Mechanism of egg recognition in defenses against conspecific brood parasitism: American coots (<Emphasis Type="Italic">Fulica americana</Emphasis>) know their own eggs

Hosts of avian brood parasites use a variety of defenses based on egg recognition to reduce the costs of parasitism; the most important of which is rejecting the parasitic eggs. Two basic recognition mechanisms are possible: “true recognition”, whereby hosts recognize their own eggs irrespective of their relative frequency in the clutch, and minority recognition (or “recognition by discordancy”), whereby hosts respond to the minority egg type. The mechanism of recognition has been experimentally studied in a handful of species parasitized by interspecific brood parasites, but the mechanism used in defenses against conspecific brood parasitism is unknown. I experimentally determined the mechanism of egg recognition in American coots (Fulica americana), a species with high levels of conspecific brood parasitism, egg recognition, and rejection. I swapped eggs between pairs of nests to alter frequencies of host and “parasite” eggs and then used two criteria for recognition: egg rejection and nonrandom incubation positions in the clutch. Eight of 12 nests (66%) given equal frequencies of host and parasite eggs showed evidence of true recognition. In contrast, only one of eight (12.5%) nests where host eggs were in the minority showed evidence of recognition by discordancy. The nonrandom incubation positions of parasitic eggs indicates that birds sometimes recognize parasitic eggs without rejecting them and provides a means of assessing recognition on a per nest basis in species with large clutches. Adaptive recognition without rejection may also be an important evolutionary stepping stone to the evolution of egg rejection in some taxa.     

Mechanisms of avian egg recognition: Which egg parameters elicit responses by rejecter species?

Summary Some species of North American passerines nearly always reject nonmimetic eggs placed in their nests and have apparently evolved this behavior in response to brood parasitism. Experiments presented here examined the specific egg parameters to which rejecter species respond, the relative tolerances rejecters show towards nonmimetic eggs and the degree to which rejection is limited to eggs of the brown-headed cowbird (Molothrus ater), the only parasitic bird widespread in North America.Relative to cowbird eggs, American robin (Turdus migratorius) eggs are larger, blue rather than white and immaculate rather than spotted. Experiments using 10 egg models at 137 nests showed that robins respond to each of these differences (Figs. 2 and 3) but do not usually reject an egg that deviates from their own by only one difference. Eggs that differ in ant two of the three parameters are usually rejected. This built-in tolerance reduces the likelihood that robins will reject their own eggs if these are atypical in size or coloration.Small egg size was the most important parameter eliciting rapid rejections (i.e. within 1 day), probably because differnces in size can be detected by both visual and tactile perception. By contrast, small egg size was the least important parameter determining whether eggs were eventually rejected (i.e. within 5 days, Tables 2 and 3). In terms of their eventual response, robins may be more sensitive to egg coloration than to size because the latter parameter is less reliable in distinguishing between robin and cowbird eggs.Experiments were also carried out at 37 nests of the gray catbird (Dumetella carolinensis), whose immaculate blue-green egg is only slightly larger than a cowbird egg. Catbirds are much more responsive to white ground color than to maculation (Table 4), perhaps because color is more reliable in distinguishing between catbird and cowbird eggs.Rejecter species exhibit degrees of tolerance towards foreign eggs that are proportional to the divergence between their eggs and those of the cowbird. Birds with eggs strongly divergent from cowbird eggs benefit from being relatively tolerant because they avoid rejecting their own eggs but still act against cowbird eggs. Species with cowbird-like eggs must be relatively intolerant to maximize the chances that cowbird eggs are rejected.Experiments show that rejection is not specific to cowbird eggs. Thus, birds have apparently responded evolutionarily to brood parasitism by developing recognition of their own eggs, rather than by developing recognition and rejection specific to parasitic eggs.     

Foreign egg retention by avian hosts in repeated brood parasitism: why do rejecters accept?

