Population Of Painted Turtles Research Articles

Mother knows best: nest-site choice homogenizes embryo thermal environments among populations in a widespread ectotherm.

Species with large geographical ranges provide an excellent model for studying how different populations respond to dissimilar local conditions, particularly with respect to variation in climate. Maternal effects, such as nest-site choice greatly affect offspring phenotypes and survival. Thus, maternal behaviour has the potential to mitigate the effects of divergent climatic conditions across a species' range. We delineated natural nesting areas of six populations of painted turtles (Chrysemys picta) that span a broad latitudinal range and quantified spatial and temporal variation in nest characteristics. To quantify microhabitats available for females to choose, we also identified sites within the nesting area of each location that were representative of available thermal microhabitats. Across the range, females nested non-randomly and targeted microhabitats that generally had less canopy cover and thus higher nest temperatures. Nest microhabitats differed among locations but did not predictably vary with latitude or historic mean air temperature during embryonic development. In conjunction with other studies of these populations, our results suggest that nest-site choice is homogenizing nest environments, which buffers embryos from thermally induced selection and could slow embryonic evolution. Thus, although effective at a macroclimatic scale, nest-site choice is unlikely to compensate for novel stressors that rapidly increase local temperatures. This article is part of the theme issue 'The evolutionary ecology of nests: a cross-taxon approach'.

Modeling Onset of Hourly Nesting Activity in a Freshwater Turtle Using Abiotic Variables and Physiological Capacity

Nesting is an essential, yet variable, reproductive behavior in most oviparous organisms. Although many factors conceivably influence nesting behaviors, it is unclear which factors strongly influence terrestrial nest timing in aquatic nonavian reptiles. As climate is changing rapidly, understanding the relative influences of biotic and abiotic factors on nesting behaviors may yield important information on future changes in daily and seasonal nesting activity. We collected hourly data to examine the significance of local weather conditions to the timing of within-season nesting activity in a large population of Painted Turtles (Chrysemys picta). We quantified nesting activity as the ratio of females who nested to all females who could nest in each hour, adjusting the size of the denominator to include the time required to shell a subsequent egg clutch. We then used zero-inflated models to identify potential weather predictors of presence/absence of nesting activity and strength of nesting responses (i.e., the fraction of turtles nesting that could nest). Higher temperatures and rainfall predicted stronger nesting responses, whereas lower temperatures and no rainfall predicted the absence of nesting activity, indicating that both temperature and rainfall are important cues in within-season nesting phenology. Our study enhances our understanding of abiotic influences on the terrestrial nesting behavior of aquatic organisms.

Breadth of the thermal response captures individual and geographic variation in temperature‐dependent sex determination

Abstract Population‐scale responses of key ecological traits to local environmental conditions provide insight into their adaptive potential. In species with temperature‐dependent sex determination (TSD), short‐term, individual developmental responses to the incubation environment have long‐term consequences for populations. We took a model‐based approach to study within‐ and among‐population variation in the physiological components of TSD in 12 populations of painted turtles (Chrysemys picta). We used laboratory and field incubation data to quantify variation in thermal reaction norms at both population and clutch scales, focusing on the pivotal temperature that produces a 1:1 sex ratio (P) and the transitional range of incubation temperatures (TRTs) that produce mixed sex ratios. Defying theoretical expectations, among‐population variation in P was not convincingly explained by geography or local thermal conditions. However, within some populations, P varied by >5°C at the clutch scale, indicating that the temperature sensitivity of gonadal differentiation can vary substantially among individual nesting females. In addition, the TRT was wider at lower latitudes, suggesting responsiveness to local incubation conditions. Our results provide a potential explanation for discrepancies observed between constant‐temperature experimental results and outcomes of fluctuating incubation conditions experienced in natural nests, exposing important knowledge gaps in our understanding of local adaptation in TSD and identifying shortcomings of traditional laboratory studies. Understanding individual variation and the timing of gonadal differentiation is likely to be far more useful in understanding local adaptation than previously acknowledged. A free Plain Language Summary can be found within the Supporting Information of this article.

