Clutch size variation in passerine birds: The nest predation hypothesis

The fecundity costs of building domed nests in birds

Animal nests provide a beneficial environment for offspring development and as such, contribute to fitness. Gathering and transporting materials to construct nests is energetically costly, but the life history trade-offs associated with the types of nests built are largely unknown. Who contributes to building the nest could also mediate these trade-offs, as building a nest as a couple is expected to be less costly per individual than building alone. Using a comparative analysis of 227 songbird species globally, we found a fecundity cost associated with the type of nest a species builds. Species that build domed nests produce fewer broods per year than species building cups or platforms. Dome nesting species also have larger clutch sizes than open nesting species, but only when the nest is built by a couple and not when females build nests alone. This suggests that building domed nests represents a trade-off with investment in young, especially when females are solely responsible for nest building. More broadly, our results could explain macroevolutionary patterns, such as the recent finding that females, building on their own, more often build open cups rather than domed nests.

Global drivers of variation in cup nest size in passerine birds

The size of a bird's nest can play a key role in ensuring reproductive success and is determined by a variety of factors. The primary function of the nest is to protect offspring from the environment and predators. Field studies in a number of passerine species have indicated that higher-latitude populations in colder habitats build larger nests with thicker walls compared to lower-latitude populations, but that these larger nests are more vulnerable to predation. Increases in nest size can also be driven by sexual selection, as nest size can act as a signal of parental quality and prompt differential investment in other aspects of care. It is unknown, however, how these microevolutionary patterns translate to a macroevolutionary scale. Here, we investigate potential drivers of variation in the outer and inner volume of open cup nests using a large dataset of nest measurements from 1117 species of passerines breeding in a diverse range of environments. Our dataset is sourced primarily from the nest specimens at the Natural History Museum (UK), complemented with information from ornithological handbooks and online databases. We use phylogenetic comparative methods to test long-standing hypotheses about potential macroevolutionary correlates of nest size, namely nest location, clutch size and variables relating to parental care, together with environmental and geographical factors such as temperature, rainfall, latitude and insularity. After controlling for phylogeny and parental body size, we demonstrate that the outer volume of the nest is greater in colder climates, in island-dwelling species and in species that nest on cliffs or rocks. By contrast, the inner cup volume is associated solely with average clutch size, increasing with the number of chicks raised in the nest. We do not find evidence that nest size is related to the length of parental care for nestlings. Our study reveals that the average temperature in the breeding range, along with several key life-history traits and proxies of predation threat, shapes the global interspecific variation in passerine cup nest size. We also showcase the utility of museum nest collections-a historically underused resource-for large-scale studies of trait evolution.

Weather and nest cavity characteristics influence fecundity in mountain chickadees

Background

Examining direct and indirect effects on reproduction at multiple scales allows for a broad understanding of species' resilience to environmental change. We examine how the fecundity of the mountain chickadee (Poecile gambeli), a secondary cavity-nesting, insectivorous bird, varied in relation to factors at three scales: regional weather conditions, regional- and site-level food availability, site-level community dynamics, and nest-level cavity characteristics. We hypothesized that earlier laying dates and higher fecundity (clutch size, nest survival, brood size) would be associated with milder climatic conditions, increased food from insect outbreaks, lower densities of conspecifics and nest predators (red squirrel; Tamiasciurus hudsonicus), and safer (smaller, higher) cavities.

Methods

We collected data on laying date, clutch size, brood size, nest fate (success/failure), and cavity characteristics from 513 mountain chickadee nests in tree cavities in temperate mixed coniferous-broadleaf forest in interior British Columbia, Canada, from 2000 to 2011. We surveyed annual abundances of mountain chickadees and squirrels using repeated point counts, and mountain pine beetle (Dendroctonus ponderosae) and lepidopteran defoliators by monitoring host trees and by using regional-scale aerial overview forest insect survey data. We used weather data (temperature, rain, snow) from a local Environment and Climate Change Canada weather station. We modeled laying date, clutch size, daily nest survival, and brood size as a function of predictors at regional-, site-, and nest-scales.

