Nest Insulation Research Articles

Parenting in the city: effects of urbanization on incubation behaviour and egg temperature in great tits, Parus major

With 68% of the world's population expected to live in cities by 2050, it is crucial to understand how animals respond to urbanization. Urban areas are associated with changes in microclimate, light, noise and human activity, which can affect animal physiology, behaviour and fitness. In particular, urbanization may affect how parents allocate time and energy between parental care and self-maintenance. In birds, a critical aspect of parental care is incubation, where parents must maintain egg temperatures for successful offspring development. However, incubation is energetically costly, and anthropogenic disturbances may affect parental decisions related to this cost. Determining whether urbanization affects incubation is crucial for understanding the differences between urban and nonurban populations in their reproductive performance and fitness. We measured incubation behaviour and egg temperature in one urban and one forest population of great tits, a uniparental incubator, using temperature loggers. We found that, compared to forest females, urban females spent more time incubating, took shorter off-bouts and ended their last daily off-bouts at a later hour. Furthermore, time spent off the nest, the number and duration of off-bouts and daily start of activity were related to ambient temperature; start of activity was related to clutch size; and time spent off the nest was related to the day of incubation. Despite urban females spending more time on the nest, eggs of both forest and urban females experienced relatively similar average egg temperatures, and urban eggs tended to experience greater variability in egg temperature. Furthermore, there were no differences between sites for any fitness-related offspring variable. Our results suggest that urbanization can influence the thermal environment of developing avian offspring by causing urban adults to be inefficient incubators through increased activity off the nest in the evening, differences in nest insulation and/or suboptimal incubation behaviour compared to forest counterparts. • Urbanization appears to influence parental incubation behaviour of great tits. • Urban birds spent more time warming their eggs compared to birds in the forest. • However, outcomes for offspring were similar. • Urban eggs and forest eggs experienced similar temperatures.

Nest ornaments and feather composition form an extended phenotype syndrome in a wild bird

Many species throughout the animal kingdom construct nests for reproduction. A nest is an extended phenotype—a non-bodily attribute—of the individual building it. In some bird species, including our study population of blue tits (Cyanistes caeruleus), conspicuous feathers or other material are placed on top of the nest. These so-called nest ornaments do not contribute to nest insulation, but are hypothesised to have a signalling function. Here, we apply the concept of behavioural syndromes, with focus on between-individual variation (repeatability) and between-individual correlations, to the study of avian nest construction. We find that nest ornamentation is a moderately repeatable trait in female blue tits, which suggests it is an extended phenotype of the female. Furthermore, the tendency to ornament the nest covaries across females with another aspect of her extended phenotype, the composition of the nest lining material, and these two traits thus form an extended phenotype syndrome. Assuming the correlation is reflected on a genetic level, it implies that nest ornamentation and composition of the nest lining do not evolve in isolation; one aspect may be an evolutionary by-product of selection on the other aspect and their overall flexibility to respond to change is reduced.Significance statementThe avian nest is an extended phenotype (a non-bodily attribute) of its builder with potentially multiple functions in terms of insulation and signalling. In particular, many bird species’ nests contain nest ornaments, feathers or other materials that are placed on top of the nest and that stand out from the nest material due to their colour and/or size. We quantified between-individual variation (repeatability) of nest ornamentation behaviour in a wild population of blue tits and between-individual covariation (syndrome) of nest ornamentation to other features of nest construction. We find that nest ornamentation is a repeatable trait limited to females in our study population. The tendency to ornament the nest covaries across females with another aspect of her extended nest phenotype, the composition of the nest lining material. These correlated traits thus form an extended phenotype syndrome. It hence becomes crucial to recognise that a study of a single aspect of nest construction in isolation captures only a part of the complexity, as one aspect may have evolved as a correlated response of selection on the other aspect. Moreover, such a syndrome implies limited flexibility in the range of adaptive response.

