Avian Life History Research Articles

Nest survival of Crested Lark (Galerida cristata) in intensively used habitats in Central Poland

Breeding success of birds depends on survival during two critical stages of the reproductive period: First, egg laying and incubation, and then nestling and fledgling. This essential element of avian life history mirrors an evolutionary adaptation of parents to existing environmental conditions. The main factors that determine nest survival involve weather, predation and sibling competition. Despite numerous studies documenting their impact on breeding success, only a few have analysed the issue with regard to ground-nesting birds and estimated their survival functions. Therefore, we decided to examine nest survival probability of the Crested Lark (Galerida cristata) in a population that occupies newly established roadside habitats in Central Poland. The analysis is based on 141 nests monitored between 2018 and 2022. We found that the mean survival probability of the whole nesting period was 0.376, and it was higher during the egg stage (0.836) than during the nestling period (0.392). The Cox proportional hazards model shows that nest survival in the nestling stage is mainly affected by the time of breeding and the distance from the road edge, and also by mean temperature of the whole nesting period. In other words, the probability of a brood failure in the nestling stage decreases as the breeding season advances, whereas it increases among nests located closer to the road edge. Furthermore, during the whole nesting period, nest failure decreases as the temperature rises. Our study suggests that the Crested Lark, being a multi-brood and thermophilic species inhabiting temperate regions, may benefit more if its nesting attempts begin later during the season when breeding conditions become more favourable. However, we also take into consideration a hypothesis that nesting in grassy habitats near road edges may prove to be an ecological trap, attracting high densities of nesting birds but leading to their low nest success.

BIOLOGÍA REPRODUCTIVA DEL CHINCHERO ENANO O CHINCHERO EXCAVADOR XIPHORHYNCHUS FUSCUS

Despite recent advances, the knowledge scarcity on Furnariidae forest species reproduction continues to bias ideas and limit the understanding of avian life history. We studied the reproductive biology of the Lesser Woodcreeper (Xiphorhynchus fuscus; Dendrocolaptinae), which nests in tree cavities, to contribute observations toward ideas about evolution and ecology. In the Atlantic Forest of Argentina (2007–2022), we inspected 26 nests (total: 333 visits), color-banded nine adults, conducted 180 h of focal observations at 11 nests, and analyzed patterns of nest survival and parental care using linear models in R. Lesser Woodcreepers laid two or three white eggs (mean±SE = 2.8±0.1; N = 16) on a bed of bark flakes. One adult (possibly female) carried out construction, incubation, feeding, and nest hygiene, but we frequently observed a second adult (possibly male) singing from 'preferred' perches, sometimes approaching the nest tree in response to alarm calls from the nest. Attending adults brought bark to their nests throughout incubation (19 days) and occasionally during nestling (21 days; range: 20–23). In the absence of adults, eggs and young nestlings were left covered with bark. Nest attention (daytime percentage an adult was inside the cavity) was 63±3% during incubation, fell to 38±9% in the first 4 days after hatching, and approximately 0% after nestlings were 10 days old and had open pin feathers. Nestlings were fed almost exclusively arthropods, and the feeding rate increased as brooding declined. Nest survival was 0.984 (95% CI: 0.970–0.992) daily or 0.50 (0.26–0.70) for the nesting period. For four of six broods, when nestlings fledged, a second adult (possibly a male) participated minimally. Our observations cover the mating system and division of parental care in Furnariidae, the advantages of covering eggs and nestlings, and the possibility that males increase parental care during the vulnerable fledgling stage.

Honorary Lifetime Membership Award: Ellen D. Ketterson.

Honorary Lifetime Membership Award: Ellen D. Ketterson.

Reproductive Success of a Tropical Barn Swallow Hirundo rustica Population Is Lower Than That in Temperate Regions.

Temperate-tropical comparisons of avian life history traits are helpful to understand the different selective pressures placed on birds by different climate zones. Although there have been many comparative studies targeting multiple species in different regions, there are few comparative studies on the reproductive successes of the same species between tropical and temperate regions. In this study, we monitored the breeding activities of the Barn Swallow (Hirundo rustica) simultaneously at a single tropical site and a single temperate site in China, compared the breeding performances of the two populations, and investigated the effects of weather conditions on reproductive success separately. The clutch and brood sizes of the Barn Swallow at the topical site were significantly smaller than those at the temperate site. Furthermore, the breeding success of the Barn Swallow at the tropical site was significantly lower than that at the temperate site. The mean daytime temperature had a negative effect on the clutch size and brood size at both sites; it had a negative effect on nestling survival at the tropical site, but not the temperate site. This study will help us understand the adaptation strategies of widely distributed bird species in different environments, and how climate change will affect birds in different climate zones.