Great reed warblers (Acrocephalus arundinaceus) are frequently parasitized by egg-mimetic common cuckoos (Cuculus canorus) in Hungary, and these hosts reject about a third of parasitic eggs. The timing of parasitism is important, in that the probability of rejection decreases with advancing breeding stages in this host. Also, egg rejection is more common when a clutch is parasitized by a single foreign egg, compared to parasitism by multiple eggs. We repeatedly parasitized great reed warbler clutches with moderately mimetic foreign eggs, either with (1) one foreign egg (single parasitism) and, after 3 days, by all foreign eggs (multiple parasitism), or (2) all foreign eggs and, 3 days later, by only one foreign egg. Hosts ejected 26–53 % of the experimental parasitic eggs in the first stage of the repeated parasitism, but almost all eggs were accepted in the second stage, irrespective of whether the clutch was singly or multiply parasitized. Video-taping of the behavioural responses of hosts to experimental parasitism revealed no evidence for sensory constraints on foreign-egg recognition, because hosts recognized and pecked the parasitic eggs as frequently in the second stage of repeated parasitism, as they did in the first stage. We suggest that the relative timing of parasitism (laying vs. incubation stage), rather than learning to accept earlier-laid foreign eggs, results in higher acceptance rates of cuckoo eggs in repeated parasitism, because there is decreasing natural cuckoo parasitism on this host species and, hence, less need for antiparasitic defences, with the advancing stages of breeding.     

The cost of host egg damage caused by a brood parasite: experiments on great spotted cuckoos (Clamator glandarius) and magpies (Pica pica)

Adult great spotted cuckoos, Clamator glandarius, frequently damage one or more eggs of their magpie host, Pica pica, without removing or eating them. The presence of damaged host eggs could signal parasitism thereby increasing the probability that the parasitic egg is ejected. This hypothesis was tested by experimentally introducing a model cuckoo egg with or without damaged host eggs. Magpie responses to experimental parasitism did not differ significantly between treatments implying that damaged host eggs are not used by magpies to assess parasitism. We followed the fate of magpie eggs naturally damaged by the great spotted cuckoo or experimentally damaged by us. Host response was very similar for naturally or experimentally damaged host eggs, but varied significantly according to the type of egg damage, eggs being removed more frequently when pecked than crushed, while cracked eggs were never removed. However, the egg damage that most readily causes egg removal is albumen leakage. Received: 30 November 1998 / Received in revised form: 7 June 1999 / Accepted: 12 June 1999     

Tactics of parasitic American coots: host choice and the pattern of egg dispersion among host nests

Summary I examined the tactics adopted by a conspecific brood parasite, the American coot (Fulica americana), and the degree to which these tactics reflect sources of mortality for parasitic eggs. Only 8% of parasitic eggs produced independent offspring, compared to a 35% success rate for non-parasitic eggs, and most mortality was due to egg-rejection by hosts or the consequences of laying eggs too late in the host's nesting cycle. Parasites usually laid parasitically before initiating their own nests and usually parasitized immediate neighbours. Parasites did not remove host eggs before laying their own egg, and egg disappearance in general was not more common at parasitized nests. I found no evidence for non-random host choice, either on the basis of stage of the host's nesting cycle or the host's brood size. The absence of adaptive host choice is likely a consequence of the fact that, due to host limitation, only a small proportion of parasites had meaningful variation among potential hosts to choose from. The pattern of egg dispersion among host nests by individual parasites appears to be a compromise between constraints imposed by host limitation and the increased success obtained from spreading eggs among nests. Most females laying fewer than five parasitic eggs laid them in a single host nest while females laying five or more eggs normally parasitized two or more hosts. An examination of egg rejection and survival rates showed that parasites would maximize success by laying a single egg per host nest, and the pattern of laying several eggs per host nest is likely a consequence of host limitation. However, no egg that was the fifth laid, or later, parasitic egg in a host nest was ever successful and this probably explains why most females laying five or more eggs parasitized more than one host.     