The Use of Mobile Nesting Platforms to Reduce Electrocution Risk to Ferruginous Hawks

Electrocution on low-voltage power lines is a well-known source of mortality for raptors. Electrocution risk is particularly high for large raptors that can bridge components and make lethal contact, and for raptors living in open landscapes where power poles can be attractive nest or perching sites. Thus, large, grassland-obligate species like the Ferruginous Hawk (Buteo regalis), a legally endangered and threatened species in Alberta and Canada, respectively, are at high risk of electrocution. We used mobile (temporary) versions of artificial nest platforms (ANP) to relocate Ferruginous Hawks nesting on power poles at two sites on Canadian Forces Base Suffield, in Alberta, Canada. At one of these sites an adult female and a fledgling were electrocuted in 2014 and 2015, respectively, prior to introduction of a mobile ANP. In 2016, we relocated this nest to a mobile ANP 23 m away from the original nest pole, and the nesting attempt was successful. In 2017, we installed nest deterrents and insulation on the original nest pole and we installed a permanent ANP 465 m away. From 2017 to 2019, we moved the mobile ANP in stages toward the permanent ANP, with Ferruginous Hawks either nesting on or occupying the mobile ANP each year. We anticipate Ferruginous Hawks transferring to the permanent ANP soon. We also attempted our mobile-ANP-to-permanent-ANP approach at a similar power pole elsewhere on the base in 2016. In 2017, a Ferruginous Hawk pair nested on the permanent ANP so we removed the mobile ANP. This pair did not return in 2018 or 2019. Despite mixed results, use of mobile ANPs as a tool to move nesting raptors away from power poles that are not avian-safe may be a viable electrocution mitigation strategy.

Wall Structure Affects Insulation of Nests Built by Thrushes

Incubation and rearing of nestlings are energetically demanding activities that can be affected by insulation conferred by the nest wall. Although thermal properties of nests are widely studied, no reports to date have dealt with the relative effects of the various structural parts of the nest wall. This experimental study investigated the effects of air movement, and for the first time, the effects of wall structure, on nest insulation for the Common Blackbird Turdus merula and Song Thrush Turdus philomelos of the Turdidae. Insulatory values, i.e. difference in cooling rate measured simultaneously inside and outside a nest, and internal cooling rates within the nest cup, were determined using temperature loggers under still-air and moving-air conditions. Moving-air increased insulatory values and internal cooling rates in both species. Insulatory values were positively correlated with the nest base thickness, but not nest side wall thickness. Removal of the outer nest wall and cup lining to reveal the internal mud cup of thrush nests reduced the insulatory value compared to the whole nest, the effect of which was exacerbated under moving-air conditions. These results may reflect that the nest wall confers huge insulatory advantages and especially so during periods of air movement. However, it is unclear whether this is related to the presence of particular nest materials that confer insulation, or to the amount of air trapped in the nest wall. Alternatively, the thermal conductivity of the materials lining the cup wall may be important.

Construction materials affect the hydrological properties of bird nests

AbstractBird nests are highly variable structures that represent extended phenotypes, which may have a range of roles during avian reproduction, but all nests serve as the location for incubation of eggs. Nest characteristics should counter any adverse weather conditions that could potentially reduce hatchability or chick survival. Thermal properties of nests are relatively well studied, but the effects of moisture on nest insulation are largely unknown. Wet nests could reduce thermal insulation or deleteriously cool the eggs or nestlings. In this study, a novel method of simulated rainfall was applied to open‐cup nests from nineteen different passerine species. The amount of water absorbed by the nest and the time it took to dry were recorded. To determine the factors affecting these characteristics, nests were deconstructed to provide data on materials used. Compared with smaller nests, larger nests absorbed more water and took longer to dry. The amount of water absorbed was a function of the proportion of grass, moss, roots and hair present in the nest. The time taken for the nest to dry completely was affected by the proportion of grass, moss, and hair in the nest. When tested individually, the materials all had different rates of water absorption and took different times to dry; moss absorbed the most water and took the longest time to dry. We concluded that simulated rainfall was a realistic approach to understanding the hydrological characteristics of nests. Structural considerations for nest construction may affect the hydrological properties of nests, but ex situ studies provide no information about the possible shelter provided to a nest by its immediate surroundings. Further research into nest function should consider the importance of exposure to water to nest construction and function.

Air movement affects insulatory values of nests constructed by Old World Warblers

Avian nests provide a location for incubation, and in many cases rearing of nestlings. These energetically demanding activities have meant that nest insulation has been the subject of many studies but few to date have dealt with how variation among species in nest construction materials could affect nest insulation. This study investigated the insulation of nests constructed by three species of Old Word warblers (Sylviidae), which vary in size and composition. Insulatory values, i.e. difference in cooling rate of temperature loggers placed inside and outside a nest, and internal cooling rates within the nest cup were determined using temperature loggers under still-air and moving-air conditions. Insulatory values determined in still-air conditions of a laboratory were significantly different among nests of the different species but not when the values were determined within the smaller volume of a wind tunnel. Moving-air increased insulatory values by an order of magnitude but also increased internal cooling rates in all species. Insulatory values were positively correlated with the nest base thickness. Moving-air increased the cooling rate of the external temperature logger much more than the internal logger, which inflated the insulatory value of a nest wall in moving-air. Reasons for these results may reflect the thermal properties of the materials individually or in combination. Future testing of nest insulation should be under standard conditions that limit air movement but the role of the nest location in situ should be investigated in future research.