Results and Conclusions

Measures of fecundity varied dramatically across years and spatial scales. At the regional (study-wide) scale, chickadees laid earlier and larger first clutches in warmer springs with minimal storms, and daily nest survival (DSR) increased with a 2-year lag in growing season temperature. Despite a doubling of mountain chickadee density that roughly accompanied the outbreaks of mountain pine beetle and lepidopteran defoliators, we found little evidence at the site scale that fecundity was influenced by insect availability, conspecific density, or predator density. At the nest scale, DSR and brood size increased with clutch size but DSR declined with nest cavity size indicating a positive reproductive effect of small-bodied cavity excavators. Double-brooding, rare in chickadees, occurred frequently in 2005 and 2007, coinciding with early breeding, high food availability from insect outbreaks, and warm spring temperatures with 0-1 spring storms. Our results support the idea that fecundity in secondary cavity-nesting species is impacted directly and indirectly by weather, and indirectly through changes in community dynamics (via cavity resource supply). We stress the importance of adopting holistic, community-level study frameworks to refine our understanding of fecundity in opportunistic and climate-sensitive species in future.

Breeding ecology of Kessler’s thrush (Turdus kessleri) in the western Sichuan plateau

The breeding of most high-altitude birds remains poorly known. We studied the breeding ecology of Kessler’s thrush (Turdus kessleri) and documented reproductive information throughout the nestling periods in the western Sichuan plateau. The data included natural nest sites, nest components, nest size, egg-laying dates, egg morphology, egg size, clutch size, egg incubation, nestling brooding and feeding, nestling morphology and growth, and reproductive outcome. The study found that T. kessleri used the old nest to breed. As the nestling grew, the female’s nestling brooding time decreased, and the feeding frequency of parent birds increased at first and then decreased. The frequency of clearing feces was positively correlated with the feeding frequency. The difference in the feeding frequency of both parents may be attributed to their division of labor, with a distinct difference between their investments. The parent birds’ cost of reproduction per nestling varies from one breeding period to the next. Predation by natural enemies is the main factor leading to reproductive failure in T. kessleri. This suite of life-history and behavioral strategies enables fledgling T. kessleri to cope with the harsh environments of mountains at higher altitudes.

Drivers of nest survival in the Tawny-bellied Seedeater Sporophila hypoxantha (Aves: Thraupidae): time-specific factors are more related to success than ecological variables

Tawny-bellied Seedeater (Sporophila hypoxantha) is an endangered (in Brazil) grassland dependent species, whose breeding success remains poorly known hampering conservation measures on its breeding grounds. Here we analyze the nest survival of the species in relation to temporal and environmental variables over three breeding seasons in hilly dry grasslands in southern Brazil. The apparent nest success was 40%, and MARK survival 20%. Predation was the main cause of failure, affecting 55% of the unsuccessful nests, followed by desertion, infestation by Philornis fly larvae, cattle trampling and burning. The productivity was 1.77 young per pair. Best models include time-specific factors (nest age and time of breeding season), reflected by a gradual reduction in nest survival over the nesting cycle, accompanied by an increase in temperature, subcutaneous larvae infestation, and predation. Nest site characteristics did not influence nest survival. Predation is more prevalent in the nestling rearing period than during incubation. This tendency may be caused by an increase in the activity in the nests, as is predicted by the Skutch hypothesis. The conclusion that time-specific factors influence nest survival more than ecological variables is important to plan on seasonally dependent conservation and management measures.