Life history variation between two Eurasian tree sparrow Passer montanus populations at different altitudes

Abstract Altitudinal gradients create environmental variation that can strongly affect avian life history strategies. To fully understand this issue, a comparison of the demography of populations of the same species over large altitudinal differences is required. Here, we investigated the breeding ecology of two populations of Eurasian tree sparrow (Passer montanus), one in the temperate zone of central China at an altitude of 30 m and another in the alpine zone of the northeastern Tibetan plateau at an altitude of 3430 m. The breeding ecology of this species was rarely studied at such a high altitude before. Our result revealed that high-altitude tree sparrows started breeding later, had a shorter breeding period (80 versus 140 days), produced smaller broods (1.2 versus 2.5 chicks) and had significantly smaller clutches (4.0 versus 4.9 eggs) but larger eggs (4272 versus 3443 mm3) when compared with their low-altitude counterparts. Besides, tree sparrows at our high-altitude site had longer nestling periods (14.5 versus 13.4 days) than those at the low-altitude site. High-altitude tree sparrows tend to have reduced fecundity but allocate more energy into each offspring to confront the stressful conditions at high altitudes. The observed patterns are consistent with adaptive life history strategies.

Plasticity of snowy plover incubation behaviors in response to risks of nest predation

Abstract Nest predation influences population dynamics and is thought to exert strong selection on the evolution of avian life history. Because parental behaviors can attract the attention of nest predators, incubating birds are predicted to decrease conspicuous behaviors at the nest-site and increase incubation constancy when risks of nest predation are high. We examined whether snowy plovers Charadrius nivosus responded to predator-specific risks of nest predation, using the number of off bouts and daily nest attendance (proportion of time spent incubating) as responses. We quantified risks using predator-specific hazard rates of nest mortality, which varied daily and were based on habitat characteristics at each nest. We assessed the influence of predator-specific risks of nest mortality on incubation behaviors using an individual-centering approach, allowing us to explain variation in incubation behaviors within- and among-breeding pairs. We found increased number of off bouts and nest attendance within-breeding pairs in response to increasing risks of nest predation by foxes (Vulpes spp.) and gulls (Larus spp.), but not coyotes (Canis latrans) and common ravens (Corvus corax). Among breeding pairs across habitats, we found increased nest attendance in response to higher risks of nest predation by foxes, but not coyotes, gulls, or ravens. Breeding pairs differed in the amount of behavioral plasticity they exhibited in response to risks of nest predation. Our results suggest that risks of nest predation differentially influence behavioral responses of snowy plovers depending on the predator species, and the amount of behavioral plasticity may depend on the characteristics of breeding adults.

Clutch size of passerines increases with latitude in China, but egg size is conserved

The size and number of eggs are important components of reproductive strategies, with implications for offspring and parental fitness. Birds typically produce larger clutches in more poleward latitudes, in response to the strong seasonal peak of food availability required to raise young. However, changes in egg size impact the interpretation of this pattern, and latitudinal variation in egg size remains poorly explored. Here we comparatively investigate this issue at both interspecific and intraspecific levels, using datasets from 298 populations of 154 passerine species breeding in mainland China. Clutch size was larger toward the north, but egg size was relatively constant and therefore total clutch volume (clutch size × egg size) increased. No negative correlation was evident between egg size and clutch size along latitudinal gradients, suggesting the lack of a direct trade‐off between the two traits. This may be explained by latitudinally increased food availability and concentrated annual reproductive investment due to seasonal time constraints on raising more broods, arising from the later onset of reproduction at higher latitudes. In addition, the latitudinal patterns of egg size and clutch size appear to be independent of parental investment in terms of the lengths of incubation and nestling periods, which were similar for lower‐ and higher‐latitude birds. Our results shed light on the evolution of avian life histories across latitudinal gradients.