Experimental support for the use of egg uniformity in parasite egg discrimination by cuckoo hosts

Common cuckoo (Cuculus canorus) parasitism drastically reduces the reproductive success of their hosts and selects for host discrimination of cuckoo eggs. In a second stage of anti-parasite adaptation, once cuckoos can lay eggs that mimic those of their hosts, a high uniformity of host egg appearance within a clutch may favour cuckoo egg discrimination. Comparative evidence provides indirect support for this hypothesis although experimental support is currently lacking. Here, we studied the effect of experimentally decreased uniformity of host egg appearance on cuckoo egg discrimination by great reed warbler (Acrocephalus arundinaceus) hosts in a population in which long-term cuckoo parasitism has led to high levels of cuckoo–host egg mimesis. We manipulated host clutch uniformity by adding extra spots to fresh host eggs just after they were laid. Rejection of non-mimetic experimental eggs added to these nests was compared with those in control nests in which uniformity was not altered. Previously, by over-painting real spots in a control group of nests, we showed a negligible effect of our paints on hosts’ perception of their eggs. We show that for the great reed warbler, non-mimetic experimental eggs were relatively more tolerated in experimental nests, i.e. with lower uniformity (40%) than in control nests (5%). This is the first experimental study, to our knowledge, which demonstrates a reduced discrimination of foreign eggs as a consequence of an increase of egg phenotypes variation perception in a cuckoo host.     

Egg adoption can explain joint egg-laying in common eiders

Hypotheses regarding the evolution and maintenance of intraspecific nest parasitism were tested with data collected during a 3-year study of common eiders (Somateria mollissima) breeding near Churchill, Manitoba. The nest parasitism rate was highest (42.4% of nests) during the year with the highest nest density and the best environmental conditions, and lowest (20.2% of nests) in the year with the lowest nest density and the poorest environmental conditions. Over the nesting season, parasitic eggs were laid at the same time as normally laid eggs. Most parasitic eggs (>75%) were laid before the host female laid her third egg. The majority of the parasitic eggs were the first or second egg produced by the parasitic female. When a parasitic egg was laid before or on the same day as the host female initiated her clutch, the probability of her first egg being depredated before incubation was significantly lowered. First- and second-laid eggs suffered a high rate of predation probably because nesting females do not attend their clutch until their second or third egg is laid. Hypotheses that some females use intraspecific nest parasitism to parasitize the parental care of other females were inconsistent with these data. Egg adoption is a likely explanation for the prevalence of females incubating parasitic eggs in this population. Received: 30 September 1997 / Accepted after revision: 6 May 1998     

Spectral tuning and perceptual differences do not explain the rejection of brood parasitic eggs by American robins (Turdus migratorius)

By laying their eggs in the nests of other birds, avian brood parasites impose the cost of rearing young upon their hosts. The recognition and rejection of foreign eggs are primary host defenses against costly brood parasitism. Hosts of parasitic brown-headed cowbirds (Molothrus ater) challenge coevolutionary theory because most cowbird hosts accept parasitic eggs despite their drastically different appearance from the hosts’ own eggs. American robins (Turdus migratorius) are one of only 10 % of the over 200 potential cowbird host species to robustly reject parasitic eggs, but the mechanisms driving the sensory bases of foreign egg rejection remain elusive. Our research combined avian visual perceptual modeling and behavioral experimentation to investigate chromatic cues eliciting parasitic egg rejection in American robins. We assessed the effects of perceivable background color differences between real host and model parasite eggs, across all four avian photoreceptors, on rates of rejection of model eggs spanning in color across the entire avian spectral sensitivity range, and including immaculate model eggs matching the natural colors of robin and cowbird eggs. The results suggest that egg rejection in robins is driven by the overall perceivable difference in color between own and artificial eggs, and input from all four single-cone avian photoreceptors affects the rejection decision. The results, however, also reveal that when viewed by the avian eye, natural cowbird eggs appear more similar in background color to robin eggs than predicted by the high rejection rate of these parasitic eggs. This suggests that robins respond specifically to parasitism by cowbirds, despite an apparent lack of sensory tuning toward the detection of the background color of cowbird eggs.     

Post-ejection nest-desertion of common cuckoo hosts: a second defense mechanism or avoiding reduced reproductive success?