Infrared thermography provides insight into the thermal properties of bird nests

Thermal properties of nests have been investigated using a variety of techniques. Infrared (IR) thermography has the advantage of being a non-invasive technique allowing the integrity of the nest wall to be retained during measurement. This study investigated the insulative properties of nests of the Eurasian Bullfinch (Pyrrhula pyrrhula), the Common Blackbird (Turdus merula) and the Song Thrush (Turdus philomelos) using IR thermography. Nests were inverted over a heat source and the temperature of the external nest surface was recorded. Bullfinch nests were less insulated than thrush nests. Including foil inside the nest cup decreased the amount of convection through the open walls of Bullfinch nests. Removal of the outer nest and cup lining of thrush nests only slightly decreased the degree of insulation offered by the nest indicating an important insulative role for the substantial ‘mud cup’ in these nests. The results suggested that the nest wall is not sealed and convection currents may a play a significant role in nest insulation. In conjunction with a steady-state heat source IR thermography is useful in assessing the insulative properties of bird nests.

Wild Peromyscus adjust maternal nest-building behaviour in response to ambient temperature

The heat dissipation limit (HDL) hypothesis suggests that energy output during lactation in mammals might be constrained by their ability to dissipate heat. This hypothesis predicts that wild mammals ought to adjust nest insulation in response to heat load, but these predictions have rarely been tested in wild mammals. Here we developed a simple score of nest-building for wild deer mice (Peromyscus maniculatus (Wagner, 1845)) on an ordinal scale from 0 to 4, based on three qualitative and easy to observe aspects of nest-building behaviour: bedding quality, nest shape, and mouse visibility. We used this measure to track 472 nest-building observations across 14 wild P. maniculatus that were brought into captivity and housed under pseudoambient temperatures across one reproductive event. Our observations of nest-building behaviour of the genus Peromyscus Gloger, 1841 provide varying support for the HDL hypothesis; there is a negative effect of ambient temperature on nest-building behaviour and lactating females became more sensitive to temperature as days post partum increased. However, females generally build more elaborate nests in lactation than other reproductive states and there are no effects of litter size, total pup mass, or days post partum on nest scores during lactation. Our observations have broad implications for quantifying behaviours in nest-building species and metabolic relationships in wild mammals.

First nesting account and breeding biology of “Gould's Inca” (Coeligena torquata omissa) in Manu National Park, Peru

ABSTRACT We present the first detailed nesting biology information for the highland Andean hummingbird Gould's Inca (Coeligena torquata omissa) at 2,200–3,000 m in Manu National Park, SE Peru. We found nine mossy cup-shaped nests lined with red fern scales containing nestlings or 1–2 synchronously hatching eggs. We obtained 17 days of internal nest temperatures from one nest, which exhibited wide circadian variation (range = 14.9–36.2 °C). We show that C. t. omissa meets the challenges of life and incubation at high altitudes by undertaking numerous (20.3 ± 0.4 trips/morning, range = 14–27) and short morning off-bouts (5.2 ± 0.2 min, range = 1–24 min). In the morning, the focal bird warmed the nest 1.7 times faster than in the afternoon where incubation bouts are long (17.8 ± 0.6 min, range = 4–56) and less frequent (10.2 ± 0.3 trips/afternoon). Incubation off-bout frequency and durations were not influenced by external temperature, suggesting strong nest insulation. On-bout durations increased with exter...

Nest Insulation in the European Robin (Erithacus Rubecula): Effects of Latitude and Construction Materials

The various roles that birds’ nests play are widely discussed and there is increasing interest in developing a better understanding of how they achieve these roles. The insulatory properties of nests have been investigated in a variety of songbird species and can vary with temperature and latitude. However, data are only available for a limited number of species. Here, we study the variations of the insulatory properties of European Robin ( Erithacus rubecula) nests in conjunction with the morphology of the nests and composition of the materials used in construction. As well as quantifying the materials used to construct these nests for the first time, we test the hypothesis that there is a latitudinal effect on nest insulation. Robin nests have a cup structure that is quite distinct, and easily separated, from the nest's base. Although the cup and nest's base shared similar construction materials discriminant analysis showed that they can be distinguished on the basis of their differing proportions of leaves. Insulatory values correlated with the mass of leaves in the cup but not latitude. Unlike other songbirds of comparable size that nest in nestboxes, European Robins appear to be able to use plant-derived materials rather than animal-derived materials to effectively insulate their nests.

