elucidation of the life history of R. loewi has fundamental implications for understanding the evolution of cynipid wasps.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 153–172.     

Host plant toxicity affects developmental rates in a polyphagous fruit fly: experimental evidence

Various, non-exclusive mechanisms have been invoked to explain the observed association between host plant use and speciation in insect species. In the Afrotropical genus Ceratitis (Diptera: Tephritidae), morphological, molecular, and biochemical data suggest that evolutionary radiation of stenophagous clades originates from their ability to exploit toxic hosts. To test whether, and to what extent, the development and fitness of Ceratitis fasciventris , a polyphagous congener known to infest over 43 host species, is adversely affected by host plant toxicity, we compared the rates of development, survival, and reproduction of captive bred individuals in four media that differ in alkaloid concentration. Despite reduced pupal and adult sizes, C. fasciventris larvae developing under low alkaloid concentrations successfully developed to the adult stage, probably as a result of accelerated pupation and ensuing restricted exposure to the toxic environment. High alkaloid concentrations, however, impaired their developmental process and prevented subsequent reproduction. The adverse effects of host plant toxicity on larval development in polyphagous fruit flies indicate that high alkaloid concentrations pose a significant constraint on host use by polyphagous Ceratitis species.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 728–737.     

Genetics of host-plant preference in the comma butterfly Polygonia c-album (Nymphalidae), and evolutionary implications

In the Lepidoptera, sex-linked genes have been found to be of importance for species differences in, for example, host-plant preference, and have been implicated in ecological speciation. Variation within species is typically not sex-linked. However, in the comma butterfly Polygonia c-album (Nymphalidae) an X-linked gene has been found to play a major role in determining differences in host-plant use between two well separated populations. For this reason, we studied the role of sex-linked genes for host-plant preference within a single Swedish population of this species. Three generations of females with known pedigrees were studied in the laboratory, and they were given a choice between Urtica dioica and Salix caprea in flight cages. We found strong variation among females and significant genetic variance for host-plant preference, but no evidence for major importance of sex linkage of host-plant preference on this local scale. To what extent the observed genetic variation was due to additive genes and/or effects of major genes was not clear from the maximum likelihood analysis. In a follow-up study we sampled females over a larger area. We found strong variation among females, but not among localities, suggesting an open population structure with strong gene flow. From the combined stock, a selection experiment was performed over 2 years and six generations. The selection lines diverged after the first generation of selection and remained separate, but did not diverge further, suggesting a low degree of narrow-sense heritability and that the genetic differences may be effects of major genes. We discuss these results in relation to the possible role of genetics in the radiation of the Lepidoptera and other phytophagous insects.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 755–765.     

Pollination of a slippery lady slipper orchid in south-west China: Cypripedium guttatum (Orchidaceae)

Cypripedium guttatum was studied in north-west Yunnan at 3490 m a.s.l. The flowers are rewardless 'kettle traps'. The structure of the lip, where pollinators are temporarily kept prisoner, and the method of their capture, are unusual in being Paphiopedilum - rather than Cypripedium -like. The deceptive orchid does not mimic any of the diverse flowers concurrently blooming in the habitat, all being visited by the polylectic pollinators of C. guttatum , viz . Lasioglossum virideglaucum, L. clypeinitens and L. sauterum , besides two additional probable pollinators and four non-pollinating visitors (all Halictidae; three new species). The bees got caught when they tried to climb onto the staminode and their forelegs slid down its slippery downward ridges, causing them to tumble to the pouch bottom. To leave, they had to climb a tunnel leading past the stigma to the anthers where a pollen smear was acquired while extruding themselves from the narrow exit. The similarities with myiophylous Paphiopedilum are discussed in view of the possibility that they may foreshadow evolutionary transitions between melittophily and myiophily found in slipper orchids.  © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 148 , 251–264.     