How much calcium to shell out? Eggshell calcium carbonate content is greater in birds with thinner shells, larger clutches and longer lifespans

The avian eggshell is a bio-ceramic structure that protects the embryo. It is composed almost entirely of calcium carbonate and a small amount of organic material. An optimal amount of calcium carbonate in the eggshell is essential for the embryo's development, yet how the ratio of calcium carbonate to organic matter varies between species has not been investigated. Calcium is a limiting resource for most birds, so its investment in their eggs should be optimized for a bird's life history. We measured the relative calcium carbonate content of eggshells in 222 bird species and tested hypotheses for how this trait has evolved with the life-history strategies of these species and other traits of their respective egg physiologies. We found that (i) eggshell calcium carbonate content was positively correlated with species having thinner eggshells and smaller than expected eggs relative to incubating parental mass, (ii) species with small mean clutch sizes had lower calcium carbonate content in their eggshells, and (iii) for species with larger clutch sizes, eggshell calcium carbonate content was negatively correlated with their mean lifespan. The pattern of lower eggshell calcium carbonate in longer lived, larger clutched birds suggests that calcium provision to the eggshell has long-term costs for the individual.

Multiple nesting attempts and long breeding seasons of Mimus gilvus (Aves: Mimidae) in southeastern Brazil

This study describes aspects of the life history of the Tropical MockingbirdMimusgilvus(Vieillot, 1808), including the breeding period, clutch size, nestlings and a list of plants used for nesting. Nests were monitored in an area of Restinga (sand-coastal plain) habitat in a protected area in southeastern Brazil. The data from 181 nests during five breeding seasons (2010-2014) showed that the Tropical Mockingbird has a long breeding season (26.1 ± 2.6 weeks) with up to two peaks of active nests from August to March. The breeding pairs made up to four nesting attempts in the same breeding season. The mean (± SD) clutch size was 2.4 ± 0.6 eggs (n = 169). The mean (± SD) incubation period was 14 ± 0.6 days, and the nestling remained in the nest for 14.5 ± 2.2 days. The nests were built on thirty plant species, andProtiumicicariba(DC.) Marchand. was the plant species most commonly used for nesting. The breeding parameters of the Tropical Mockingbird are similar to those of other Mimidae species. The knowledge gained from this study makes the Tropical Mockingbird a good choice for future studies, particularly for testing ecological and evolutionary hypotheses regarding life history attributes, habitat selection and parental investment.

European Starling (Sturnus vulgaris) reproduction undeterred by predator scent inside nest boxes

Indirect predator cues near nests have been shown to enhance perceived predation risk and associated antipredator behaviours in breeding animals across taxa and particularly with birds. We hypothesized that scent from the raccoon (Procyon lotor (Linnaeus, 1758)) inside nest boxes would, despite being an evolutionarily unique predator, enhance perceived risk to the European Starling (Sturnus vulgaris (Linnaeus, 1758)), thus reducing use of treated sites and reproduction. During early spring, starlings selected from nest boxes treated with equal volumes of predator scent, a novel odour, or water (n = 40 boxes per treatment). We evaluated effects of treatment on reproductive traits via generalized linear models. Starlings established nest bowls in 61% of nest boxes (predator scent, n = 27 boxes; novel odour, n = 24 boxes; control (water), n = 22 boxes); clutches were laid in 68 boxes. We observed no effects of treatment on the likelihood of a clutch (≥1 egg) or nest failure. Further, we found no treatment effects on date of first egg, clutch size, or hatchling number. We conclude that starling antipredator response to enhanced, indirect risk of nest predation is contingent upon a combination of predator cues, as well as direct or indirect experience with nest predation.

Breeding biology of Tyrannus melancholicus (Aves: Tyrannidae) in a restinga reserve of southeastern Brazil

Existing knowledge of the breeding success and life history characteristics of most Neotropical bird species is scarce. Here, we help fill this gap by analyzing aspects of the breeding biology of the Tropical Kingbird Tyrannusmelancholicus (Vieillot, 1819), which is a good model for this kind of study as it is a common species occurring in various environments, including urban areas, but little is known about its life history. We provide results concerning the breeding period, clutch size, incubation and nestling periods, description of nests, eggs and nestlings, and the plants used for nest sites by this species. Fifty-four nests were monitored over two seasons (2012–2014) in a protected area in southeastern Brazil. Nesting began at the end of the dry season and the beginning of the rainy season. The frequency of active nests varied according to variations in rainfall for each breeding season analyzed. The means and standard deviations of the incubation period (14.2 ± 1.9 days), nestling period (15.1 ± 0.8 days) and clutch size (2.5 ± 0.7 eggs) were similar to values reported for other Neotropical passerines. Twenty-one plant species used as nest trees and for the construction of the nests were identified. The results show that T.melancholicus is not highly selective when choosing plant species used for nest construction.