Nest Surface Temperature Predicts Fledging Success of Blue TitsCyanistes caeruleusBut Not Great TitsParus major

Abstract. Studies investigating nest function in birds show that individual preferences and environmental temperature can affect the type and amount of materials used in their construction and, thus, how well insulated they are. Levels of insulation of bird nests may be important because this could impact on heat loss by adults and eggs during incubation, and by nestlings during rearing, which may in turn affect individual fitness. Here we used infrared (IR) thermography to measure the surface temperature of nests of Great Tits Parus major and Blue Tits Cyanistes caeruleus in situ during incubation to test the hypothesis that nest insulation predicts reproductive success. Previous studies of thermodynamics during incubation have focussed on factors, e.g. egg temperature, which do not directly measure the thermal conditions in the nest itself. Rarely applied to studies of avian incubation, IR thermography has yet to be used to quantify the thermal properties of nests with the incubating bird in situ. The r...

Do Great TitsParus majorNesting at High Altitudes Build Better Insulated Nests?

We investigated nest building behaviour of great tits Parus major in relation to altitudinal temperature gradients in Lower Austria. We hypothesised that colder environments at higher altitudes would select for a nest construction that increases thermal nest insulation. We collected data on nest architecture from 11 nests and on nest components from 33 nests along an altitudinal range (390 m) in 2010 to 2012, and experimentally measured specific insulation quality in 11 highly variable nests. Highly insulated nests included significantly more feathers than less well-insulated nests. Despite significantly lower temperatures at high altitude than at low altitude sites before clutch initiation, nest insulation quality did not increase with altitude. Moreover, we found that the date of clutch initiation was positively correlated with altitude. Birds may adjust to lower spring temperatures at high altitude by delaying clutch initiation, thus avoiding the need to invest in gathering highly insulating material.

Influence of incubation recess patterns on incubation period and hatchling traits in wood ducks Aix sponsa

Parental effects are influential sources of phenotypic variation in offspring. Incubation temperature in birds, which is largely driven by parental behavior and physiology, affects a suite of phenotypic traits in offspring including growth, immune function, stress endocrinology, and sex ratios. The importance of average incubation temperature on offspring phenotype has recently been described in birds, but parental incubation behaviors like the duration and frequency of recesses from the nest can be variable. There are few studies describing how or if thermal variation as a result of variable incubation affects offspring phenotype. We incubated wood duck Aix sponsa eggs under three different incubation regimes, based on patterns that occur in nature, which varied in off‐bout duration and/or temperature. We measured incubation period, morphometrics at hatching, and monitored growth and body condition for nine days post hatch. When average incubation temperature was allowed to drop from 35.9°C to 35.5°C as a result of doubled off‐bout duration, we found a significant 2 d extension in incubation period, but no effects on duckling hatch mass, or growth and body condition up to nine days post hatch. However, when average incubation temperatures were equivalent (35.9°C), doubling the duration of the simulated off‐bouts did not influence incubation period or any post hatch parameters. Our results suggest that if incubating parents can maintain favorable thermal environments in the nest via altered behavior (e.g. manipulating nest insulation) and/or physiology (e.g. heat production), parents may be able to avoid the costs of longer incubation periods resulting from increased off‐bout duration.

The effects of nest size and insulation on thermal properties of tree swallow nests

The nest environment can have important influences on incubation behavior and nestling development in birds. Nest thermal properties, particularly nest composition and size, can have a major influence on heat loss. To examine the role of nest size and insulation on clutch cooling rates, we collected tree swallow Tachycineta bicolor nests and measured the cooling rate of eggs in a controlled thermal environment. We also examined the thermal benefits of nest feathers by comparing the cooling rates of nests with and without feathers. Nests with more feather insulation and heavier, deeper cupped nests cooled at slower rates. In addition, nests with feathers cooled at much slower rates than did the same nests without feathers. Our results show that nest insulation and size play important roles in nest cooling rates, which may ultimately affect incubation costs and thus reproductive performance.

Is Carrying Feathers a Sexually Selected Trait in House Sparrows?

