Plumage redness predicts breeding onset and reproductive success in the House Finch: a validation of Darwin's theory

Darwin (1871) and later Fisher (1958) suggested that sexual selection can drive the evolution of ornamental traits in monogamous species when female preferences for these traits allow well-ornamented males to begin breeding earlier in a season and, as a result, gain reproductive advantages over poorly ornamented males. However, few studies have been conducted to test this fundamental concept upon which much of the sexual selection theory for monogamous species has been based. In this study, we examined the relationship between breeding onset, reproductive success, and male ornamentation in the House Finch Carpodacus mexicanus , a species in which males display bright carotenoid-based plumage pigmentation. In previous work, it has been shown that bright male House Finches are preferred as social mates by females and, as a result, begin nesting earlier in the season than do drab orange and yellow males. Here we show that, by initiating breeding earlier in the season, brightly colored males fledge more offspring in a season than do drab males. Thus, differential timing of breeding generates considerable variance in reproductive success among male House Finches and contributes to sexual selection for male plumage ornamentation in this species.     

Changes in concentrations of circulating heat‐shock proteins in House Finches in response to different environmental stressors

Heat‐shock proteins (HSP) are molecular chaperones that play key roles in the maintenance of cellular homeostasis under variable environmental conditions. Although HSP are frequently used in studies of wild vertebrates as indicators of stress, no one to date has assessed responses of HSP60, HSP70, and HSP90 in the same species to different environmental stressors. We studied changes in the circulating concentrations of HSP60, HSP70, and HSP90 in wild‐caught House Finches (Haemorhous mexicanus) in response to multiple and sequential stressful environments, including high temperatures, transportation, and pathogen exposure. House Finches sampled during a period of low‐environmental stress with moderate ambient temperatures had low levels of HSP60 and modest levels of HSP70 and HSP90 compared to birds sampled during a presumably more stressful period with high temperatures. After exposure to high‐ambient temperatures, transportation in a vehicle, and exposure to Mycoplasma gallisepticum, captive finches were found to have increasingly higher levels of HSP60. HSP70 tended to rise in response to each stressor, but to drop in the weeks between stress challenges. HSP90 levels increased significantly only in response to pathogen challenge. Our observations suggest that HSP60 and HSP70 are indices of a range of stressors in House Finches, whereas HSP90 primarily reflects health state.     

Egg temperature and initial brood patch area determine hatching asynchrony in Magellanic penguin Spheniscus magellanicus

In birds, the adaptive significance of hatching asynchrony has been under debate for many years and the parental effects on hatching asynchrony have been largely assumed but not often tested. Some authors suggest that hatching asynchrony depends on the incubation onset and many factors have been shown to influence hatching asynchrony in different species. Our objective was to analyze the exact timing of the onset of incubation and if this affects hatching asynchrony; and, in addition, which other factors (brood patch development, incubation position, adult body condition, intra‐clutch egg dimorphism, laying date and year) affect hatching asynchrony in Magellanic penguins Spheniscus magellanicus. We first estimated the eggshell temperature at which embryo development starts, with a non‐destructive and novel method. We then recorded individual egg temperatures in 61 nests during incubation, and related them, and other breeding parameters, to hatching asynchrony. We also observed incubation positions in 307 nests. We found a significant positive relationship between hatching asynchrony and the temperature that the first‐laid egg experienced during egg laying and between hatching asynchrony and the initial brood patch area. We also found a negative relationship between hatching asynchrony and the difference in temperature between second and first‐laid eggs within a clutch, measured after the egg‐laying period was finished. We ruled out position of the eggs during incubation, adult body condition, egg volume, laying date, and study year as factors influencing hatching asynchrony. The egg temperature during laying and the difference in temperature between eggs of a clutch are determinants of hatching asynchrony in Magellanic penguins.     