EVOLUTION OF AVIAN COOPERATIVE BREEDING: COMPARATIVE TESTS OF THE NEST PREDATION HYPOTHESIS

Comparative analyses carried out on two different phylogenies of cooperatively and noncooperatively breeding Australian passerine birds (parvorder Corvida) were unable to detect a significant difference in nest predation rates after controlling for body mass and risk of predation due to location of the nest (nest safety). Nest predation rates, however, decrease as nest safety and body mass increase. We suggest that cooperative breeding does not bring about a current net change in rates of nest predation among Australian passerines. Species breeding cooperatively may have developed antipredator strategies that produce results similar to those adopted by noncooperatively breeding species. The function of cooperative breeding may lie outside of antipredator strategies.

The Effects of Supplementary Food on the Breeding Performance of Eurasian Reed Warblers Acrocephalus scirpaceus; Implications for Climate Change Impacts

Understanding the mechanisms by which climate variation can drive population changes requires information linking climate, local conditions, trophic resources, behaviour and demography. Climate change alters the seasonal pattern of emergence and abundance of invertebrate populations, which may have important consequences for the breeding performance and population change of insectivorous birds. In this study, we examine the role of food availability in driving behavioural changes in an insectivorous migratory songbird; the Eurasian reed warbler Acrocephalus scirpaceus. We use a feeding experiment to examine the effect of increased food supply on different components of breeding behaviour and first-brood productivity, over three breeding seasons (2012-2014). Reed warblers respond to food-supplementation by advancing their laying date by up to 5.6 days. Incubation periods are shorter in supplemented groups during the warmest mean spring temperatures. Nestling growth rates are increased in nests provisioned by supplemented parents. In addition, nest predation is reduced, possibly because supplemented adults spend more time at the nest and faster nestling growth reduces the period of vulnerability of eggs and nestlings to predators (and brood parasites). The net effect of these changes is to advance the fledging completion date and to increase the overall productivity of the first brood for supplemented birds. European populations of reed warblers are currently increasing; our results suggest that advancing spring phenology, leading to increased food availability early in the breeding season, could account for this change by facilitating higher productivity. Furthermore, the earlier brood completion potentially allows multiple breeding attempts. This study identifies the likely trophic and behavioural mechanisms by which climate-driven changes in invertebrate phenology and abundance may lead to changes in breeding phenology, nest survival and net reproductive performance of insectivorous birds.

Fecundity selection theory: concepts and evidence

Fitness results from an optimal balance between survival, mating success and fecundity. The interactions between these three components of fitness vary depending on the selective context, from positive covariation between them, to antagonistic pleiotropic relationships when fitness increases in one reduce the fitness of others. Therefore, elucidating the routes through which selection shapes life history and phenotypic adaptations via these fitness components is of primary significance to understanding ecological and evolutionary dynamics. However, while the fitness components mediated by natural (survival) and sexual (mating success) selection have been debated extensively from most possible perspectives, fecundity selection remains considerably less studied. Here, we review the theoretical basis, evidence and implications of fecundity selection as a driver of sex-specific adaptive evolution. Based on accumulating literature on the life-history, phenotypic and ecological aspects of fecundity, we (i) suggest a re-arrangement of the concepts of fecundity, whereby we coin the term 'transient fecundity' to refer to brood size per reproductive episode, while 'annual' and 'lifetime fecundity' should not be used interchangeably with 'transient fecundity' as they represent different life-history parameters; (ii) provide a generalized re-definition of the concept of fecundity selection as a mechanism that encompasses any traits that influence fecundity in any direction (from high to low) and in either sex; (iii) review the (macro)ecological basis of fecundity selection (e.g. ecological pressures that influence predictable spatial variation in fecundity); (iv) suggest that most ecological theories of fecundity selection should be tested in organisms other than birds; (v) argue that the longstanding fecundity selection hypothesis of female-biased sexual size dimorphism (SSD) has gained inconsistent support, that strong fecundity selection does not necessarily drive female-biased SSD, and that this form of SSD can be driven by other selective pressures; and (vi) discuss cases in which fecundity selection operates on males. This conceptual analysis of the theory of fecundity selection promises to help illuminate one of the central components of fitness and its contribution to adaptive evolution.