AbstractMales in several avian groups carry specialised materials as part of their courtship display. Females may vary their investment in reproduction in relation to their mates' attractiveness. The house sparrow (Passer domesticus) population on Dassen Island, South Africa, builds domed nests in the branches of trees. These nests are more or less spherical structures with a deep cup lined largely with feathers. When males collect feathers, they call to females and display the feather before it is added to the nest. We examined whether carrying feathers provides females with an index of male quality, which correlates with their subsequent investment in parental care. Additionally, we studied the potential importance of feathers in nest insulation. Feathers were collected mainly by males. Males also brought larger feathers, and deposited more feathers in nests, than females. Number of trips with feathers – which increased after feathers were experimentally removed from nests – and number of feathers brought varied among males. Volume of feathers influenced females' investment in reproduction and positively correlated with clutch size and chick feeding rates. We found more feathers during incubation and immediately after chicks hatched, when nest heat requirements peak. Furthermore, number of trips with feathers and number and size of feathers were greater during incubation. Our results suggest that this feather‐carrying behaviour by male house sparrows influences maternal reproduction investment and could be a kind of parental care per se by supplementing nest insulation.

Eggs in the Freezer: Energetic Consequences of Nest Site and Nest Design in Arctic Breeding Shorebirds

Birds construct nests for several reasons. For species that breed in the Arctic, the insulative properties of nests are very important. Incubation is costly there and due to an increasing surface to volume ratio, more so in smaller species. Small species are therefore more likely to place their nests in thermally favourable microhabitats and/or to invest more in nest insulation than large species. To test this hypothesis, we examined characteristics of nests of six Arctic breeding shorebird species. All species chose thermally favourable nesting sites in a higher proportion than expected on the basis of habitat availability. Site choice did not differ between species. Depth to frozen ground, measured near the nests, decreased in the course of the season at similar non-species-specific speeds, but this depth increased with species size. Nest cup depth and nest scrape depth (nest cup without the lining) were unrelated to body mass (we applied an exponent of 0.73, to account for metabolic activity of the differently sized species). Cup depth divided by diameter2 was used as a measure of nest cup shape. Small species had narrow and deep nests, while large species had wide shallow nests. The thickness of nest lining varied between 0.1 cm and 7.6 cm, and decreased significantly with body mass. We reconstruct the combined effect of different nest properties on the egg cooling coefficient using previously published quantitative relationships. The predicted effect of nest cup depth and lining depth on heat loss to the frozen ground did not correlate with body mass, but the sheltering effect of nest cup diameter against wind and the effects of lining material on the cooling coefficient increased with body mass. Our results suggest that small arctic shorebirds invest more in the insulation of their nests than large species.

Nest material selection affects nest insulation quality for the European ground squirrel (Spermophilus citellus)

Natural nests of the European ground squirrel (Spermophilus citellus) are constructed almost exclusively of fresh fescue (Festuca pseudovina: Poaceae). We performed laboratory experiments to understand the functional significance of preference of wild squirrels for nest material. We examined the factors contributing to nest quality by monitoring the construction and analyzing the composition of squirrel nests. As in the wild, squirrels showed strong preference for fescue during the laboratory tests and preferred fresh material to dry material. Because preference for fresh material was not expected, because high water content reduces insulation, we manipulated the moisture content of nests made from either fresh or dry fescue to determine how nest quality and moisture level contribute to insulation. We found that the insulation property of fresh grass nests was superior to nests constructed from dry grass only. Intracellular water in the nest material did not affect thermal conductance and insulation significantly. We concluded that fresh fescue provided a more flexible material that allows squirrels to construct nests with better insulation under both field and laboratory conditions.

The structure and function of nests of Long‐Tailed Tits Aegithalos caudatus

Summary The aim of this study was to investigate the structure and thermoregulatory function of nests of the Long‐Tailed Tit, Aegithalos caudatus. The feather lining of Long‐Tailed Tit nests represents a major portion (41%) of the total nest mass. The mass of feathers varied among nests and declined through the breeding season, but there was no seasonal loss of nest insulation quality because of increasing ambient temperatures. In an experiment to investigate the seasonal decline in the feather mass of nests, feathers were added to nests at an early stage of the lining phase of nest construction. Nest structure and insulating properties were then examined following nest completion. The total mass of feathers in treatment and control nests did not differ significantly and there was no significant difference in their nest insulation quality. Our results demonstrate that Long‐Tailed Tits adjust their nest‐building behaviour according to the nest's thermal environment. Moreover, nest structure appears to be adjusted to prevailing environmental conditions rather than being a function of feather availability or time constraints.