Unpredictable perturbation reduces breeding propensity regardless of pre‐laying reproductive readiness in a partial capital breeder

Theoretically, individuals of migratory species should optimize reproductive investment based on a combination of timing of and body condition at arrival on the breeding grounds. A minimum threshold body mass is required to initiate reproduction, and the timing of reaching this threshold is critical because of the trade‐off between delaying breeding to gain in condition against the declining value of offspring with later reproductive timing. Long‐lived species have the flexibility within their life history to skip reproduction in a given year if they are unable to achieve this theoretical mass threshold. Although the decision to breed or not is an important parameter influencing population dynamics, the mechanisms underlying this decision are poorly understood. Here, we mimicked an unpredictable environmental perturbation that induced a reduction in body mass of Arctic pre‐breeding (before the laying period) female common eiders Somateria mollissima; a long‐lived migratory seaduck, while controlling for individual variation in the pre‐laying physiological reproductive readiness via vitellogenin (VTG) – a yolk‐targeted lipoprotein. Our aim was to causally determine the interaction between body condition and pre‐laying reproductive readiness (VTG) on breeding propensity by experimentally reducing body mass in treatment females. We first demonstrated that arrival body condition was a key driver of breeding propensity. Secondly, we found that treatment and VTG levels interacted to influence breeding propensity, indicating that our experimental manipulation, mimicking an unpredictable food shortage, reduced breeding propensity, regardless of the degree of pre‐laying physiological reproductive readiness (i.e. timing of ovarian follicles recruitment). Our experiment demonstrates that momentary environmental perturbations during the pre‐breeding period can strongly affect the decision to breed, a key parameter driving population dynamics.     

Sensory control of clutch size in the Common Swift Apus apus

Clutch size varies among individuals in most bird species. A widespread assumption is that such variation results from variable timing in the disruption of ovarian follicular growth that brings, with a few days' lag, egg‐laying to an end. Currently, there is empirical evidence that this is the case in Blue Tits Cyanistes caeruleus but not in Zebra Finches Taeniopygia guttata, in which the timing of follicular disruption has been shown to be invariant. Here, I investigate clutch size regulation of Common Swifts Apus apus. Using experimental egg removal, I show that, assuming the female gets enough egg contact, the determination of clutch size occurs at noon solar time on the day the first egg of a clutch is laid. In spite of individual variation in clutch size, there was no hint of any variability in the timing of clutch‐size determination. In this species therefore, the timing of the signal disrupting follicular growth appears invariant. The physiological mechanism that controls clutch size is discussed, including the existence of an endogenous circadian clock and potential zeitgeber, the developmental range of clutch size for the species, the sensory nature of the input that triggers follicular disruption, and the stress of laying extra eggs.     

Long‐term phenological shifts and intra‐specific differences in migratory change in the willow warbler Phylloscopus trochilus

Climate change can influence many aspects of avian phenology and especially migratory shifts and changes in breeding onset receive much research interest in this context. However, changes in these different life‐cycle events in birds are often investigated separately and by means of ringing records of mixed populations. In this long‐term study on the willow warbler Phylloscopus trochilus, we investigated timing of spring and autumn migration in conjunction with timing of breeding. We made distinction among individuals with regard to age, sex, juvenile origin and migratory phase. The data set comprised 22‐yr of ringing records and two temporally separated data sets of egg‐laying dates and arrival of the breeding population close to the ringing site. The results reveal an overall advancement consistent in most, but not all, phenological events. During spring migration, early and median passage of males and females became earlier by between 4.4 to 6.3 d and median egg‐laying dates became earlier by 5 d. Male arrival advanced more, which may lead to an increase in the degree of protandry in the future. Among breeding individuals, only female arrival advanced in timing. In autumn, adults and locally hatched juvenile females did not advanced median passage, but locally hatched juvenile males appeared 4.2 d earlier. Migrating juvenile males and females advanced passage both in early and median migratory phase by between 8.4 to 10.1 d. The dissimilarities in the response between birds of different age, sex and migratory phase emphasize that environmental change may elicit intra‐specific selection pressures. The overall consistency of the phenological change in spring, autumn and egg‐laying, coupled with the unchanged number of days between median spring and autumn migration in adults, indicate that the breeding area residence has advanced seasonally but remained temporally constant.     

