The effect of wind on the rate of heat loss from avian cup-shaped nests

Forced convection can significantly influence the heat loss from birds and their offspring but effects may be reduced by using sheltered micro-sites such as cavities or constructing nests. The structural and thermal properties of the nests of two species, the spiny-cheeked honeyeater (Acanthagenys rufogularis) and yellow-throated miner (Manorina flavigula), were measured in relation to three wind speeds. Nest dimensions differ between the two species, despite the similar body mass of the incubating adults, however nest conductance is comparable. As wind speed increases, so does the rate of heat loss from the nests of both species, and further still during incubation recesses. The significance of forced convection through the nest is a near-doubling in heat production required by the parent, even when incubating at relatively low wind speeds. This provides confirmation that selecting a sheltered nest site is important for avian reproductive success.     

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 diameter(2) 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.     

Structural support, not insulation, is the primary driver for avian cup-shaped nest design

The nest micro-environment is a widely studied area of avian biology, however, the contribution of nest conductance (the inverse of insulation) to the energetics of the incubating adult and offspring has largely been overlooked. Surface-specific thermal conductance (W °C(-1) cm(-2)) has been related to nest dimensions, wall porosity, height above-ground and altitude, but the most relevant measure is total conductance (G, W °C(-1)). This study is the first to analyse conductance allometrically with adult body mass (M, g), according to the form G = aM(b). We propose three alternative hypotheses to explain the scaling of conductance. The exponent may emerge from: heat loss scaling (M(0.48)) in which G scales with the same exponent as thermal conductance of the adult bird, isometric scaling (M(0.33)) in which nest shape is held constant as parent mass increases, and structural scaling (M(0.25)) in which nests are designed to support a given adult mass. Data from 213 cup-shaped nests, from 36 Australian species weighing 8-360 g, show conductance is proportional to M(0.25). This allometric exponent is significantly different from those expected for heat loss and isometric scaling and confirms the hypothesis that structural support for the eggs and incubating parent is the primary factor driving nest design.     

Nest reuse in arctic‐breeding shorebirds: an analysis of potential benefits and factors affecting the occurrence of this rare behavior

During a 13‐yr study near Utqia?vik (formerly Barrow), Alaska, we documented the prevalence of nest reuse in eight arctic‐breeding shorebirds. We evaluated whether nest reuse saved individuals time and energy, enhanced nest survival, or was related to nest density. We documented 208 (6.2%) cases of nest reuse among 3336 nesting attempts. Nest reuse occurred in all years but the first and in all species, with greatest reuse in semipalmated sandpiper (10.9%) and American golden‐plover (10.0%). While most cases of nest reuse occurred with conspecifics, many cases of heterospecific nest reuse were also observed, indicating high niche overlap in nest site preferences among species. We found that individuals reusing old nests may have benefited by nesting earlier, but nest reuse did not generally enhance nest survival. A significant positive relationship was also found between nest reuse and nest density at the community level and for four of the eight species, suggesting high inter‐ or intraspecific competition combined with limited suitable nest sites may force individuals to reuse old nests. Our observations also suggest that upland nesting species may be the most dependent on old nest sites. Preferential development of these sites may therefore have a previously unknown detrimental effect on these species, although further study is needed to better determine the impact of such habitat loss.     

Contributions of feather microstructure to eider down insulation properties

Insulation is an essential component of nest structure that helps provide incubation requirements for birds. Many species of waterfowl breed in high latitudes where rapid heat loss can necessitate a high energetic input from parents and use down feathers to line their nests. Common eider Somateria mollissima nest down has exceptional insulating properties but the microstructural mechanisms behind the feather properties have not been thoroughly examined. Here, we hypothesized that insulating properties of nest down are correlated to down feather (plumule) microstructure. We tested the thermal efficiency (fill power) and cohesion of plumules from nests of two Icelandic colonies of wild common eiders and compared them to properties of plumules of wild greylag goose Anser anser. We then used electron microscopy to examine the morphological basis of feather insulating properties. We found that greylag goose down has higher fill power (i.e. traps more air) but much lower cohesion (i.e. less prone to stick together) compared to common eider down. These differences were related to interspecific variation in feather microstructure. Down cohesion increased with the number of barbule microstructures (prongs) that create strong points of contact among feathers. Eider down feathers also had longer barbules than greylag goose down feathers, likely increasing their air‐trapping capacity. Feather properties of these two species might reflect the demands of their contrasting evolutionary history. In greylag goose, a temperate, terrestrial species, plumule microstructure may optimize heat trapping. In common eiders, a diving duck that nests in arctic and subarctic waters, plumule structure may have evolved to maximize cohesion over thermal insulation, which would both reduce buoyancy during their foraging dives and enable nest down to withstand strong arctic winds.     