Hosts of the common cuckoo (Cuculus canorus), an avian brood parasite, develop antiparasite defense mechanisms to increase their reproductive success. Ejection of the parasite egg and desertion of the parasitized nest are the most typical adaptations in response to brood parasitism, but nest desertion may also occur in response to partial clutch reduction, independently from parasitism. Some great reed warblers (Acrocephalus arundinaceus) showed both mechanisms in the same incidence of cuckoo parasitism: in 18% of successful ejections of the parasite eggs, they deserted their nests. We studied if such cases of post-ejection nest-desertion are caused by brood parasitism or reduced clutch value. We experimentally parasitized clutches consisting of five or three host eggs with two painted conspecific eggs to mimic parasitic eggs, as multiple parasitism is frequent in the area. Although hosts ejected these parasitic eggs in both clutch categories (100% and 67% for the larger and smaller inital clutch sizes, respectively), we found that after manipulation, post-ejection nest-desertion frequently occurred at small (3-egg) clutches (40%), but rarely at large (5-egg) clutches (17%). The same phenomenon also occurred when unparasitized 3-egg clutches were reduced by two eggs, but not when 5-egg clutches were reduced in the same way. A logistic regression model revealed that only initial clutch size affected nest desertion of parasitized nests in our experiments. Therefore, we conclude that post-ejection nest-desertion is not a second antiparasite mechanism, which might serve as a redundant antiparasite defense, but a reaction to typically small and further decreased clutch size.     

House sparrows selectively eject parasitic conspecific eggs and incur very low rejection costs

Most host species of obligate interspecific brood parasites are under strong selection because such parasitism, e.g., that involving evictor nestmates, is highly costly. Egg rejection is one of the most efficient host defences against avian brood parasites. Many hosts have thus evolved egg-recognition ability and rejection behaviour. However, this defensive mechanism has not evolved in most species where only intraspecific brood parasitism occurs, probably because (1) the eggs of conspecific females are very similar in appearance, making egg rejection less likely to emerge, and (2) such parasitism is frequently less costly than interspecific parasitism. Using a captive population of house sparrows (Passer domesticus) with a low breeding density, we here provide new evidence showing that this species actually has a fine capacity to discriminate conspecific eggs and to eject them (44.2% of foreign eggs ejected) while incurring very low rejection costs (4.2% of own eggs ejected). This result contradicts those previously found in high-density house sparrow populations in which very high rejection costs and very high clutch desertion rates were reported, probably as a consequence of intraspecific competition and infanticide provoked by the high breeding density. The house sparrow has only rarely been reported as the host of an interspecific brood parasite, which implies that it is a newly described example of an altricial species in which egg ejection has evolved and is maintained in response to intraspecific brood parasitism.     

Electrophoretic variants of egg white transferrin indicate a low rate of intraspecific brood parasitism in colonial cliff swallows in the Sierra Nevada,California

Summary The frequency of intraspecific brood parasitism in two colonies of cliff swallows (Hirundo pyrrhonota) nesting in the eastern Sierra Nevada in California was assessed through an electrophoretic analysis of egg white (albumin) proteins. Albumin proteins are maternally derived and are presumed to directly reflect maternal genotype. Thus, a comparison of protein banding patterns produced by eggs collected from a single clutch allows any egg laid by a female other than the putative mother to be detected. Eggs were collected from 13 cliff swallow nests in 1984 and 41 nests in 1987, a total of 54 nests. Of the clutches collected in 1984, one showed evidence of intraspecific brood parasitism, giving a nest parasitism rate of 7.6%. In 1987, one of 41 nests contained a parasitic egg, for a parasitism rate of 2.4%. Over both years combined the rate of intraspecific brood parasitism was 2 of 54 nests, or 3.7%. These parasitism rates are much lower than those estimated for Nebraskan cliff swallows (22–43%) and 2nd-year purple martins (36%). Possible explanations for the discrepancy between parasitism rates in Sierran cliff swallows and other groups of swallows are discussed.Correspondence to: A.P. Smyth     

Should the redstart Phoenicurus phoenicurus accept or reject cuckoo Cuculus canorus eggs?