Thermoregulation in a North Temperate Population of Midland Painted Turtles (Chrysemys picta marginata): Temporal Patterns and Intersexual Differences

The regulation of body temperature (Tb) is important for energy acquisition in ectotherms but may be challenging and costly. We studied the thermal biology of a north temperate population of Midland Painted Turtles (Chrysemys picta marginata) in a small pond in central Michigan. Cycling of mean daily body temperature (Tb) began as early as March and continued through October. The thermoregulatory setpoint range was 25–31°C, and turtles largely maintained Tb near or within the Tset range throughout the day between May and September. Mean hourly Tb was slightly ( 0.5°C) higher on sunny days when compared to cloudy days. Turtles had relatively low investment in thermoregulation during the fall months potentially as an energy conservation measure prior to hibernation. Thermal exploitation values (Ex) indicated that our turtles spent as much, or more, time within the Tset range than individuals in other north temperate Painted Turtle populations and other reptile species studied to date, which was most likely due to the relatively high thermal quality of our aquatic environment. We did not find differences between males and females in terms of thermal exploitation, which suggests little or no differences in thermal energy needs that might be associated with potential energetic allocations to reproductive activities (e.g., ovarian development or mate location in males).

Phenotypic plasticity of nest timing in a post-glacial landscape: how do reptiles adapt to seasonal time constraints?

Life histories evolve in response to constraints on the time available for growth and development. Nesting date and its plasticity in response to spring temperature may therefore be important components of fitness in oviparous ectotherms near their northern range limit, as reproducing early provides more time for embryos to complete development before winter. We used data collected over several decades to compare air temperature and nest date plasticity in populations of painted turtles and snapping turtles from a relatively warm environment (southeastern Michigan) near the southern extent of the last glacial maximum to a relatively cool environment (central Ontario) near the northern extent of post-glacial recolonization. For painted turtles, population-level differences in reaction norm elevation for two phenological traits were consistent with adaptation to time constraints, but no differences in reaction norm slopes were observed. For snapping turtle populations, the difference in reaction norm elevation for a single phenological trait was in the opposite direction of what was expected under adaptation to time constraints, and no difference in reaction norm slope was observed. Finally, among-individual variation in individual plasticity for nesting date was detected only in the northern population of snapping turtles, suggesting that reaction norms are less canalized in this northern population. Overall, we observed evidence of phenological adaptation, and possibly maladaptation, to time constraints in long-lived reptiles. Where present, (mal)adaptation occurred by virtue of differences in reaction norm elevation, not reaction norm slope. Glacial history, generation time, and genetic constraint may all play an important role in the evolution of phenological timing and its plasticity in long-lived reptiles.

Do trade-offs between predation pressures on females versus nests drive nest-site choice in painted turtles?

Predation strongly influences reproductive behaviours because reproducing individuals must balance mortality risks to themselves and to their offspring. In many freshwater turtles, the nest predation risk decreases with nest distance from water, whereas the predation risk to females increases farther from water. To determine whether predation pressure influences the distance from water at which female turtles nest, we measured predation pressure on nesting females and on nests, as well as the distances of nests to water, in two populations of painted turtles. Using models, we found that female survival in both populations was high and did not vary with distance from water. Nest survival was also uncorrelated with nest distance to water, although it was significantly lower than adult survival in both populations and was only 1.2% in one population. Our results suggest that nest sites are not predictably safe from predators. Instead, turtles may hedge their bets by nesting over a wide range of distances from water because any distance is risky for nests and no distance is particularly risky for the nesting female. We suggest that other factors, such as suitable incubation conditions and/or post-emergence hatchling survival, probably play a larger role than predation in driving nest-site choice in painted turtles.

Swimming against the tide: resilience of a riverine turtle to recurrent extreme environmental events.

Extreme environmental events (EEEs) are likely to exert deleterious effects on populations. From 1996 to 2012 we studied the nesting dynamics of a riverine population of painted turtles (Chrysemys picta) that experienced seven years with significantly definable spring floods. We used capture-mark-recapture methods to estimate the relationships between more than 5 m and more than 6 m flood events and population parameters. Contrary to expectations, flooding was not associated with annual differences in survival, recruitment or annual population growth rates of the adult female segment of the population. These findings suggest that female C. picta exhibit resiliency to key EEE, which are expected to increase in frequency under climate change.