Nest site attributes and temporal patterns of northern flicker nest loss: effects of predation and competition

To date, most studies of nest site selection have failed to take into account more than one source of nest loss (or have combined all sources in one analysis) when examining nest site characteristics, leaving us with an incomplete understanding of the potential trade-offs that individuals may face when selecting a nest site. Our objectives were to determine whether northern flickers (Colaptes auratus) may experience a trade-off in nest site selection in response to mammalian nest predation and nest loss to a cavity nest competitor (European starling, Sturnus vulgaris). We also document within-season temporal patterns of these two sources of nest loss with the hypothesis that flickers may also be constrained in the timing of reproduction under both predatory and competitive influence. Mammalian predators frequently depredated flicker nests that were: lower to the ground, less concealed by vegetation around the cavity entrance and at the base of the nest tree, closer to coniferous forest edges and in forest clumps with a high percentage of conifer content. Proximity to coniferous edges or coniferous trees increased the probability of nest predation, but nests near conifers were less likely to be lost to starlings. Flickers may thus face a trade-off in nest site selection with respect to safety from predators or competitors. Models suggested that peaks of nest predation and nest loss to eviction occurred at the same time, although a competing model suggested that the peak of nest loss to starlings occurred 5 days earlier than the peak of mammalian predation. Differences in peaks of mammalian predation and loss to starlings may constrain any adjustment in clutch initiation date by flickers to avoid one source of nest loss.     

Long‐term and large‐scale analyses of nest predation patterns in Australian songbirds and a global comparison of nest predation rates

Juvenile mortality is one of crucial drivers of life‐history evolution, and predation is the main cause of nest loss in birds. Thus, understanding how nest predation and failure vary in nature is important for understanding life history evolution and, moreover, for effective conservation. We used published data and unpublished records to study factors influencing nest predation and total failure in 138 populations of 90 species of Australian songbirds. Daily predation (average 2.0% d^?1) and failure rates (2.9%) increased from temperate regions to the tropics, over the last four decades, and were lowest in temperate south‐western Australia. Predation and failure were higher in smaller species, and failure rates were lower in species with closed nests than in species with open nests. There was no effect of nest height or nest site (ground, shrub, canopy) or social organization on nest predation or failure rates. Nest predation caused on average 72% of total nest failure, similar to other tropical, subtropical, and temperate areas. Our study spanning from the tropics to temperate regions and using > 10 000 nests confirmed that tropical birds faced higher nest failure rates. We identified an increase in nest depredation rates in the last four decades in Australia, suggesting that a large‐scale ecological phenomenon must be responsible. It may include increases in predator abundances and/or ranges, possibly connected with human‐caused habitat change. A global comparison of nest failure rates confirmed that predation is the main source of nest mortality in songbirds worldwide. We discuss implications of our results for the evolution of reproductive strategies and for the conservation of Australian birds.     

Island shape,size, and isolation affect nest‐site selection by Little Terns

ABSTRACT Populations of many seabirds and other species that nest along coasts are declining due to habitat degradation and loss. An improved understanding of the species‐specific factors that determine nest density across a landscape is therefore critical for conservation efforts. We examined factors that affected the density (number per hectare) and abundance (number at a sampling site) of nests of Little Terns (Sternula albifrons) on the Sinai Peninsula, Egypt. Terns preferred to nest on islands rather than the mainland, with islands constituting 64% of the area surveyed, but containing 99% of the 439 tern nests we found. Nest densities were highest on islands that were small, located at moderate distances from the mainland, and irregularly shaped or elongated. Most nests (69%) were on islands with areas < 3 ha, although these islands represented < 5% of total island area, and islands with the highest nest densities were 80–300 m from the mainland. Terrestrial predators were more likely to occur on larger islands, visiting three of the largest four islands. Most tern nests were within 1 m of shorelines, causing island perimeter to be a strong influence on nest density. Island shape was the only factor that significantly affected nest abundance, with more nests on islands with relatively long perimeters for their size. Our results suggest that protection or creation of relatively small, slender islands at moderate distances from shore may be an effective means of increasing the number of breeding sites for Little Terns. Although not generally considered a potential determinant of nest site preferences for seabirds, island shape is likely to be important for species that prefer sites adjacent to water, including species that nest on beaches and seaside cliffs.     