A pre‐breeding immune challenge delays reproduction in the female dark‐eyed junco Junco hyemalis

Precise timing of life‐history transitions in predictably changing environments is hypothesized to aid in individual survival and reproductive success, by appropriately matching an animal's physiology and behavior with prevailing environmental conditions. Therefore, it is imperative for individuals to time energetically costly life‐history stages (i.e. reproduction) so they overlap with seasonal peaks in food abundance and quality. Female lifetime reproductive fitness is affected by several factors that influence energy balance, including arrival date, timing of egg production, and energetic condition. Therefore, any extra energetic costs during reproduction may negatively affect timing of egg production, and ultimately a female's fitness. For example, mounting an immunological response elicits a high energetic cost, and this transfer of resources towards cell and immune system maintenance could have direct negative effects on reproductive timing. In order to determine whether an immune challenge delays onset of breeding (i.e. egg production), we administered either a humoral immune challenge (keyhole limpet hemocyanin (KLH)) (treatment) or physiological saline (control) to free‐living female dark‐eyed juncos Junco hyemalis in the period immediately prior to egg‐laying (～4 weeks). We found that KLH‐injected females artificially delayed clutch initiation when compared to control females. These data help to refine our understanding of how free‐living birds allocate resources between reproduction and self‐maintenance processes during the critical pre‐laying period of the annual cycle.     

Novel insights into relationships between egg corticosterone and timing of breeding revealed by LC‐MS/MS

Inter‐ and intra‐clutch variation in egg corticosterone (CORT), the major glucocorticoid in birds, may provide insights into how maternal stress levels vary with the timing of breeding and with laying order. Common analytical methods (e.g. immunoassays), however, suffer from cross‐reaction with other steroids, leading to potential overestimation of CORT concentrations which can obscure true hormone–environment relationships and complicate among‐study comparisons. We here apply a new LC‐MS/MS technique, which has recently been shown to avoid the problem of cross‐reactivity due to its high specificity, to quantify CORT concentrations in yolk and albumen in clutches of lesser black‐backed gulls Larus fuscus. We found that CORT concentration exhibited a previously unreported U‐shaped relationship with time of breeding, which we explain as a potential interplay of two forces exerting extra strain on the early and late breeders. Furthermore, results showed an increase in CORT with laying order indicating the energetic expense of egg production. The levels of CORT assessed in this study were significantly lower than those previously reported in studies using immunoassays for CORT analysis. This supports the fact that incorporating chromatography effectively reduces overestimation of CORT due to cross‐reactivity with other steroid hormones, particularly in egg yolk.     

The visual fields of two ground‐foraging birds,House Finches and House Sparrows,allow for simultaneous foraging and anti‐predator vigilance

In birds, differences in the extent and position of the binocular visual field reflect adaptations to varying foraging strategies, and the extent of the lateral portion of the field may reflect anti‐predator strategies. The goal of this study was to describe and compare the visual fields of two ground‐foraging passerines, House Finch Carpodacus mexicanus and House Sparrow Passer domesticus. We found that both species have a binocular field type that is associated with the accurate control of bill position when pecking. Both species have eye movements of relatively large amplitude, which can produce substantial variations in the configuration of the binocular fields. We propose that in these ground foragers, their relatively wide binocular fields could function to increase foraging efficiency by locating multiple rather than single food items prior to pecking events. The lateral fields of both species are wide enough to facilitate the detection of predators or conspecifics while head‐down foraging. This suggests that foraging and scanning are not mutually exclusive activities in these species, as previously assumed. Furthermore, we found some slight, but significant, differences between species: House Sparrow binocular fields are both wider and vertically taller, and the blind area is wider than in House Finches. These differences may be related to variations in the degree of eye movements and position of the orbits in the skull.     