Effective heritability of targets of sex-ratio selection under environmental sex determination

Selection is expected to maintain primary sex ratios at an evolutionary equilibrium. In organisms with temperature-dependent sex determination (TSD), targets of sex-ratio selection include the thermal sensitivity of the sex-determining pathway (hereafter, sex determination threshold) and nest-site choice. However, offspring sex may be canalized for nests located in thermally extreme environments; thus, genetic variance for the sex determination threshold is not expressed and is invisible to direct selection. The concept of 'effective heritability' accounts for this dependence and provides a more realistic prediction of the expected evolutionary response to selection in the wild. Past estimates of effective heritability of the sex determination threshold, which were derived from laboratory data, suggested that the potential for the sex determination threshold to evolve in the wild was extremely low. We re-evaluated original estimates of this parameter by analysing field-collected measures of nest temperatures, vegetation cover and clutch sex ratios from nests in a population of painted turtles (Chrysemys picta). We coupled these data with measurements of broad-sense heritability of the sex determination threshold in C. picta, using an experiment that splits clutches of eggs between a constant temperature (i.e. typical laboratory incubation) and a daily fluctuating temperature (i.e. similar to natural nests) with the same mean. We found that (i) the effective heritability of the sex determination threshold appears to have been historically underestimated and the effective heritability of nest-site choice has been overestimated and (ii) significant family-by-incubation treatment interaction exists for sex for C. picta between constant- and fluctuating-temperature regimes. Our results suggest that the thermal sensitivity of the sex-determining pathway may play a larger, more complex role in the microevolution of TSD than traditionally thought.

Climate and predation dominate juvenile and adult recruitment in a turtle with temperature‐dependent sex determination

Conditions experienced early in life can influence phenotypes in ecologically important ways, as exemplified by organisms with environmental sex determination. For organisms with temperature-dependent sex determination (TSD), variation in nest temperatures induces phenotypic variation that could impact population growth rates. In environments that vary over space and time, how does this variation influence key demographic parameters (cohort sex ratio and hatchling recruitment) in early life stages of populations exhibiting TSD? We leverage a 17-year data set on a population of painted turtles, Chrysemys picta, to investigate how spatial variation in nest vegetation cover and temporal variation in climate influence early life-history demography. We found that spatial variation in nest cover strongly influenced nest temperature and sex ratio, but was not correlated with clutch size, nest predation, total nest failure, or hatching success. Temporal variation in climate influenced percentage of total nest failure and cohort sex ratio, but not depredation rate, mean clutch size, or mean hatching success. Total hatchling recruitment in a year was influenced primarily by temporal variation in climate-independent factors, number of nests constructed, and depredation rate. Recruitment of female hatchlings was determined by stochastic variation in nest depredation and annual climate and also by the total nest production. Overall population demography depends more strongly on annual variation in climate and predation than it does on the intricacies of nest-specific biology. Finally, we demonstrate that recruitment of female hatchlings translates into recruitment of breeding females into the population, thus linking climate (and other) effects on early life stages to adult demographics.

Inheritance of nesting behaviour across natural environmental variation in a turtle with temperature-dependent sex determination

Nesting behaviour is critical for reproductive success in oviparous organisms with no parental care. In organisms where sex is determined by incubation temperature, nesting behaviour may be a prime target of selection in response to unbalanced sex ratios. To produce an evolutionary change in response to sex-ratio selection, components of nesting behaviour must be heritable. We estimated the field heritability of two key components of nesting behaviour in a population of painted turtles (Chrysemys picta) with temperature-dependent sex determination by applying the 'animal model' to a pedigree reconstructed from genotype data. We obtained estimates of low to non-detectable heritability using repeated records across all environments. We then determined environment-specific heritability by grouping records with similar temperatures for the winter preceding the nesting season, a variable known to be highly associated with our two traits of interest, nest vegetation cover and Julian date of nesting. The heritability estimates of nest vegetation cover and Julian date of nesting were qualitatively highest and significant, or nearly so, after hot winters. Additive genetic variance for these traits was not detectable after cold winters. Our analysis suggests that the potential for evolutionary change of nesting behaviour may be dependent on the thermal conditions of the preceding winter, a season that is predicted to be especially subject to climate change.

Climate Change and Temperature‐Dependent Sex Determination: Can Individual Plasticity in Nesting Phenology Prevent Extreme Sex Ratios?