Hole-nesting habits of redstarts Phoenicurus phoenicurus make laying difficult for parasitic cuckoo Cuculus canorus females and eviction of host eggs difficult for the cuckoo hatchling, causing fitness costs of cuckoo parasitism to be lower than those reported for open nesting hosts. Redstarts have recognition problems when confronted with real cuckoo eggs showing a perfect mimicry with their own eggs since they never eject when parasitized with perfect mimetic cuckoo eggs but instead desert the nest. Here we use a cost-benefit model to assess the effects of parasitism costs and the probability of being parasitized to estimate the reproductive success of redstarts when accepting or rejecting in the presence or absence of parasitism. Baseline data for model calculations come from this and a previous study on a cuckoo parasitized redstart population in Finland. When desertion implies a loss of 50%, we found that below a threshold value of 20% parasitism redstarts should accept cuckoo eggs since the costs of rejection exceed the benefits, whereas above this threshold they should reject. Interestingly, as the cost of desertion increases the threshold value, it should pay the redstart to reject increasingly at an exponential rate. Our field observations on natural parasitism and experiments with artificial cuckoo eggs confirmed the predictions from the model when hatching failures of the cuckoo were taken into account. Therefore, the low cost imposed by cuckoo parasitism in the system, and the presumably high cost of desertion as a response to parasitism favours acceptance over rejection for a wide range of parasitism pressures. This finding could explain the low rejection rate of real cuckoo eggs found in the redstart despite the presumably long history of a coevolutionary relationship with the cuckoo in Finland.     

Brood parasitism disproportionately increases nest provisioning and helper recruitment in a cooperatively breeding bird

Obligate avian brood parasites lay their eggs in nests of other species (hosts), which raise parasitic young. Parasitic nestlings are likely to influence host’s parental behaviours as they typically beg for food more vigorously than young host for a given hunger level. However, few studies have tested this idea, with conflicting results. These prior studies were largely limited to biparental hosts, but little is known about the effect of brood parasitism on parental behaviours in hosts that breed cooperatively. We followed a multimodel approach to examine the effect of brood parasitism on nest provisioning and helper recruitment in the baywing (Agelaioides badius), a cooperative breeder parasitised by screaming (Molothrus rufoaxillaris) and shiny (Molothrus bonariensis) cowbirds. Multimodel inference results indicated that feeding visits increased with nestling age, cooperative group size and number of cowbird nestlings in the brood. Brood size had little influence on feeding visits, which further suggests that baywings adjusted their provisioning effort in response to cowbird parasitism. In addition, nests parasitised artificially with shiny cowbird eggs or hatchlings recruited more helpers than unmanipulated nests having only host or screaming cowbird young. Our results provide novel evidence that brood parasitism and cooperative breeding interact in determining the levels of nest provisioning.     

Helpers and egg investment in the cooperatively breeding acorn woodpecker: testing the concealed helper effects hypothesis

In cooperatively breeding acorn woodpeckers (Melanerpes formicivorus), helper males have a large positive effect on fledging success in good acorn crop years but only a small positive effect in poor acorn crop years, while helper females exhibit the opposite pattern. Based on these findings, we tested the “concealed helper effects” hypothesis, which proposes that laying females reduce investment in eggs (with respect to their size, number, or quality) in a way that confounds helper effects and results in an absence of a relationship between helpers and breeding success. Results generally failed to support the hypothesis. Mean egg size was positively related to temperatures during the 10 days prior to egg-laying and negatively related to the food supply as indexed by the prior fall’s acorn crop, but there were no significant differences vis-à-vis helpers except for interactions with the acorn crop that only partly corresponded to those predicted. With respect to clutch size, females laid larger clutches when assisted by female helpers, opposite the pattern predicted. Although our results suggest that egg size is adjusted to particular ecological circumstances, we conclude that neither egg nor clutch size is adjusted in a way that confounds the apparent effects of helpers, as proposed by the concealed helper effects hypothesis.     