Best environmental predictors of breeding phenology differ with elevation in a common woodland bird species

Temperatures in mountain areas are increasing at a higher rate than the Northern Hemisphere land average, but how fauna may respond, in particular in terms of phenology, remains poorly understood. The aim of this study was to assess how elevation could modify the relationships between climate variability (air temperature and snow melt‐out date), the timing of plant phenology and egg‐laying date of the coal tit (Periparus ater). We collected 9 years (2011–2019) of data on egg‐laying date, spring air temperature, snow melt‐out date, and larch budburst date at two elevations (~1,300 m and ~1,900 m asl) on a slope located in the Mont‐Blanc Massif in the French Alps. We found that at low elevation, larch budburst date had a direct influence on egg‐laying date, while at high‐altitude snow melt‐out date was the limiting factor. At both elevations, air temperature had a similar effect on egg‐laying date, but was a poorer predictor than larch budburst or snowmelt date. Our results shed light on proximate drivers of breeding phenology responses to interannual climate variability in mountain areas and suggest that factors directly influencing species phenology vary at different elevations. Predicting the future responses of species in a climate change context will require testing the transferability of models and accounting for nonstationary relationships between environmental predictors and the timing of phenological events.     

Serotonin modulates locomotory behavior and coordinates egg‐laying and movement in Caenorhabditis elegans

Biogenic amines have been implicated in the modulation of neural circuits involved in diverse behaviors in a wide variety of organisms. In the nematode C. elegans, serotonin has been shown to modulate the temporal pattern of egg‐laying behavior. Here we show that serotonergic neurotransmission is also required for modulation of the timing of behavioral events associated with locomotion and for coordinating locomotive behavior with egg‐laying. Using an automated tracking system to record locomotory behavior over long time periods, we determined that both the direction and velocity of movement fluctuate in a stochastic pattern in wild‐type worms. During periods of active egg‐laying, the patterns of reversals and velocity were altered: velocity increased transiently before egg‐laying events, while reversals increased in frequency following egg‐laying events. The temporal coordination between egg‐laying and locomotion was dependent on the serotonergic HSN egg‐laying motorneurons as well as the decision‐making AVF interneurons, which receive synaptic input from the HSNs. Serotonin‐deficient mutants also failed to coordinate egg‐laying and locomotion and exhibited an abnormally low overall reversal frequency. Thus, serotonin appears to function specifically to facilitate increased locomotion during periods of active egg‐laying, and to function generally to modulate decision‐making neurons that promote forward movement. © 2001 John Wiley & Sons, Inc. J Neurobiol 49: 303–313, 2001     

Delayed egg‐laying and shortened incubation duration of Arctic‐breeding shorebirds coincide with climate cooling

Biological impacts of climate change are exemplified by shifts in phenology. As the timing of breeding advances, the within‐season relationships between timing of breeding and reproductive traits may change and cause long‐term changes in the population mean value of reproductive traits. We investigated long‐term changes in the timing of breeding and within‐season patterns of clutch size, egg volume, incubation duration, and daily nest survival of three shorebird species between two decades. Based on previously known within‐season patterns and assuming a warming trend, we hypothesized that the timing of clutch initiation would advance between decades and would be coupled with increases in mean clutch size, egg volume, and daily nest survival rate. We monitored 1,378 nests of western sandpipers, semipalmated sandpipers, and red‐necked phalaropes at a subarctic site during 1993–1996 and 2010–2014. Sandpipers have biparental incubation, whereas phalaropes have uniparental incubation. We found an unexpected long‐term cooling trend during the early part of the breeding season. Three species delayed clutch initiation by 5 days in the 2010s relative to the 1990s. Clutch size and daily nest survival showed strong within‐season declines in sandpipers, but not in phalaropes. Egg volume showed strong within‐season declines in one species of sandpiper, but increased in phalaropes. Despite the within‐season patterns in traits and shifts in phenology, clutch size, egg volume, and daily nest survival were similar between decades. In contrast, incubation duration did not show within‐season variation, but decreased by 2 days in sandpipers and increased by 2 days in phalaropes. Shorebirds demonstrated variable breeding phenology and incubation duration in relation to climate cooling, but little change in nonphenological components of traits. Our results indicate that the breeding phenology of shorebirds is closely associated with the temperature conditions on breeding ground, the effects of which can vary among reproductive traits and among sympatric species.     