Experimentally reduced feather microbial loads improve reproductive performance in captive Zebra Finches

AbstractWe performed a laboratory experiment on caged, domesticated Australian Zebra Finches (Taeniopygia guttata castanotis) to assess the effects of antimicrobial treatment applied to birds’ plumage on the feather microbial loads of breeding pairs and their associated reproductive performance. Treatments included an experimental group that experienced frequent misting with a broad-spectrum antimicrobial, as well as 2 control groups. Microbial loads (colony forming units [CFUs]) sampled from the belly feathers of breeders were higher posthatching than prehatching, and experimental pairs had lower loads (one-half to one-third the CFU counts) than controls at all sampled breeding-cycle stages. Clutch size and hatching success did not differ among treatments, but experimental pairs fledged nearly 50% more offspring (mean = 3.5 per clutch) than controls (mean = 2.3 per clutch) due to experiencing much lower nestling mortality. Offspring development rate, as assessed by one metric, varied among treatments: beak color score sampled midway in the development of adult phenotype revealed that offspring of experimental pairs had more rapid development. Findings imply that experimentally reduced density of feather microbial loads on breeders decreased reproductive costs in the posthatch phase, resulting in increased offspring survival and condition. At the time offspring fledged, uropygial gland volume tended to be greater among breeders that had experienced antimicrobial application throughout the breeding cycle. These results are consistent with a growing literature showing that the feather microbiome contributes significantly to the evolution of avian life histories and behavior and indicate that Zebra Finches are a useful laboratory model for investigating in vivo effects of the feather microbiome on avian hosts.

Postfledging survival and movements of juvenile Gray-backed Thrushes (Turdus hortulorum)

The postfledging period is a critical phase of avian life history. In the initial few weeks after fledging, juveniles face various hazards and develop necessary life skills. Knowledge of the postfledging period is important for understanding the fate of juvenile birds. From May to July of 2015, we tracked 22 Gray-backed Thrush (Turdus hortulorum) juveniles from 20 nests in a population in northeastern China using radiotelemetry. Juveniles dispersed from their natal sites for a mean (± SE) of 21.0 ± 2.4 d after fledging. The initial dispersal distance was 560.9 ± 109.4 m (the distance between the natal area and the first postdispersal area). The natal range size of the juveniles had a mean of 1.7 ± 0.5 ha (95% minimum convex polygons). Ten of 22 (45.5%) of the monitored fledglings died within the first 4 weeks after fledging, and predation was the primary cause of mortality. The Cox proportional-hazards regression model showed that there was a positive correlation between tarsus length and the survival rate of fledglings, while the influences of fledgling weight, date of fledging, and brood size on survival were not significant. Our results indicate that mammalian predators were the main threat to juveniles in the first few days after fledging when the juveniles had limited mobility. After the first week postfledging, raptors became the most important source of juvenile mortality.

Tibetan birds lay larger but fewer eggs in a clutch.

How do offspring size and number vary along elevational gradients? This is a striking but rarely addressed question in life-history evolution. Here we comparatively explore it using phylogenetically paired passerine birds of lowland China vs. the Tibetan plateau spanning an elevational range of 18-4500m. Toward the Tibetan plateau, egg size increased, clutch size decreased and total clutch volume (= clutch size × egg size) did not change, when accounting for major confounding factors. Larger eggs and smaller clutches can be a response to harsh abiotic conditions at higher elevations and the corresponding reduction in food resources required for raising young, respectively. There was a negative correlation between egg size and clutch size after controlling for elevation effect on either trait, suggesting that the trade-off contributed to the expression of these two traits across elevations, given the lack of an elevational trend in total clutch volume. Tibetan birds had a shorter breeding season, prolonged incubation and nestling period than their lowland counterparts. While fewer clutches over the short breeding season may facilitate the increase in egg size, the concentration of annual reproductive investment did not seem to act on clutch size; despite the smaller clutches, hard environments may impose greater costs of parental care on Tibetan birds. The current research provides insight into the evolution of avian life histories across elevations.

Preformative molt, plumage maturation, and age criteria for 7 species of manakins

We developed aging criteria for 7 species of manakins (Pipridae) from the Manu Biosphere Reserve, Peru, based on patterns of plumage maturation and wing-feather replacement following their preformative molt, and summarize information on their morphological characteristics. Each species underwent a partial preformative molt, which could be identified using the presence of molt limits in the greater coverts. Some male Band-tailed Manakin (Pipra fasciicauda), Round-tailed Manakin (Ceratopipra chloromeros), Cerulean-capped Manakin (Lepidothrix coeruleocapilla), and Yungas Manakin (Chiroxiphia boliviana) showed evidence of delayed plumage maturation, allowing for age classification up to the third annual cycle, whereas Blue-crowned Manakin (L. coronata caelestipileata), Fiery-capped Manakin (Machaeropterus pyrocephalus), and Green Manakin (Cryptopipo holochlora viridor) appeared to attain definitive plumage after their second molt cycle. Morphometrics showed strong overlap and were less useful for separation of age and sex classes. Our findings add to the growing list of studies that suggest many tropical passerines can be aged using primarily molt limits. Data on molt and plumage maturation are still needed for the vast majority of tropical birds in order to inform conservation-based research and studies of avian life history.