Under temperature-dependent sex determination (TSD), temperatures experienced by embryos during development determine the sex of the offspring. Consequently, populations of organisms with TSD have the potential to be strongly impacted by climatic warming that could bias offspring sex ratio, a fundamental demographic parameter involved in population dynamics. Moreover, many taxa with TSD are imperiled, so research on this phenomenon, particularly long-term field study, has assumed great urgency. Recently, turtles with TSD have joined the diverse list of taxa that have demonstrated population-level changes in breeding phenology in response to recent climate change. This raises the possibility that any adverse impacts of climate change on populations may be alleviated by individual plasticity in nesting phenology. Here, we examine data from a long-term study on a population of painted turtles (Chrysemys picta) to determine whether changes in phenology are due to individual plasticity and whether individual plasticity in the timing of nesting has the capacity to offset the sex ratio effects of a rise in climatic temperature. We find that individual females show plasticity in the date of first nesting each year, and that this plasticity depends on the climate from the previous winter. First nesting date is not repeatable within individuals, suggesting that it would not respond to selection. Sex ratios of hatchlings within a nest declined nonsignificantly over the nesting season. However, small increases in summer temperature had a much stronger effect on nest sex ratios than did laying nests earlier in the season. For this and other reasons, it seems unlikely that individual plasticity in the timing of nesting will offset the effects of climate change on sex ratios in this population, and we hypothesize that this conclusion applies to other populations with TSD.

Sources and Significance of Among-Individual Reproductive Variation in a Northern Population of Painted Turtles (Chrysemys picta)

Painted Turtles (Chrysemys picta) are often used to test life-history theory. However, within populations, the factors that contribute to among-individual variation in egg size and clutch size are poorly understood, and an understanding of the biotic and abiotic parameters that contribute to this variation is important when framing patterns of maternal investment in a life-history context. We examined proximate sources of reproductive variation in a northern population of Painted Turtles, we attempted to frame these sources of variation in a life-history context, and we evaluate which optimality model most adequately explains patterns of reproductive allocation in populations of small-bodied turtles. We used multiple linear regression on data from 168 first clutches of marked females that nested at a long-term study site in Algonquin Park, Ontario, Canada, in 2004. We found that mean egg mass was positively related to maximum plastron length (MPL) and female age, and negatively related to clutch size and water temperature prior to oviposition. Clutch size was positively related to MPL and carapace height, and negatively related to mean egg mass, and the number of clutches laid in the season. Body size (MPL) was the most important predictor of each reproductive parameter, and residual analysis indicated that egg mass was more conserved than clutch size across the range of female body sizes sampled in this study. Thus, egg size may be optimized as a body size-specific function, and in light of this, we suggest that ‘phenotype-habitat matching’ may occur in C. picta. If a female's phenotype (e.g., body size) influences the selective environment of her eggs and hatchlings (e.g., if larger females generally nest farther away from water), then the optimal strategy of maternal investment should vary among maternal phenotypes. The positive correlation between egg mass and body size that was observed in the present study can be explained in adaptive terms under hypotheses based on the concept of phenotype–habitat matching.

Overwintering Habitats of a Northern Population of Painted Turtles (Chrysemys picta): Winter Temperature Selection and Dissolved Oxygen Concentrations

We compared shell temperatures of Painted Turtles (Chrysemys picta) overwintering in two different ponds in Algonquin Park, Canada, over one winter. Movements under the ice occurred from late November into late December or early January, and based on thermal data, turtles were likely not buried in the substrate. Over the course of the winter, one pond became severely hypoxic, whereas the other remained more highly oxygenated. Turtles in the hypoxic pond exhibited significantly lower shell temperatures than did the turtles in the normoxic environment, despite significantly cooler average water temperatures in the normoxic pond. Painted Turtles submerged in normoxic water can assimilate O2 through extrapulmonary means, and cutaneous O2 uptake at low temperatures would significantly delay the onset of metabolic acidosis. Therefore, we suggest that, in the present study, turtles in the hypoxic environment likely could not assimilate O2 cutaneously; thus, these individuals may have voluntarily selected low temperatures to delay metabolic acidosis and conserve energy stores for the active season. Our limited data suggest that Painted Turtles possess behavioral adaptations to detect and respond to aquatic hypoxia.