Begging form and growth pattern of nestlings correlate with parental food-allocation patterns in the Horned Lark (Eremophilaalpestris)

Life-history theory assumes that selection favors parents that can maximize their reproductive success via behavioral strategies. As brood size determines the reproductive value of each nestling, parents may adjust their food-allocation patterns according to brood size. We test this assumption in the Horned Lark (Eremophila alpestris (L., 1758)). Our findings revealed that nestling begging forms varied with brood size, by gaping in one-chick broods and postural activity in two- and three-chick broods. Accordingly, parental food-allocation patterns differed in different-sized broods. In one-chick broods, parents increased feeding rates with the gaping duration of nestling. In two-chick broods, parents did not change food-allocation patterns according to nestlings’ begging. In three-chick broods, however, they fed later-hatched nestlings more even when early-hatched nestlings begged more intensely. Horned Larks exhibited obvious sexual differences in parenting style and ability, which resulted in nestlings from two- and three-chick broods changing their begging intensity according to the sex of the provisioning adult. Furthermore, nestling growth pattern diverged with brood sizes, with body mass growing faster in one-chick broods than in two- and three-chick broods. Growth rate of beak gape and tarsus length did not differ significantly among brood sizes, but beak gape was larger and tarsus length was shorter in one-chick broods than in larger broods at fledging. Our results thus support the idea that parents may use food allocation to regulate sibling rivalry, which in turn cause nestlings to beg food in different forms and grow in different patterns so that their reproductive success can be enhanced.

Elevational trends in life histories: revising the pace-of-life framework

Life-history traits in birds, such as lifespan, age at maturity, and rate of reproduction, vary across environments and in combinations imposed by trade-offs and limitations of physiological mechanisms. A plethora of studies have described the diversity of traits and hypothesized selection pressures shaping components of the survival-reproduction trade-off. Life-history variation appears to fall along a slow-fast continuum, with slow pace characterized by higher investment in survival over reproduction and fast pace characterized by higher investment in reproduction over survival. The Pace-of-Life Syndrome (POLS) is a framework to describe the slow-fast axis of variation in life-history traits and physiological traits. The POLS corresponds to latitudinal gradients, with tropical birds exhibiting a slow pace of life. We examined four possible ways that the traits of high-elevation birds might correspond to the POLS continuum: (i) rapid pace, (ii) tropical slow pace, (iii) novel elevational pace, or (iv) constrained pace. Recent studies reveal that birds breeding at high elevations in temperate zones exhibit a combination of traits creating a unique elevational pace of life with a central trade-off similar to a slow pace but physiological trade-offs more similar to a fast pace. A paucity of studies prevents consideration of the possibility of a constrained pace of life. We propose extending the POLS framework to include trait variation of elevational clines to help to investigate complexity in global geographic patterns.

Reproductive biology of Synallaxis albescens (Aves: Furnariidae) in the cerrado of central Brazil

Understanding the causes and consequences of variation in reproductive strategies is a central theme in studies of avian life history evolution. This study describes the reproductive biology of Synallaxis albescens (Furnariidae) in the cerrado biome of central Brazil. We monitored 35 nests during the 2003 to 2011 breeding seasons, visiting them every 2-4 days. Synallaxis albescens breeds from mid-September to mid-January, builds a retort-shaped nest, and generally lays three immaculate white eggs. Eggs weighed 1.75 g and measured 19.7 by 14.4 mm. Most nests studied were in open cerrado or shrub grassland at an average height above the ground of 0.3 m, with a preference for Davilla elliptica (Dilleniaceae) shrubs as a nesting substrate. Incubation period averaged 18.1 days, while the nestling period averaged 13.6 days. Of 16 closely monitored nests, four were successful (25%), 11 were depredated (69%), and one was abandoned. Predation was similar during incubation (45%) and nestling (55%) phases. In general, the breeding biology of S. albescens was similar to that described previously for this species and for related Furnariidae.