Relative effects of nest size and site on the risk of predation in open nesting passerines

Large nests may incur fitness cost in terms of conspicuousness to predators, but the effect of nest size on predation risk can be confounded by effects of nest site and parental characteristics. I examined relative effects of nest size and placement by experimentally exchanging subsets of inactive nests baited with artificial clutches, among three open-cup nesting passerine species characterized by different nest size, placement and predation rate. The prediction that increasing nest size (original nest replaced by nest of the larger species) would increase predation, while decreasing nest size (replacement by nest of the smaller species) would decrease predation, relative to control (replacement by conspecific nest) was not supported in any species. The prediction that predation should be higher for large nests compared to small ones, even after exchanging nests among species-specific sites, was not supported. Predation rate differed among species (combined site/nest effect) before manipulation, whereas only the effect of nest site was significant after manipulation. This means that predation differed between species-specific nest sites, irrespective of the nest placed at these sites, but not between large (thrush) and small (warbler) nests, irrespective of their placement. Results do not suggest that nest predation selects directionally for smaller nest size. This conclusion could be specific to the study system characterized by high nest densities and high predation rate.     

Beach erosion and nest site selection by the leatherback sea turtle Dermochelys coriacea (Testudines: Dermochelyidae) and implications for management practices at Playa Gandoca, Costa Rica

Leatherback sea turtles (Dermochelys coriacea) nest on dynamic, erosion-prone beaches. Erosive processes and resulting nest loss have long been presumed to be a hindrance to clutch survival. In order to better understand how leatherbacks cope with unstable nesting beaches, I investigated the role of beach erosion in leatherback nest site selection at Playa Gandoca, Costa Rica. I also examined the potential effect of nest relocation, a conservation strategy in place at Playa Gandoca to prevent nest loss to erosion, on the temperature of incubating clutches. I monitored changes in beach structure as a result of erosion at natural nest sites during the time the nest was laid, as well as in subsequent weeks. To investigate slope as a cue for nest site selection, I measured the slope of the beach where turtles ascended from the sea to nest, as well as the slopes at other random locations on the beach for comparison. I examined temperature differences between natural and relocated nest sites with thermocouples placed in the sand at depths typical of leatherback nests. Nests were distributed non-randomly in a clumped distribution along the length of the beach and laid at locations that were not undergoing erosion. The slope at nest sites was significantly different than at randomly chosen locations on the beach. The sand temperature at nest depths was significantly warmer at natural nest sites than at locations of relocated nests. The findings of this study suggest leatherbacks actively select nest sites that are not undergoing erosive processes, with slope potentially being used as a cue for site selection. The relocation of nests appears to be inadvertently cooling the nest environment. Due to the fact that leatherback clutches undergo temperature-dependent sex determination, the relocation of nests may be producing an unnatural male biasing of hatchlings. The results of this study suggest that the necessity of relocation practices, largely in place to protect nests from erosion, should be reevaluated to ensure the proper conservation of this critically endangered species.     

No evidence for recognition errors in Acrocephalus warblers

Failure to recognise own eggs (recognition errors) may be an important selective force behind acceptance of parasitic eggs, leading to a balance between rejecters and acceptors in a host population (the equilibrium hypothesis). We predicted that recognition errors should occur frequently among host species with intermediate rejection rates, whose rejection behaviour shows many conditional responses. The reed warbler Acrocephalus scirpaceus and great reed warbler A. arundinaceus fulfil these requirements. These two species were therefore used in an experiment where host birds were exposed to a common cuckoo Cuculus canorus dummy, either <2 m or 5–10 m from the nest, at fishponds in southern Moravia (Czech Republic). The hosts responded to the cuckoo dummy, great reed warblers being much more aggressive than reed warblers, and both species being more aggressive towards the dummy when it was close to the nest than when it was farther away. We furthermore predicted that there should be more eggs rejected (ejected or nest abandoned) due to recognition errors among hosts exposed to a dummy close to the nest than among both those exposed to a dummy farther away from the nest and towards controls not exposed to cuckoo dummies. When comparing egg loss between groups of birds that were exposed to a cuckoo dummy with those that were not, we found no significant difference. However, partial egg loss was frequent among hosts in the studied population, most probably due to cuckoo depredation. We discuss why there were no detectable recognition errors in the studied population, when other researchers have claimed to have found such errors in host populations elsewhere.     