Thermoregulation When The Growing Season Is Short: Sex-Biased Basking Patterns in a Northern Population of Painted Turtles (Chrysemys Picta)

In reptiles, thermoregulation is important because it alters the rate of many physiological processes. Thermoregulation may be especially important to reproductive females that inhabit regions where the growing season is short, because the amount of thermal energy experienced during the season may limit the amount of energy devoted to egg production. We studied basking behavior of Painted Turtles (Chrysemys picta) in Algonquin Park, Ontario, during the period of follicular recrudescence, a time of year when females allocate energy to developing follicles. Based on the notion that females bask (in part) to increase the amount of energy they allocate to developing follicles, we tested whether basking duration was greater in females than in males. Between 14 and 21 August 2003, we found that females basked longer than males on three of seven days, but males never basked significantly longer than females. Within sex, male but not female body size was positively related to basking duration. Our study suggests that the energetic demands of egg production result in an increased basking duration for females in this northern population. Males may bask to reach a certain temperature then return to water because of potential mating opportunities. Future studies should combine body temperature measurements with behavioral observations to elucidate further the reasons for sex-biased basking.

Optimal offspring provisioning when egg size is “constrained”: a case study with the painted turtle Chrysemys picta

Classic egg size theory predicts that, in a given environment, there is a level of maternal investment per offspring that will maximize maternal fitness. However, positive correlations among egg size and female body size are observed within populations in diverse animal taxa. A popular explanation for this phenomenon is that, in some populations, morphological constraints on egg size, such as ovipositor size (insects) or pelvic aperture width (lizards and turtles), limit egg size. Egg size may therefore increase with female body size due to body size‐specific constraints on investment per offspring, coupled with selection towards an optimal egg size. We use 17 years of data from a population of painted turtles Chrysemys picta to evaluate this hypothesis. In accordance with our predictions, we find that (1) morphological constraints on egg size are apparent only in relatively small females, similarly (2) egg mass exhibits a strong asymptotic relationship with female body size, suggesting egg mass is optimized only at large body sizes, (3) clutch size, not egg mass, varies with female condition, and (4) clutch size varies more than egg mass across years. Contrary to our predictions, we observe that (5) the egg mass‐clutch size tradeoff is not less pronounced at large body sizes. Our data do not fully support the traditional hypothesis, and recent models suggest that this hypothesis is indeed overly simplistic. When the selective environment of a female's offspring is influenced by her phenotype, optimal egg size may vary among maternal phenotypes. This concept can explain correlations among egg size and body size in many taxa, as well as the patterns observed in the present study. In this paradigm, a tight coupling of aperture width (or other ‘constraints’) and egg size may occur in small females, even when such morphological features are not causally related to variation in egg size. In this spirit, we question validity of invoking morphological constraints to explain covariation among egg size and female body size.

Thermoregulation as a function of thermal quality in a northern population of painted turtles, Chrysemys picta

Body temperature affects nearly all processes of ectotherms. Reptiles do not generate sufficient body heat to regulate their body temperature internally and therefore use behavioural thermoregulation. We determined whether thermoregulatory effort varied among seasons in an environment where large temporal differences in environmental temperatures (Te) exist. We took 31 297 internal body temperature (Tb) measurements from 18 painted turtles ( Chrysemys picta (Schneider, 1783)) throughout their active season. We estimated Te with physical models and water temperatures. We measured the range of preferred body temperatures (Tset) in a thermal gradient. Tset was 21.3–25.0 °C. We used Tb, Te, and Tset to calculate standard thermoregulation indices (Ex and de – db). An Ex of 40.7% and a de – db of 2.4 °C indicated that painted turtles are moderate thermoregulators, despite inhabiting a high-cost environment. Effort to regulate Tb increased as the thermal quality of the habitat decreased. Thermoregulatory effort was higher when Tset could not be achieved. Painted turtles put more effort in thermoregulation in the early season than in the rest of the season. This within-species pattern follows the pattern seen among species. This study is the first to measure Tb internally and to apply standard thermoregulation indices to free-ranging turtles.