Variation in laying date in relation to spring temperature in three species of tits (Paridae) and pied flycatchers Ficedula hypoleuca in southernmost Sweden

This study documents the advancement of laying dates in three species of tits (Paridae) in southernmost Sweden during recent decades, and the absence of a similar response in the pied flycatcher Ficedula hypoleuca. It is based on several different nestbox studies; the oldest one starting in 1969. During 1969 to 2012, mean spring temperatures in the study area increased by between 0.06 and 0.08°C per year, depending on the period considered. Great tits Parus major, blue tits Cyanistes caeruleus and marsh tits Poecile palustris, which generally start egg laying between the last week of April and the first week of May, all advanced laying date at a similar rate during the study period (0.25 d yr^–1). This indicates that these species were similarly affected by increasing temperatures. When accounting for mean spring temperature variation, we still found an advancement of laying date over the study period, mostly due to such relationships among marsh and blue tits. This result could reflect ongoing microevolution favouring earlier laying, but could also be a result of other factors such as increased intra‐ or inter‐specific competition for early breeding. Pied flycatchers, which generally lay during the third week of May, did not significantly advance the date of egg laying despite that the long‐term trend in the increase in ambient temperature during the 30‐d period preceding the start of egg laying was similar for pied flycatchers compared to the tit species.     

Individual repeatability in laying behaviour does not support the migratory carry‐over effect hypothesis of egg‐size dimorphism in Eudyptes penguins

Penguins of the genus Eudyptes are unique among birds in that their first‐laid A‐egg is 54–85% the mass of their second‐laid B‐egg. Although the degree of intra‐clutch egg‐size dimorphism varies greatly among the seven species of the genus, obligate brood reduction is typical of each, with most fledged chicks resulting from the larger B‐egg. Many authors have speculated upon why Eudyptes penguins have evolved and maintained a highly dimorphic 2‐egg clutch, and why it is the first‐laid egg that is so much smaller than the second, but only recently has a testable, proximate mechanism been proposed. In most species of Eudyptes penguins females appear to initiate egg‐formation at sea during return migration to breeding colonies. In macaroni penguins E. chrysolophus, females with a shorter pre‐laying interval ashore (and thus presumably greater overlap between migration and egg‐formation) lay more dimorphic eggs, suggesting a physiological conflict may constrain growth of the earlier‐initiated A‐egg. This migratory carry‐over effect hypothesis (MCEH) was tested in eastern rockhopper penguins E. chrysocome filholi on Campbell Island, New Zealand, by recording the arrival and lay dates, body sizes, and egg masses of transponder‐tagged females over two years. Females with longer pre‐laying intervals laid less dimorphic clutches, as predicted by the MCEH. However, repeated measures of individual females revealed that within‐individual variation in egg‐size dimorphism between years was unrelated to within‐individual variation in pre‐laying interval. Egg masses, and to a lesser extent egg‐size dimorphism, were highly repeatable traits related to body size and body mass. These results and a detailed consideration of the MCEH suggest that egg‐size dimorphism in Eudyptes penguins is unlikely to be caused by a migratory carry‐over effect.     