Sources of variation in the nesting success of understory tropical birds

Survival of offspring is a key fitness component and, for birds, the threat of predation on nests is especially influential. Data on rates of nest success from tropical regions are comparatively few, conservation‐relevant, and essential for assessing the validity of models comparing the life histories and behavior or birds across latitudinal gradients. We monitored over 2 000 nests in the lowland forests of central Panama and, using the logistic exposure to model the fate of nests, explored the importance of variation in rate of nest success according to type of nest, height of nests, among years, in early versus late nests, and at different stages of the nest cycle. Analyses of over 1 400 nests for 18 species revealed considerable variation among species in the daily survival rate of nests (range among 18 species=0.91 to 0.98), but nest type and stage of the nesting cycle were generally influential on the probability of nest success. Cavity or enclosed nesters experienced greater nest success than open cup nesters and rates of nest loss were generally greatest in the nestling stage. We found limited evidence that height of nests affected probability of success, but no indication that timing of nesting effort was influential. Despite the occurrence of a severe ENSO event during our sampling, annual variation in nest success was not consistent among species. Interspecific variation in the rates and patterns of nest predation in our study, coupled with reports of high rates of nest loss at temperate latitudes, lead us to question long standing assumptions about latitudinal trends in rates of nest loss. We urge further work to understand the implications of nest predation on the evolutionary ecology of tropical birds.     

Heat loss and site-dependent fecundity in chinstrap penguins (Pygoscelis antarctica)

We study the effects of heat loss and nest site quality on fecundity in a chinstrap penguin (Pygoscelis antarctica) colony on Deception Island, Antarctica. During the austral summers of 1990–1991, 1993–1994, and 1995–1996, 441 randomly selected nests were analyzed. Penguins breeding in the center of subcolonies hatched significantly earlier and had larger broods and than those nesting near or at the edge of the subcolonies. These differences, however, were significantly affected by interaction between year and nest location, being highly significant in colder years, when peripheral nests produce fewer young. Analysis of the rate of heat loss showed that penguins breeding at the edge of subcolonies lost heat twice as rapidly as those breeding in the interior of the subcolony. In a re-occupation experiment, evacuated center nests were re-occupied almost ten times as rapidly as edge nests. An analysis of mean fecundity in the period 1991–1996 and mean wind chill suggested that most of the variability in fecundity among years was related to differences in the rate of heat loss. Subcolonies tend to be as circular as possible, thereby decreasing the proportion of edge nests as the size of subcolony increases. Our results support the site-dependent fecundity hypothesis.     

Potential influences of climate and nest structure on spotted owl reproductive success: a biophysical approach

Many bird species do not make their own nests; therefore, selection of existing sites that provide adequate microclimates is critical. This is particularly true for owls in north temperate climates that often nest early in the year when inclement weather is common. Spotted owls use three main types of nest structures, each of which are structurally distinct and may provide varying levels of protection to the eggs or young. We tested the hypothesis that spotted owl nest configuration influences nest microclimate using both experimental and observational data. We used a wind tunnel to estimate the convective heat transfer coefficient (h(c)) of eggs in 25 potential nest configurations that mimicked 2 nest types (top-cavity and platform nests), at 3 different wind speeds. We then used the estimates of h(c) in a biophysical heat transfer model to estimate how long it would take unattended eggs to cool from incubation temperature (～36°C) to physiological zero temperature (PZT; ～26°C) under natural environmental conditions. Our results indicated that the structural configuration of nests influences the cooling time of the eggs inside those nests, and hence, influences the nest microclimate. Estimates of time to PZT ranged from 10.6 minutes to 33.3 minutes. Nest configurations that were most similar to platform nests always had the fastest egg cooling times, suggesting that platform nests were the least protective of those nests we tested. Our field data coupled with our experimental results suggested that nest choice is important for the reproductive success of owls during years of inclement weather or in regions characterized by inclement weather during the nesting season.     