Male short-tailed field voles (Microtus agrestis) build better insulated nests than females.

Nest construction is an extremely widespread behaviour. In small endotherms the nest serves primarily to provide insulation, and thereby retard heat loss of the constructor, or its offspring. In arctic and temperate regions many small mammals build nests to protect themselves from low ambient temperatures. We measured the physical properties of nests built by short-tailed field voles Microtus agrestis that were kept in captivity under cold conditions. The most important factor influencing nest insulation was nest wall thickness; however, nests with thick walls also contained more nesting material. Insulative capacity of the nest did not reach an asymptote up to nests containing 20 g of material. Nest insulation was not correlated with resting metabolic rate, body mass or body composition of the vole that constructed the nest. However, nests built by males had greater insulation than those made by females; males also had significantly lower food intake rates when compared to females with nests. No significant difference was observed in either fat mass or whole animal thermal conductance between males and females. Thermal conductance did increase significantly with increasing body mass, although not with resting metabolic rate. Voles with nests for prolonged periods had lower food intakes than voles without nests. The absolute saving averaged 1.9 g and was independent of body mass. This was a 28% saving on intake for a 22-g vole but only an 18% saving for a 40-g individual. When voles had nests for short periods they used the energy they saved to reduce food intake and increase body mass.

Nest Morphology and Body Size of Ross' Geese and Lesser Snow Geese

--Arctic-nesting geese build large, insulated nests to protect developing embryos from cold ambient temperatures. Ross' Geese (Chen rossii) are about two-thirds the mass of Lesser Snow Geese (C. caerulescens caerulescens), have higher mass-specific metabolic rate, and maintain lower nest attentiveness, yet they hatch goslings with more functionally mature gizzards and more protein for their size than do Lesser Snow Geese. We compared nest size (a reflection of nest insulation) in four distinct habitats in a mixed breeding colony of Ross' Geese and Lesser Snow Geese at Karrak Lake, Northwest Territories, Canada. After adjusting measurements for nest-specific egg size and clutch size, we found that overall nest morphology differed between species and among habitats. Nest size increased progressively among heath, rock, mixed, and moss habitats. When nesting materials were not limiting, nests were smaller in habitats that provided cover from wind and precipitation than in habitats that did not provide cover Ross' Geese constructed relatively larger, more insulated nests than did Lesser Snow Geese, which may hasten embryonic development, minimize energy expenditure during incubation, and minimize embryonic ooling during recesses. We suggest hat relative differences in nest morphology reflect greater selection for Ross' Geese to improve nest insulation because of their smaller size (adults and embryos), higher massspecific metabolic rate, and lower incubation constancy. Received 13 May 1996, accepted 18 March 1997. IN BIRDS, HEAT IS SUPPLIED to developing embryos primarily by body warmth of the brooding parent (Afton and Paulus 1992). Heat loss from developing embryos and metabolic costs to parents can be minimized by increasing nest insulation. Nest insulation affects growth rates of nestlings (Winkler 1993), and probably affects embryonic growth as well, particularly in precocial species. Nest morphology and insulative properties of nests probably are influenced by body size, metabolic rates, incubation constancy, ambient thermal conditions, availability of nesting materials, and risk of predation (Moller 1984). Mass-specific metabolic rate and heat-transfer rate generally increase with declining body mass (Brody 1945, Lasiewski and Dawson 1967, Templeton 1970, Hill and Wyse 1989). Consequently, smaller species must assimilate nutrients at greater rates than larger species to maintain high metabolic rates. In addition, smaller species have less capacity to store endogenous nutrients than do larger 4 E-mail: kmccrac@unixl.sncc.lsu.edu species. Thus, body size is of profound consequence to incubating parents, which must fast during incubation unless they are fed by mates or auxiliaries (von Haartman 1958, Skutch 1962, Afton and Paulus 1992). If endogenous nutrients are not sufficient to meet energy requirements during incubation sessions, parents may recess from incubation in order to feed, thereby decreasing incubation constancy and exposing nests to predators and heat loss. Afton and Paulus (1992) reported that incubation constancy is positively related to body mass among all species of waterfowl (Anatidae), particularly among geese and swans. Within species, nest-building behavior may be adapted to maximize efficiency of nest insulation in response to cumulative embryonic metabolic rates, which are products of clutch size and embryo body size. Because of the relationship between surface area and volume, small eggs and clutches have greater rates of heat transfer than larger eggs and clutches, all else being equal. These factors are particularly important under the windy conditions that pre-