Colony attendance and at‐sea distribution of thin‐billed prions during the early breeding season

Procellariiform seabirds have extreme life histories; they are very long‐lived, first breed when relatively old, lay single egg clutches, both incubation and chick‐rearing are prolonged and chicks exhibit slow growth. The early part of the breeding season is crucial, when pair bonds are re‐established and partners coordinate their breeding duties, but is a difficult period to study in burrow‐nesting species. Miniature geolocators (Global Location Sensors or GLS loggers) now offer a way to collect data on burrow attendance, as well as determine at‐sea movements. We studied the early breeding season in thin‐billed prions Pachyptila belcheri breeding at New Island, Falkland Islands. Males and females arrived back at the colony at similar times, with peak arrival in the last days of September. However, males spent more time on land during the pre‐laying period, presumably defending and maintaining the burrow and maximising mating opportunities. Males departed later than females, and carried out a significantly shorter pre‐laying exodus. Males took on the first long incubation shift, whereas females returned to sea soon after egg laying. During the pre‐laying exodus and incubation, males and females travelled at similar speeds (> 250 km d^?1) and were widely distributed over large areas of the Patagonian Shelf. Inter‐annual differences in oceanographic conditions were stronger during the incubation than during the pre‐laying exodus and were matched by stronger differences in distribution. The study thus suggests that extended trips and flexible distribution enable thin‐billed prions to meet the high energy demands of egg production and incubation despite low productivity in waters around the colony during the early summer.     

Protoporphyrin‐based eggshell pigmentation predicts hatching success and offspring sex ratio in the barn swallow

Inter‐ and intraspecific variation in eggshell colouration has long fascinated evolutionary biologists. Among species, such variation may accomplish different functions, the most obvious of which is camouflage and background matching. Within species, it has been proposed that inter‐female variation in eggshell pigmentation patterns can reflect egg, maternal or paternal traits and hence may provide cues to conspecifics about egg, maternal or paternal phenotypic quality. However, the relationship between protoporphyrin‐based eggshell pigmentation and egg or maternal/paternal traits appears to be highly variable among species. We investigated patterns of intraspecific variation in Eurasian barn swallow Hirundo r. rustica protoporphyrin‐based eggshell pigmentation, and analysed its association with egg and clutch characteristics, maternal/paternal phenotypic traits and parental feeding effort. Eggshell pigmentation pattern significantly varied between breeding colonies, was significantly repeatable in first clutches laid by the same females in different years (intraclass correlation coefficient ranging between 0.56 and 0.63), but it was not significantly associated with egg traits, such as position in the laying sequence, egg mass, yolk testosterone concentration and antioxidant capacity. It was weakly or non‐significantly associated with female and male traits (sexual ornaments), but females laying darker (higher pigment intensity) first clutches had higher hatching success, suggesting that eggshell pigment intensity may predict fitness. Male nestling feeding effort was not predicted by eggshell pigmentation. In addition, females with darker breast plumage colouration (a melanin‐based trait related to fitness) laid highly protoporphyrin‐covered eggs, suggesting the presence of a previously unappreciated link between protoporphyrin biosynthesis and plumage melanisation. Moreover, the proportion of male offspring increased in clutches originating from highly protoporphyrin‐covered eggs, suggesting that parents could acquire visual cues about their future brood sex composition before egg hatching. Our results support the idea that intraspecific signalling via eggshell pigmentation is a species‐specific rather than a general feature of avian taxa.     

Interspecific competition between introduced house finch populations and two associated passerine species

Summary House Finches (Carpodacus mexicanus), natives of western North America, have expanded their range in the eastern United States since their 1940 release in New York City. Range and the relation of House Finch population growth to the population dynamics of House Sparrows (Passer domesticus) and Purple Finches (Carpodacus purpureus) were examined, using data from the Breeding Bird Survey and the Christmas Bird Count. The House Finch population grew exponentially throughout its eastern range. Significant negative relationships in population density, relative to spatial and temporal control populations, were found between House Finches and House Sparrows in summer and winter, and between House Finches and Purple Finches in summer. Purple Finch and House Sparrow populations outside of the House Finch range appeared to have no effect on each other throughout the study. Neither changes in 74 weather variables, nor changes in forest, field or developed habitat explained the observed trends in population density. The results indicate that House Finches compete with these two species, but winter migration complicates the picture.     