Snowy plover nest survival in Kansas and effective management to counter negative effects of precipitation

Snowy plovers (Charadrius nivosus) are a species of conservation concern throughout North America and listed as a threatened species in Kansas. Management to minimize the effects of flooding and predation were implemented at Kansas breeding sites in the 1980s to encourage reproductive success. However, the effectiveness of those strategies and the effect of other variables that may influence nest survival have not been formally assessed. We used Program MARK to model the daily survival rate (DSR) of 317 snowy plover nests with 14 habitat- and management-related covariates to identify factors that influence nest survival and examine the efficacy of current management practices. In 2005 and 2006, we monitored nests and collected habitat data at the 2 known breeding sites in Kansas, Quivira National Wildlife Refuge (NWR) and Cheyenne Bottoms Wildlife Area (WA). Overall DSR was greater at Quivira NWR in 2006 (0.954) than at Cheyenne Bottoms WA (0.917) and Quivira NWR (0.942) in 2005. We developed 88 candidate models of which 4 competing models (ΔAIC_c < 2) were identified. We selected the most parsimonious model (K = 14, w_i = 0.23) as the remaining 3 included covariates deemed biologically uninformative. This model included the effect of study site and year on a quadratic time trend, and included covariates quantifying nest age; precipitation; the proportion of gravel, rock, and vegetation at nests; occurrence within an electric fence and within 20 m of a road; occurrence on a human-constructed nest mound; and adult capture during incubation. We found a strong positive relationship between the use of nest mounds and DSR, and a strong negative relationship between precipitation and DSR. We also found a strong positive relationship between DSR and the proportion of vegetation at nest sites, the occurrence of a nest within an electric fence, and adult capture at a nest. We noted a strong negative relationship between DSR and occurrence within 20 m of a road. However, we found that DSR was not sensitive to the proportion of vegetation at a nest, occurrence within an electric fence or within 20 m of a road, and to adult capture at a nest in light of covariates quantifying precipitation and the use of nest mounds. We found weak support for a positive relationship between DSR, nest age, and the proportion of gravel and rock at nests. Our results indicate that large rainfall events are a major source of snowy plover nest loss in Kansas that can be mitigated by the construction of nest mounds. Limited influence of environmental variables found to influence nest survival at other breeding sites suggests that threats to snowy plover nest survival are site specific and managers should assess local sources of nest loss prior to implementing management strategies to improve reproductive success. © 2012 The Wildlife Society.     

Experimental enlargement of nest size does not increase risk of predation or brood parasitism in the Great Reed Warbler Acrocephalus arundinaceus

We assessed whether nest size affects the probability of nest loss using dyads of large and small (large being twice the size of small) inactive Great Reed Warbler Acrocephalus arundinaceus nests placed at similar sites in Great Reed Warbler territories. Large nests were not predated significantly more frequently than small nests. Experimentally enlarged active Great Reed Warbler nests suffered non‐significantly higher predation compared with non‐manipulated control nests. Our experiments did not support the nest‐size hypothesis and suggested that nest size does not appear to be a factor affecting the risk of nest predation in this species. The probability of brood parasitism by the Common Cuckoo Cuculus canorus was also unaffected by experimental nest enlargement, supporting the commonly accepted hypothesis that the Common Cuckoo searches for suitable host nests by host activity during nest building rather than nest size.     

Coastal saltpans are a good alternative breeding habitat for Kentish plover Charadrius alexandrinus when umbrella species are present

The loss and degradation of natural habitats in coastal areas worldwide has adversely affected many waterbird species, changing their breeding distribution and reducing their productivity. Anthropogenic habitats such as saltpans can provide alternative or complementary habitats for waterbirds and mitigate the increasing human impact on natural coastal habitats. Unvegetated linear paths between salt ponds are used by ground‐nesting waterbird species to breed but their linear structure may facilitate the detection of nests by predators. This negative effect may, however, be counterbalanced by the advantages of breeding in mixed colonies. To evaluate the importance and the risks of breeding in saltpans we used the Kentish plover Charadrius alexandrinus breeding in coastal saltpans of southern Portugal as a model species. Specifically, we assessed the role of nest‐site characteristics, predation and nesting proximity to species with aggressive antipredator behaviour (black‐winged stilt Himantopus himantopus) on their breeding success. Kentish plovers selected nest‐sites on the edges of paths, with about 20% of water around the nest, and a mean visibility of more than 72%; however nest‐site characteristics were not correlated with nesting success. Predation was the main cause of nest loss in saltpans (42%); carrion crows Corvus corone were responsible for most daylight nest predation (58%) and red fox (Vulpes vulpes; 73%) for night predation. An 8‐yr monitoring plan of a Kentish plover population showed a linear increase in their breeding success as their breeding season increasingly overlapped with that of the black‐winged stilt. An experiment with artificial nests showed a significant increase in the number of exposure days (7 to 12) when these nests were within close distance of black‐winged stilt nests. Overall, our results showed that saltpans are an important alternative breeding habitat for the Kentish plover, especially for the maintenance of mixed species colonies.     