Activity patterns of Hawaiian forest birds in a fragmented and continuous landscape

Activity patterns are a key component of avian life history and behavior, and decisions about how activity periods are structured can have important fitness implications. Despite the importance of activity patterns, individual variability in activity is poorly studied in small birds. We used automated radio telemetry to track Hawaiian forest birds from three dietary guilds in a continuous forest, and in a similar forest that was naturally fragmented in the 1800s by lava flows but supports stable populations of native birds. We generated kernel density profiles of activity (KDEs) over the full tracking period of each bird and used the KDEs to derive seven activity metrics that characterized daily activity patterns. We identified three distinct activity strategies using a cluster analysis of the activity metrics and assessed day‐to‐day variability in activity strategy to understand intraindividual consistency in activity behavior. We also evaluated the influence of landscape structure, dietary guild and sex on the suite of activity metrics using constrained ordination. Most birds (61%) exhibited high levels of sustained activity throughout the day, while 23% showed irregular activity most of the day followed by a peak in the late afternoon, and 16% were mainly active in the morning. Though activity patterns varied by sex and dietary guild, landscape structure had the strongest influence on activity patterns. Birds sustained high levels of activity throughout the day in the fragmented landscape regardless of dietary guild, suggesting they needed to be more active to obtain necessary resources. However, activity patterns of birds in the fragmented landscape varied more day‐to‐day, indicating they retained a method for adjusting behavior to match internal and external conditions. This study reveals how a diverse suite of tropical birds has adapted to long‐term fragmentation in Hawaiʻi and demonstrates the utility of automated radio telemetry for studying the activity strategies of birds.

The relationships between breeding success and nest site selection for some Passeriformes in Ikh Nart Nature Reserve, Mongolia

Offspring survival is critical to bird conservation and understanding avian life history. We monitored 73 nests belonging to six Passeriformes species in Ikh Nart Nature Reserve, Mongolia in 2017 and 2018. The study focused on nest site selection for breeding visitors and resident avian species in relation to habitat types found in arid environments, such as rocky outcrops and open steppe. Egg laying period varied among the species. Those that nested in the rocky outcrops realized higher reproductive success. The offspring survival decreased in ground-nesting birds after hatching. Resident species, including Rock Sparrow (Petronia petronia), Mongolian Finch (Bucanetes mongolica), and Horned Lark (Eremophila alpestris), showed lower reproductive success during the first half of the season, while breeding visitors, including Northern Wheatear (Eremophila alpestris), Isabelline Wheatear (Oenanthe isabellina), and Pied Wheatear (Oenanthe pleschanka), saw higher reproductive success. Rocky outcrops near water sources in arid environments play an important part in the breeding success of rock-dwelling birds.

An evaluation of transmitter effects on adult and juvenile Common Terns using leg-loop harness attachments

Marking birds with transmitters allows for the collection of data that are critical for fully understanding avian life history, but researchers must also be confident that performing such studies is as safe as possible for transmittered individuals. While much could be learned from tracking juveniles across dependency periods and first migration, doing so would require a harness-based attachment method which has not been evaluated on any species of juvenile tern. Therefore, we monitored the reproductive success and behavior of adult Common Terns (<em>Sterna hirundo</em>) and the growth and behavior of juvenile Common Terns after attaching transmitters to adults and juveniles with leg-loop harnesses made of elastic cord. We found that transmittered adults had similar reproductive success to untransmittered controls (hatching success for nests of transmittered adults = 0.553; nests of control adults = 0.665). Transmittered adults also expressed minimal behavioral differences from untransmittered controls when the groups were compared via paired treatment-control nest observations along with observations away from the nest. Transmittered juveniles had similar fledging success and growth rates to untransmittered control juveniles (fledging success for transmittered juveniles = 0.766; control juveniles = 0.817). Transmittered juveniles exhibited slight differences in behavior from controls, with increased rates of preening, although these differences did not appear to be detrimental. Finally, monitoring efforts during the breeding season following transmitter deployment found no difference in the return rate, nesting attempt rate, or hatching success rate based on treatment (P > 0.05). However, despite evidence of an individual retaining its transmitter into fall migration, no individuals retained their transmitters when resighted the following breeding season. While our results show that leg-loop harnesses made of elastic cord present a potential option for transmitter attachment to both adult and juvenile Common Terns, additional testing could provide further insight into potential long-term impacts.