Causes and consequences of pre‐laying weight gain in a food‐caching bird that breeds in late winter

Animals that reside at high latitudes and altitudes year‐round often use cached food to survive over the winter months, but a few species also rely on stored food to sustain them during the breeding season when the nutritional requirements of females are higher than normal. Gray jays Perisoreus canadensis rely on perishable cached food during the winter and females begin breeding in late winter when fresh food is rarely available. To examine pre‐laying patterns of weight gain, as well as the causes and consequences of weight gain among individuals, we weighed females regularly throughout the pre‐laying period. Females began increasing their weight approximately nine days prior to their first egg date, and on average increased their body weight by 25%, which is on par with other bird species that rely on non‐cached food. Final pre‐laying weight was positively influenced by the percent of conifers on territories, providing some support for previous results showing that coniferous trees are better able to preserve cached food. We also found that both final pre‐laying weight and the rate of weight gain were positively related to female age, supporting the hypothesis that female caching ability improves with age. With increasing final weight, females tended to lay larger clutches and hatched more nestlings, despite the fact that final weight was not influenced by weight at the beginning of the weighing period. Our results confirm that gray jays are able to reach breeding condition while relying primarily on food stored before winter, and suggest a novel mechanism by which habitat‐mediated carry‐over effects and female age may influence reproductive performance in a food‐caching animal.     

Within‐ and between‐season repeatability of eggshell colouration in the great reed warbler Acrocephalus arundinaceus

While avian eggshell colouration has attracted biologists for decades, little is known about its variation within individuals. The main goal of this study was to explore within‐ and between‐season repeatability of eggshell appearance in the great reed warbler Acrocephalus arundinaceus. To achieve this, we measured eggshell reflectance of first and replacement clutches of individual females within one breeding season, and the reflectance of their first clutches in two following breeding seasons. As environmental conditions may affect egg colouration, repeatability was estimated from linear mixed‐effects models where we initially included temperature and rainfall during egg laying, as well as year, clutch order and egg‐laying date as fixed effects, and clutch identity nested in females as random effects. Eggshell appearance within clutches showed moderate repeatability, while both within‐ and between‐season repeatability in clutch colouration for individual females was low. Our findings indicate that low intra‐clutch variation may have a function within the context of brood parasitism, i.e. in facilitating host recognition of alien eggs. With variable eggs between successive clutches, however, host females may need to relearn the appearance of their eggs with every clutch they lay. This could represent a significant constraint for host egg‐discrimination abilities. Yet, whether environmental or intrinsic physiological factors are responsible for the variation in eggshell colouration between successive clutches of the same females still remain to be discovered.     

Small‐scale spatial variation in evolvability for life‐history traits in the storm petrel

The evolutionary potential in the timing of recruitment and reproduction may be crucial for the ability of populations to buffer against environmental changes, allowing them to avoid unfavourable breeding conditions. The evolution of a trait in a local population is determined by its heritability and selection. In the present study, we performed pedigree‐based quantitative genetic analyses for two life‐history traits (recruiting age and laying date) using population data of the storm petrel over an 18‐year period in two adjacent breeding colonies (only 150 m apart) that share the same environmental conditions. In both traits, natal colony effect was the main source of the phenotypic variation among individuals, and cohort variance for recruitment age and additive genetic variance for laying date were natal colony‐specific. We found significant heritability only in laying date and, more specifically, only in birds born in one of the colonies. The difference in genetic variance between the colonies was statistically significant. Interestingly, selection on earlier breeding birds was detected only in the colony in which heritable variation in laying date was found. Therefore, local evolvability for a life‐history trait may vary within a unexpectedly small spatial scale, through the diversifying natural selection and insulating gene flow. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106 , 439–446.