Simple and farmer-friendly bumblebee conservation: Straw bales as nest sites in agricultural landscapes

Many bumblebee species are declining due to a loss of semi-natural habitats in agricultural landscapes resulting in diminished forage and nest sites. Anecdotal experience indicates that bumblebees nest in straw bales, but scientific evidence is lacking. We spent 250 h screening for bumblebee nests in 1255 straw bales and ten straw stacks belonging to 58 farms in two intensively farmed Swedish regions and recorded nests, nest traffic, and straw characteristics. We supplemented the straw screening with screening of control areas, without straw, that were selected in similar environments as the areas with straw. We observed 45 bumblebee nests (including potential nests where a single bumblebee flew in or out of the straw) of eight species/species groups, including one red-listed, in or directly adjacent to the straw at 26 of the farms. Nests were mainly found in partly decayed straw and bales placed together. We found no nests in control areas. Based on our results, we suggest that straw can be used as an easy, cheap and efficient intervention to increase the availability of bumblebee nest sites in agricultural landscapes. Considering the costs and benefits of the alternatives, we conclude that straw addition has advantages over commercial bumblebee colonies for crop pollination purposes and over artificial nest boxes for conservation purposes.     

Thermoregulation of termite mounds: what role does ambient temperature and metabolism of the colony play?

Summary: Termites are well known for their ability to regulate the environment of their nest such as temperature and humidity. The influence of fluctuating ambient temperature and mound characteristics on mean nest temperature and daily fluctuation of nest temperature was analysed quantitatively in the fungus-cultivating, mound-building termite Macrotermes bellicosus (Macrotermitinae) in the savanna of the Comoé National Park (Côte d'Ivoire). Additionally, the nest temperatures of inhabited and uninhabited mounds were compared to analyse the contribution of ambient temperature to nest temperature in relation to metabolic heat production of the termites and their fungi. Mound structure alone resulted in a relatively constant nest temperature. Abiotic heat production via solar radiation alone yielded nest temperatures that corresponded to mean ambient temperatures. However, only the production of metabolic heat by the termites and the fungi increased these temperatures to the actual nest temperature. Therefore, and due to the high heat capacities of the mounds, large colonies (mound height above 2.0 m) had higher nest temperatures than smaller ones. Only large colonies attain constant nest temperatures of 30 °C that are largely independent from ambient temperatures and optimal for the growth and development of the termites and their fungi.     

Interpopulation Comparisons of Antipredator Defense Behavior of the Thorn‐Tailed Rayadito (Aphrastura spinicauda)

The high impact of predators on breeding success has favored the evolution of diverse behavioral strategies that enhance both individual survivorship and reproductive success. Self‐ and nest‐defense against predators are inter‐related behaviors that vary according to life history traits and the risks involved in defense. In addition, interpopulation differences in the composition of predator guilds, and hence predation pressures, may result in geographic variation in antipredator defense strategies. It is generally accepted that the absence of predators could drive the loss of antipredator behavior. However, it is unclear how species respond if one type of predator (e.g., a specialized nest predator) is absent but others are present. The multipredator hypothesis suggests that the absence of only a few predators is not sufficient to drive a change or loss of antipredator behavior in a species. We examined interpopulation variation in defense behavior in the thorn‐tailed rayadito (Aphrastura spinicauda), a suboscine bird inhabiting a wide latitudinal range in southern South America. In populations where nest predators were either present or absent, we examined the responses of rayaditos to an adult predator (an owl) and to a nest predator (a marsupial). Owl‐defense behavior involved more intense and longer responses than marsupial‐defense. This suggests that adult rayaditos may allocate more energy to defend themselves than their young. However, because the presence of adult predators near the nest also prevents parents from feeding the nestlings, owl‐defense may simultaneously represent a form of nest‐defense. No differences in marsupial‐defense were found among the three sampled populations, despite the absence of native mammalian nest predators in one population. The lack of variation in nest‐defense behavior provides some support for the multipredator hypothesis, although the high interpopulation gene flow in this species could also contribute to this behavior's maintenance.