Marking Nests Increases the Frequency of Nest Depredation in a Northern Population of Painted Turtles (Chrysemys Picta)

Predators use visual and olfactory cues to locate turtle nests. Since 1999, we marked Painted Turtle (Chrysemys picta) nests at a long-term study site by inserting Popsicle™ sticks part way into the nest cavity. Because nest-marking provides a cue to potential predators, we tested whether nest-marking increases nest depredation rates. During the nesting season, 15 artificial nest pairs (N = 30 artificial nests in total) were created by digging and refilling holes (presumably emulating nest excavation by turtles) at a nesting site. Nests in each pair were 45 cm apart, but only one nest in each pair was marked with a Popsicle™ stick, and no eggs were placed in either hole. After one week, depredation was observed in nine of the 15 nest pairs, and all depredation events were directed towards marked nests. A Binomial Test revealed that this pattern was significantly nonrandom. It is possible that predators were responding to olfactory cues left by Popsicle™ sticks, and given that mammalian predators are common at our study site, we cannot rule out the possibility that such olfactory-oriented predators depredated artificial nests. However, we suspect that Common Ravens (Corvus corax) and American Crows (Corvus brachyrhynchos; nest predators that are visually oriented) were the primary predators in this study. Future experiments should use turtle eggs in both marked and unmarked nests to evaluate whether the markers represent a significant mortality factor for Painted Turtle eggs.

Proximate constraints on reproductive output in a northern population of painted turtles: an empirical test of the bet-hedging paradigm

For long-lived, iteroparous organisms whose annual reproductive success is low and unpredictable, the “bet-hedging” life-history paradigm predicts that an increase in resource acquisition should result in an increase in stored lipids and not an increase in reproductive output. We tested whether reproductive patterns in a population of midland painted turtles ( Chrysemys picta marginata Agassiz, 1857) are consistent with this prediction. Assuming that annual temperature variation is a proxy for variation in resource acquisition, we hypothesized that if harvested resources are generally sequestered for future reproduction, then there should be no relationship between clutch frequency (the number of clutches laid per season) and the temperature experienced when no follicular growth occurred. We hypothesized further that if temperature constrains clutch frequency by limiting the amount of energy that can be allocated to reproduction, then clutch frequency would be related only to the temperature experienced during the period in which follicular growth occurred (fall temperature). We found that clutch frequency was primarily related to fall temperature, which suggests that the amount of thermal energy experienced during periods of follicular development limits the amount of stored energy that can be allocated to developing follicles. This pattern of reproductive allocation is consistent with the bet-hedging paradigm.

Comparison of harvested and nonharvested painted turtle populations

Painted turtles (Chrysemys picta) are commercially harvested in large numbers in Minnesota for sale to biological supply companies and the pet trade. We investigated the possible effects of this harvest by comparing size, demography, and catch rates of painted turtles in 12 harvested and 10 nonharvested painted turtle populations in 2001 and 2002. We correlated turtle catch rates to harvest status, and harvested lakes had a lower catch-per-unit-effort than nonharvested lakes. Harvest had minimal effect on the size of turtles captured, and we found no significant differences in the count of male:female:juvenile turtles among lakes of different harvest status. We suggest that painted turtle populations likely have been impacted by harvester activities, but it was unclear whether the current harvest is sustainable. Further work is needed to determine whether there are any long-term effects on painted turtle populations.

Variation in nesting patterns affecting nest temperatures in two populations of painted turtles (Chrysemys picta) with temperature-dependent sex determination

Mechanisms maintaining sex ratios in populations with temperature-dependent sex determination (TSD) remain elusive. Although geographic variation in embryonic sex determination (i.e., pivotal temperature) has been widely investigated in reptiles exhibiting TSD, no previous studies have directly addressed geographic variation in maternal behavior affecting nest thermal conditions. I evaluated patterns of nest-site selection and its effects on thermal and hydric nest conditions for a population of painted turtles (Chrysemys picta bellii) exhibiting TSD in New Mexico. These results are compared to data collected from a well-studied, conspecific population experiencing relatively cooler climatic conditions in Illinois. Since canopy vegetation cover reduces nest temperatures in Illinois, I expected females in New Mexico to nest under high amounts of canopy vegetation cover. However, females from New Mexico placed nests under significantly less canopy vegetation cover, but closer to standing water, than did females from Illinois. Experimental nests in New Mexico demonstrated that increased canopy vegetation cover and soil moisture reduced nest temperatures. By nesting close to standing water rather than under canopy vegetation cover, females in New Mexico nested in habitats more closely associated with maximizing moisture around nests rather than reducing nest temperatures through shading. Mean July nest temperatures were similar for both populations. Since nest hydric conditions affect hatching success and hatchling size in C. picta, nesting patterns in New Mexico may primarily reflect selection for microhabitats affecting offspring survivorship or size.