Impact of nesting mortality on avian breeding phenology: a case study on the red-backed shrike (Lanius collurio)

The seasonal timing of avian reproduction is supposed primarily to coincide with favourable feeding conditions. Long-term changes in avian breeding phenology are thus mostly scrutinized in relation to climatic factors and matching of the food supplies, while the role of nesting mortality is largely unexplored. Here we show that higher seasonal mean daily mortality rate leads to a shift in the distribution of breeding times of the successful nests to later dates in an an open-nesting passerine bird, the red-backed shrike Lanius collurio. The effect appeared to be strong enough to enhance or counteract the influence of climatic factors and breeding density on the inter-annual variation in mean hatching dates. Moreover, the seasonal distribution of reproductive output was shifted to larger, or smaller, broods early in the season when the nesting mortality increased, or decreased, respectively, during the season. We suggest that population level changes in timing of breeding caused by a general advancement of spring and of the food supplies might be altered by the seasonality in nesting mortality. Hence, we argue that consideration of nesting mortality is of major importance for understanding long-term trends in avian phenology, particularly in species capable of renesting.

Growth rate variation among passerine species in tropical and temperate sites: an antagonistic interaction between parental food provisioning and nest predation risk

Causes of interspecific variation in growth rates within and among geographic regions remain poorly understood. Passerine birds represent an intriguing case because differing theories yield the possibility of an antagonistic interaction between nest predation risk and food delivery rates on evolution of growth rates. We test this possibility among 64 Passerine species studied on three continents, including tropical and north and south temperate latitudes. Growth rates increased strongly with nestling predation rates within, but not between, sites. The importance of nest predation was further emphasized by revealing hidden allometric scaling effects. Nestling predation risk also was associated with reduced total feeding rates and per-nestling feeding rates within each site. Consequently, faster growth rates were associated with decreased per-nestling food delivery rates across species, both within and among regions. These relationships suggest that Passerines can evolve growth strategies in response to predation risk whereby food resources are not the primary limit on growth rate differences among species. In contrast, reaction norms of growth rate relative to brood size suggest that food may limit growth rates within species in temperate, but not tropical, regions. Results here provide new insight into evolution of growth strategies relative to predation risk and food within and among species.

Indirect predator effects on clutch size and the cost of egg production

Predator-induced changes in physiology and behaviour may negatively affect a prey's birth rate. Evidence of such indirect predator effects on prey demography remains scarce in birds and mammals despite invertebrate and aquatic studies that suggest ignoring such effects risks profoundly underestimating the total impact of predators. We report the first experimental demonstration of indirect predator effects on the annual 'birth' rate resulting from negative effects on the size of subsequent clutches laid by birds. We manipulated the probability of nest predation and measured the size of subsequent clutches and multiple indices of the mother's physiological condition, while controlling for food availability, date and stage of breeding. Females subject to frequent experimental nest predation laid smaller subsequent clutches and were in poorer physiological condition, particularly regarding non-resource-based indices (e.g. oxidative stress and glucocorticoid mobilization) consistent with both a response to the threat of predation and an increased cost of egg production.