Breeding biology and nest survival of the Rusty-backed Antwren (Formicivora rufa)

Breeding biology data are crucial for avian life history theory, but this information is unavailable for nearly half of the current living tropical bird species. Predation is the main driver of nest survival among tropical birds, and spatial and temporal factors may affect nest predation risk and survival. In this study, we describe the breeding biology of the Rusty-backed Antwren (Formicivora rufa), a bird that inhabits a coastal sandy plain in southeastern Brazil, and examine the effects of date and microhabitat features on nest survival. We found 47 nests over 2 breeding seasons. The nesting season lasted 7–8 months. The mean (± SD) nest-building time was 7.3 ± 1.1 d, the mean incubation period was 16 ± 1.9 d, and nestlings remained in the nest for 10 ± 0.8 d for a full nesting cycle that lasted for ∼26 d. Clutches invariably had 2 eggs and egg-laying occurred over 2 consecutive days. Males and females were both observed building nests, incubating eggs, and feeding nestlings and fledglings. We also report a nestling with a depigmented body, which may represent albinism. Most nests were depredated (68.1%, n = 32). Nest survival was constant throughout the breeding seasons and did not vary with microhabitat features (nest height above the ground, and vertical and horizontal distances between nest and shrub edges). The nesting biology of the Rusty-backed Antwren fits the general pattern typical of antbirds, and micro-spatial or temporal features cannot predict nest survival in our study population.

Disentangling the avian altricial-precocial spectrum: Quantitative assessment of developmental mode, phylogenetic signal, and dimensionality.

The altricial-precocial spectrum describes patterns of variation in avian developmental mode that greatly influence avian life histories. Appraising a given species' position on this spectrum is therefore fundamental to understanding patterns of avian life history evolution. However, evaluating avian developmental mode remains a relatively subjective task reliant on untested assumptions, including the notion that developmental strategies are distributed along a single dimension of statistical variation. Here, we present a quantitative multivariate framework that objectively discriminates among meaningfully different modes of avian development. We gathered information on seven hatchling and post-hatching traits for up to 4000 extant bird species, and find that most traits related to developmental mode show high phylogenetic signal and little intraclade variation, allowing unknown values to be reliably interpolated. Principal component analyses (PCAs) of these traits illustrate that most variation in hatchling state can be quantified along one dimension of trait space. However, our PCAs also reveal an important second dimension explaining variation in post-hatching behavior, enabling factors related to hatchling state and post-hatching behavior to be disentangled. In order to facilitate future macroevolutionary studies of variation in avian developmental strategies, as well as explorations of covariation between developmental mode and other aspects of avian biology, we present PC scores for 9993 extant avian species.

Breeding biology of the Olivaceous Flatbill (Rhynchocyclus olivaceus) in an Amazonian forest fragment of northwest Brazil

Descriptions of individual breeding bird biologies provide important insights to our understanding of avian life history strategies. The present study focused on the reproductive biology and nesting ecology of the Olivaceous Flatbill (Rhynchocyclus olivaceus) in the Parque Zoobotânico (∼100 ha) of the Universidade Federal do Acre, Brazil, between April 2015 and March 2016. We found 91 nests, of which 14 were active and 77 were abandoned. Due to multiple nest failure incidents, we only monitored a single nest throughout the breeding period from nest construction to offspring fledging. This nest was built in 14 d, eggs were incubated for 26 d, and nestlings remained in the nest for additional 21 d. The nest was classified as closed, retort-shaped, and pensile. We deconstructed one nest that had a weight of 65 g composed of twigs (31.6% of the total weight), fibers (28.9%), leaves (14.5%), bark (9.2%), bamboo pieces (9.2%), and tendrils (6.6%). We found the nests at heights of 1–9 m above the ground, with a preference for a 2–4 m stratum, built atop 25 different plant species. The 4 species that were the most frequently used as nest substrate were Protium unifoliolatum (n = 16 nests), Casearia sp. (n = 11), Siparuna guianensis (n = 10), and Couepia sp. (n = 9). The Olivaceous Flatbill breeds throughout the year, in both rainy and dry seasons, and it seems to be highly sensitive to human disturbance during the breeding season. The reproductive parameters recorded in the present study were consistent with those reported for other species of this genus, although most of the findings are novel for the Olivaceous Flatbill and provide important insights into its reproductive biology in the Amazon region.

Bridging gaps in demographic analysis with phylogenetic imputation.