Macaranga ant-plants hide food from intruders: correlation of food presentation and presence of wax barriers analysed using phylogenetically independent contrasts

Many tropical ant-plants provide specialized ant partners with food, which may attract foreign ants parasitizing the mutualism. We present evidence for the ant-plant genus Macaranga , showing that ant competition has forced host plants to hide food resources and restrict access to the mutualists. In Macaranga myrmecophytes, the influence of ant competition strongly depends on the presence of slippery 'wax barriers'. Of all Macaranga ant-plant species, 50% have waxy stems that can be climbed only by the specific ant partners and not by other ant species. We compared the presentation of food (food bodies and extrafloral nectar) between waxy and non-waxy Macaranga host plants using traditional and phylogenetic comparative methods. Consistent with the hypothesized effect of ant competition, wax-free Macaranga host species had fewer extrafloral nectaries and more often produced food bodies under recurved or tubular stipules inaccessible to other ants; closed stipules were less persistent in waxy hosts. Several traits showed phylogenetic signal, but our finding of a more promiscuous food presentation in waxy Macaranga hosts was still supported by phylogenetic comparative analyses. We conclude that competition among ants is an important factor in the evolution of myrmecophytism, and that it has given rise to traits acting as protective filter mechanisms.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 177–193.     

Pollination of the lady's slipper Cypripedium henryi Rolfe (Orchidaceae)

The pollination ecology of Cypripedium henryi Rolfe, a slipper orchid endemic to west China, was investigated, and its floral shape, size, colour, and scent were analysed. Examination of the breeding system suggests that the flowers are self-compatible, but need pollen vectors for successful reproduction. The flower is rewardless; over 15 insects belonging to Araneida, Hymenoptera, Diptera, Lepidoptera, and Coleoptera were recorded as flower visitors, but most only alighted or rested on the flower. In the total 32 h of observations over 2 years, female Lasioglossum bees were found to be the most frequent visitors and the only pollinators. They showed a high visitation frequency and, surprisingly, re-visited the same flowers frequently. Cypripedium henryi probably attracts pollinators visiting the flowers through general food deception (odour components, colour, false nectar guides), as well as special structures (slippery labellum, slippery staminode). Although three Lasioglossum species visited the flowers, only L. sauterum Fan et Ebmer was found with pollen. Lasioglossum flavohirtum Ebmer was large and climbed out from the entrance. Morphologically, L. sichuanense Fan et Ebmer could be considered as a potential pollinator, but the collected specimens were found to have no pollen of C. henryi on their bodies. It was speculated that the particular floral scent of C. henryi discouraged the entrance of L. sichuanense bees. Lasioglossum sauterum was matched morphologically to the flower, but not all of the visitations resulted in effective pollinations, as some flowers of C. henryi were frequently re-visited and the pollen mass had been taken away by bees on previous visitations. © 2008 The Linnean Society of London, Botanical Journal of the Linnean Society , 2008, 156 , 491–499.     

Reproductive characters in a gynodioecious species, Silene italica (Caryophyllaceae), with attention to the gynomonoecious phenotype

Gynodioecious populations (i.e. populations with female and hermaphrodite individuals) often contain a third phenotype with an intermediate sex expression. In Silene italica , this phenotype is characterized by a mixture of pistillate and perfect flowers and is thus gynomonoecious. To characterize sexual functions of these gynomonoecious individuals and their potential influence in the maintenance of gynodioecy, reproductive characters of the three sexual phenotypes were compared over 2 years in several families of S. italica produced by crossing. We found that gynomonoecious individuals were intermediate for flower production and female fertility characters, although they did not always significantly differ from female individuals. Perfect flowers of gynomonoecious and hermaphrodite plants were similar in size and pollen production. Gynomonoecious individuals were thus intermediate in female and in male functions. Family effects were found for most of the characters. The female advantage (i.e. the fertility of females compared to the female fertility of pollen producing plants) was not dramatically different when gynomonoecious plants were taken into account.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 87 , 583–591.     

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.