Parasite infestation and predation in Darwin's small ground finch: contrasting two elevational habitats between islands

Contrasting ecological conditions may affect the distribution, abundance and impact of parasites and predators throughout the ranges of hosts and prey. Such patterns are evident on the archipelagos of Hawaii and the Galapagos, which vary in their distribution and abundance of avian parasites within and across islands. Previous research has documented higher intensity of parasitic fly larvae (Philornis downsi) in nests of Darwin's finches on elevated islands of the Galapagos. Here we examine P. downsi intensity and predation in 71 nests of Darwin's small ground finch (Geospiza fuliginosa) on Floreana Island. We found significant differences in parasite intensity, nest predation and clutch size between the lowland (0–100 m) and highland (300–400 m) habitats. Lowland finch nests had few P. downsi parasites (mean of 8 per nest), high nest predation (44% of nests) and large clutch size (3.4). Highland finch nests showed the opposite pattern, with many P. downsi parasites (40 per nest), low nest predation (17%) and small clutch size (2.5). This study suggests that the impacts of an introduced parasite are limited by its niche requirements and resource availability within and across islands. Our findings also imply that the vulnerability of bird populations to introduced parasites and predators is linked with variation in life history strategies across habitats.

Predation on dependent offspring: a review of the consequences for mean expression and phenotypic plasticity in avian life history traits

Predation on dependent offspring (i.e., offspring that depend on parents for care) forms a critical source of natural selection that may shape a diversity of life history traits. Selection from predation risk on dependent offspring can influence life history strategies of both offspring and parents. Such selection may act on both the form of plastic responses (e.g., the shape of norms of reaction) and mean expression of traits. Consideration of both levels of responses is key to understanding the ecological and evolutionary role of predation on dependent offspring. Here, we discuss how plastic responses and mean expression of life history traits may respond to selection from predation on dependent offspring in nests of birds (i.e., nest predation). We then review the expected effects and evidence for a diversity of life history traits, including clutch size, egg size, renesting rates, onset of incubation, parental incubation behavior, development rates and period lengths, parental feeding behavior, nestling begging, and nest conspicuousness. The evidence demonstrates a broad role of nest predation on both phenotypic plasticity and mean expression of diverse traits, but evidence remains limited to a few studies on a limited variety of species for almost all traits, and much broader experimental tests are needed.

Food-supplementing parents reduces their sons' song repertoire size

Food-supplemented parents typically produce more offspring, as numerous experiments on vertebrate populations have shown. 'Propagule' (egg or neonate) size and parental care may also be affected, with implications concerning the adult quality of offspring, although few experiments have addressed whether food-supplementing one generation affects adult quality in the next. We conducted a food supplementation experiment on song sparrows (Melospiza melodia) and tested whether song repertoire size, a demonstrated indicator of male quality, differed between the adult sons of fed (food-supplemented) and unfed (non-food-supplemented) parents. Counterintuitively, fed parents produced sons with smaller adult song repertoires, who may thus be expected to contribute fewer offspring, and fewer grand-offspring, to the population. Fed and unfed parents invested equally in the total biomass of their clutches and broods, and average nestling condition was comparable, but because fed parents produced more offspring, average egg and nestling sizes were reduced. Fed and unfed parents apportioned care differently within their broods, and we suggest compensatory growth of offspring emerging from light eggs, or egg size itself, may have affected adult repertoire size. Conceivably, the conservation benefits of food-supplementing populations could attenuate over time if fed parents produce offspring of poorer quality than themselves.

The worldwide variation in avian clutch size across species and space

Traits such as clutch size vary markedly across species and environmental gradients but have usually been investigated from either a comparative or a geographic perspective, respectively. We analyzed the global variation in clutch size across 5,290 bird species, excluding brood parasites and pelagic species. We integrated intrinsic (morphological, behavioural), extrinsic (environmental), and phylogenetic effects in a combined model that predicts up to 68% of the interspecific variation in clutch size. We then applied the same species-level model to predict mean clutch size across 2,521 assemblages worldwide and found that it explains the observed eco-geographic pattern very well. Clutches are consistently largest in cavity nesters and in species occupying seasonal environments, highlighting the importance of offspring and adult mortality that is jointly expressed in intrinsic and extrinsic correlates. The findings offer a conceptual bridge between macroecology and comparative biology and provide a global and integrative understanding of the eco-geographic and cross-species variation in a core life-history trait.