Phylogenetically informed imputation methods have rarely been applied to estimate missing values in demographic data but may be a powerful tool for reconstructing vital rates of survival, maturation, and fecundity for species of conservation concern. Imputed vital rates could be used to parameterize demographic models to explore how populations respond when vital rates are perturbed. We used standardized vital rate estimates for 50 bird species to assess the use of phylogenetic imputation to fill gaps in demographic data. We calculated imputation accuracy for vital rates of focal species excluded from the data set either singly or in combination and with and without phylogeny, body mass, and life-history trait data. We used imputed vital rates to calculate demographic metrics, including generation time, to validate the use of imputation in demographic analyses. Covariance among vital rates and other trait data provided a strong basis to guide imputation of missing vital rates in birds, even in the absence of phylogenetic information. Mean NRMSE for null and phylogenetic models differed by <0.01 except when no vital rates were available or for vital rates with high phylogenetic signal (Pagel's λ > 0.8). In these cases, including body mass and life-history trait data compensated for lack of phylogenetic information: mean normalized root mean square error (NRMSE) for null and phylogenetic models differed by <0.01 for adult survival and <0.04 for maturation rate. Estimates of demographic metrics were sensitive to the accuracy of imputed vital rates. For example, mean error in generation time doubled in response to inaccurate estimates of maturation time. Accurate demographic data and metrics, such as generation time, are needed to inform conservation planning processes, for example through International Union for Conservation of Nature Red List assessments and population viability analysis. Imputed vital rates could be useful in this context but, as for any estimated model parameters, awareness of the sensitivities of demographic model outputs to the imputed vital rates is essential.

Haemosporidian prevalence, parasitaemia and aggregation in relation to avian assemblage life history traits at different elevations

The transmission of vector-borne protozoa such as parasites of the Order Haemosporida is dependent on both biotic and abiotic factors such as host life history traits and environmental conditions. This study aimed to identify the variables that determine haemosporidian prevalence, parasitaemia and aggregation within the context of elevation and avian life history traits in Central Veracruz, Mexico. We sampled 607 birds from 88 species; we used microscopy and the mtDNA cytochrome b gene to detect parasites. We found an overall prevalence of 32.3%. Haemosporidian prevalence was 21.6% in tropical sub-deciduous forest (at sea level), 38% in tropical deciduous forest (265 m above sea level (asl)), 19.4% in montane cloud forest (1630 m asl), and 51.7% in pine-oak forest (2790 m asl). The prevalence of each parasite genus was strongly influenced by elevation (a proxy of habitat type). Plasmodium showed the highest prevalence at low elevation. Haemoproteus increased in prevalence with elevation. Leucocytozoon displayed the highest prevalence at the highest elevation (pine-oak forest). Haemoproteus spp. and Leucocytozoon spp. prevalences were higher in open cup than in closed nests. Haemoproteus prevalence and haemosporidian parasitaemia were lower in solitary birds than birds with pairing and gregarious behavior. Haemosporidian aggregation decreased with elevation, yielding the significantly lowest values at the pine-oak forest. Elevation distribution patterns of prevalence for each genus were similar to those previously reported in other geographical areas (e.g., South America, Europe).

Estimating egg mass–body mass relationships in birds

Abstract The mass of a bird’s egg is a critical attribute of the species’ life history and represents a fundamental component of reproductive effort. Indeed, the tradeoff between the number of eggs in a clutch and clutch mass lies at the heart of understanding how environmental attributes such as nest predation or adult mortality influence reproductive investment. However, egg masses have not been reported for the majority of avian species. We capitalized on the strong allometric relationship between avian body mass and egg mass to produce egg mass estimates for over 5,500 species previously lacking such information. These estimates are accompanied by measures of the robustness of the regressions used to produce them (e.g., sample size, root mean square error [RMSE] of estimation, coefficient of determination, and degree of extrapolation), thus allowing independent evaluation of the suitability of any estimate to address a particular research question relating to avian life history. Most estimates (~5,000) were based on family-level egg mass–body mass regressions, with the remainder derived from other relationships such as ordinal regressions. We compared estimating regressions based on adult vs. female body masses and, after finding little difference between the 2, based our final estimates on adult masses as those were more numerous in the literature. What small differences between adult- and female-based regressions that did occur were not related to sexual size dimorphism across families. These new estimates, coupled with ~5,000 egg masses reported in the literature, provide a foundation of over 10,000 species for wider investigations assessing variation in reproductive effort in birds over a broad array of ecological and evolutionary contexts.