Egg removal and intraspecific brood parasitism in the European starling (Sturnus vulgaris)

Summary From 1983 to 1986 we monitored 284 European starling (Sturnus vulgaris) nests in New Jersey for evidence of intraspecific brood parasitism and egg removal during the laying period. Egg removal occurred significantly more often at nests where intraspecific brood parasitism was detected (12 of 35 nests, 34%) than at unparasitized nests (23 of 249 nests, 9%). Brood parasitism (92% of parasitized nests) and egg removal (74% of nests with egg removal) were most common at nests where egg laying began in April of each year (i.e., early nests). Egg removal occurred at 26 (19%) and brood parasitism at 32 (23%) of 138 early nests. Both brood parasitism and egg removal were concentrated during the first four days in the laying period when brood parasitism is most likely to be successful and when host nests are most vulnerable to parasitism (Romagnano 1987). Both parasitism and removal usually involved a single egg at each nest. We detected brood parasitism and egg removal on the same day at five of 12 nests (42%) where both were observed. Because starlings do not remove foreign eggs from their nests once they begin laying (Stouffer et al. 1987) we hypothesize that parasite females sometimes removed host eggs while parasitizing nests.     

Why do Horsfield’s bronze-cuckoo <Emphasis Type="Italic">Chalcites basalis</Emphasis> eggs mimic those of their hosts?

The Horsfield’s bronze-cuckoo (Chalcites basalis) egg closely matches the appearance of its host fairy-wren (Malurus spp.) eggs. Mimicry of host eggs by cuckoos is usually attributed to coevolution between cuckoos and hosts, with host discrimination against odd-looking eggs selecting for ever better mimicry by cuckoos. However, this process cannot explain Horsfield’s bronze-cuckoo egg mimicry because fairy-wren hosts rarely reject odd-looking eggs from their nest. An alternative hypothesis is that cuckoos have evolved egg mimicry to disguise their eggs from other cuckoos. Female cuckoos remove one egg from the nest during parasitism and would potentially benefit by selectively removing any cuckoo egg that has already been laid in the nest because otherwise, their egg will be evicted by the first nestling cuckoo along with the host clutch. We used painted, non-mimetic eggs to test whether cuckoos selectively remove odd-looking eggs during parasitism. We found that they were no more likely to remove a non-mimetic egg from a superb fairy-wren Malurus cyaneus clutch than would be expected by chance. Thus, our study does not support the cuckoo egg replacement hypothesis to explain mimicry of host eggs by cuckoos.     

Avoiding parasitism by breeding indoors: cuckoo parasitism of hirundines and rejection of eggs

Brood parasitism is costly to hosts, and, therefore, a number of anti-parasite defenses have evolved. Surprisingly, several high-quality hosts such as martins and swallows are rarely parasitized, raising the question why that is the case. We hypothesize that martins and swallows may avoid parasitism by breeding in close association with humans, and by building nests that are inaccessible for common cuckoos Cuculus canorus and other brood parasites. Here we show using egg rejection experiments that red-rumped swallows Hirundo daurica, house martins Delichon urbica, and barn swallows Hirundo rustica in Europe do not reject foreign eggs placed in their nests, while barn swallows in China often reject foreign eggs. The frequency of parasitism of barn swallows in Europe was significantly higher than in house martins relative to the expectation based on the abundance of the two species. Barn swallows in Europe that were parasitized by cuckoos more often placed their nests outdoors than expected by chance, suggesting that avoidance of cuckoo parasitism can be achieved by breeding indoors. These findings suggest that barn swallows in China have gained egg rejection behavior because they cannot avoid parasitism when breeding outdoors.     