Why do some bird species lay only one egg in their nest, and others ten? The clutch size of birds is one of the best-studied life-history traits of animals. Nevertheless, research has so far focused either on a comparative approach, relating clutch size to other biological traits of the species, such as body weight; or on a macroecological approach, testing how environmental factors, such as seasonality, influence clutch size. We used the most comprehensive dataset on clutch size ever compiled, including 5,290 species, and combined it with data on the biology and the environment of these species. This approach enabled us to merge comparative and macroecological methods and to test biological and environmental factors together in one analysis. With this approach, we are able to explain a major proportion of the global variation in clutch size and also to predict with high confidence the average clutch size of a bird assemblage on earth. For example, cavity nesters, such as woodpeckers, have larger clutches than open-nesting species; and species in seasonal environments, especially at northern latitudes, have larger clutches than tropical birds. The findings offer a bridge between macroecology and comparative biology, and provide a global and integrative understanding of a core life-history trait.

Why do some bird species lay only one egg in their nest, and others ten? An analysis combining comparative and macroecological approaches across more than half of all bird species explains the global variation in this trait with high confidence.

Breeding behaviour and ecology of the grey fantail (Rhipidura albiscapa)

The grey fantail (Rhipidura albiscapa) is a common Australian flycatcher, yet no detailed reports have been made of its breeding ecology. A population of grey fantails was studied over four seasons in the Australian Capital Territory. Males had large testes and pronounced cloacal protuberances, morphology suggestive of sperm competition. Although two polyandrous trios were observed, most individuals bred as part of a season-long monogamous pair, suggesting that extra-pair copulation may be the principal cause of sperm competition. Indeed, behavioural observations revealed that males regularly intruded other territories, targeting those with building, rather than incubating, females. Intruding males were observed harassing, attempting to copulate and successfully copulating with resident females. Males did not mate guard, but regularly attacked their mates during each building attempt. Despite the probability that extra-pair paternity is common in this species, grey fantails were monomorphic and monochromatic, with a high level of paternal care. Nest depredation was common, with 83% of all clutches depredated before fledging. High levels of male care in care in this species may be better explained by an increase in fledging success associated with high male contribution to offspring care than confidence of paternity.

Evolution of Clutch Size in Cavity‐Excavating Birds: The Nest Site Limitation Hypothesis Revisited

There are two major competing hypotheses for variation in clutch size among cavity-nesting species. The nest site limitation hypothesis postulates that nesting opportunities are more limited for weak excavators, which consequently invest more in each breeding attempt by laying larger clutches. Alternatively, clutch size may be determined by diet; the clutch sizes of strong excavators may be smaller because they are able to specialize on a more seasonally stable prey. We built a conceptual model that integrated hypotheses for interspecific variation in clutch size and tested it with comparative data on life-history traits of woodpeckers (Picidae) and nuthatches (Sittidae). In most analyses, diet explained more variation in clutch size among species than did propensity to excavate. Migratory status was positively associated with clutch size but was difficult to distinguish from diet since resident species consumed more bark beetles (a prey available in winter) and had smaller clutches than migratory species. The literature suggests that cavities are not limited in natural, old-growth forests. Although our data do not rule out nest site limitation, we conclude that annual stability of food resources has a larger impact on the evolution of clutch sizes in excavators than does limitation of nest sites.

Parent birds assess nest predation risk and adjust their reproductive strategies

Avian life history theory has long assumed that nest predation plays a minor role in shaping reproductive strategies. Yet, this assumption remains conspicuously untested by broad experiments that alter environmental risk of nest predation, despite the fact that nest predation is a major source of reproductive failure. Here, we examined whether parents can assess experimentally reduced nest predation risk and alter their reproductive strategies. We experimentally reduced nest predation risk and show that in safer environments parents increased investment in young through increased egg size, clutch mass, and the rate they fed nestlings. Parents also increased investment in female condition by increasing the rates that males fed incubating females at the nest, and decreasing the time that females spent incubating. These results demonstrate that birds can assess nest predation risk at large and that nest predation plays a key role in the expression of avian reproductive strategies.