Intraspecific brood parasitism as a conditional reproductive tactic in the treehopper<Emphasis Type="Italic"> Publilia concava</Emphasis>

In species exhibiting egg guarding as well as communal egg laying, females may adopt the strategy of laying eggs in the nests of conspecifics and leaving without providing care (termed intraspecific brood parasitism). This study is the first to describe such a behavior in the insect Publilia concava (Hemiptera: Membracidae) through field studies that followed 849 marked females across 1,828 brood associations. While brood parasitism increased the total number of eggs in a host brood, it did not reduce the overall hatching success of host broods. Solitary females exhibited a range of guarding durations while parasitic females rarely remained to guard eggs. Females exhibiting the parasitic tactic increased their lifetime number of clutches without decreasing the number of solitary clutches that they were able to initiate. Estimates of egg number for these individual broods revealed that females adopting the parasitic tactic (in addition to solitary breeding) had higher lifetime fecundity relative to females that did not parasitize. In females that exhibited both tactics (solitary and parasitic), the parasitic tactic yielded a higher rate of oviposition. The major component of oviposition rate was the time to find hosts and this time decreased with increasing host availability across 222 replicate groups. Females exhibited a shift toward the parasitic tactic when host broods were more abundant (i.e. in larger groups and later in the season). However, the time to find hosts increased as the frequency of the parasitic tactic increased, suggesting that this tactic may be maintained through negative frequency dependence. The results of this study suggest that brood parasitism may be the preferred tactic, as part of a conditional strategy, when hosts are readily available with solitary breeding being the preferred tactic when hosts are in short supply.     

Factors influencing egg size in the gammarid amphipod Gammarus insensibilis

Developmental and seasonal changes in egg volume were examined in a population of the amphipod Gammarus insensibilis Stock occurring on the south coast of England, towards its northern limit of distribution. Results showed a marked increase in egg volume during development (2.9 times by Egg Stage V), resulting from water uptake and from the conversion of yolk reserves into structural elements. The maximum rate of increase coincides with the period of organ and limb development. At hatching, after initial rupture of the egg membrane by urosome spines, egg volume increases rapidly over a short period (15 to 20 min) by a further 30% (uptake rate 3.6×10^–5 mm³s^–1), followed by a post-hatching decrease in juvenile volume. Increase in size at hatching is the result of drinking by embryos, although changes in body-wall permeability may contribute. Females carrying eggs in an advanced stage of development exhibit egg-collecting behaviour. This is seen as an adaptation to an increased likelihood of egg loss with increase in volume of the brood as hatching approaches. Seasonal changes in Stage I (early) egg size are marked in this species, with winter eggs as much as 60% greater in volume than summer eggs. Egg size is inversely related to the temperature during oocyte development. A simple model has been derived to account for the observed seasonal pattern in egg size. The consequences of seasonal variation in egg and juvenile size are considered.     

Sexual selection based on egg colour: physiological models and egg discrimination experiments in a cavity-nesting bird

It has been proposed that the blue-green bird egg colourations of many avian species may constitute a sexually selected female signal that males can use to modulate their parental investment. A fundamental untested assumption for the validation of this hypothesis is that males can accurately assess differences in the colour of eggs. A recent review suggests that this could be particularly problematic when egg clutches were located within a dimly lit nest cavity, due to limitations of the visual system in low light conditions. Here, we first used a photoreceptor noise-limited model of colour discrimination ability that accounts for visual performance under low light conditions to study whether a typical cavity-nesting passerine, the spotless starling Sturnus unicolor, can discriminate their eggs under the ambient illumination in their nest-holes. Secondly, we tested the validity of model predictions with behavioural data collected in two egg discrimination experiments performed in this species. Estimated egg detectability depended entirely on model assumptions about visual limitations linked to light intensity. Starlings would not be able to discriminate egg differences in their nests if the model was based on the assumption that light intensity limited detectability, whereas they could potentially perceive as different many possible pairwise clutch comparisons if the model assumption was that light intensity did not limit detectability. Results of behavioural experiments fitted the prediction of the visual model where light intensity did not limit detectability. Our results suggest that photoreceptor noise-limited colour models based on stimulation of single photoreceptors cannot, at present, be used to predict egg discrimination ability in spotless starlings under low light conditions. Future studies aiming to test egg discrimination constraints in the frame of the sexual selection hypothesis should therefore combine both modelling and behavioural experiments to determine which are the components of the models that produce the mismatch with the behavioural conditions.