Biology subjects

Ardia, D. R.

Publications and source records attributed to Ardia, D. R..

7 recordsLinked to original sources

bioRxiv2026

Geographical variation in plasma bactericidal ability of breeding tree swallows (Tachycineta bicolor)

Immune defenses may vary across environments due to differences in life history or abiotic and biotic characteristics, but few studies have compared the strength of immune responses across broad geographical gradients. Here we investigated how bacteria killing ability (BKA), a measure of constitutive innate immune function, varies among populations of tree swallows (Tachycineta bicolor) inhabiting different environments. We hypothesized that populations living in unpredictable environments, which also have shorter breeding seasons and thus fewer opportunities to renest in the event of reproductive failure, would invest in reproduction at the expense of immunity. To test this hypothesis, we measured BKA and monitored reproductive activity in four populations located across most of the breeding range of tree swallows, in Tennessee, New York, Wyoming and Alaska. Consistent with our predictions, BKA was lowest in Alaska and Wyoming and highest in Tennessee. In most populations BKA was also lower during incubation than during nestling provisioning, and lower in males than in females. These results suggest that the high reproductive investment seen in populations experiencing more unpredictable breeding conditions may come at a cost to immune function. While reduced immune investment could increase susceptibility to parasites, this potential cost is likely offset by lower parasite pressure in these environments, limiting the need for maintaining strong constitutive innate defenses like BKA.

ecology↗

bioRxiv2026

Developmental cold exposure increases the ability to maintain body temperature during future cold challenges in a free-living altricial bird

Developing in suboptimal temperatures can have widespread negative effects in organisms, but early exposure to thermal challenges can also trigger increased investment in thermoregulatory capacity. Within birds, the effects of cold exposure during incubation may differ between precocial and altricial species due to differences in the ontogeny of thermoregulation. To date, most of the evidence showing that embryonic cold exposure shapes future thermoregulatory capacity comes from a few precocial species in laboratory settings. Less is known about sensitivity to low-temperature incubation in altricial species which only become fully endothermic after hatching. In this study, we tested the effects of cold temperatures during embryonic development by temporarily cooling the nest boxes of free-living tree swallows (Tachycineta bicolor) during late incubation and subsequently exposing nestlings to an acute cold challenge. Cold exposure during early development made nestlings better at maintaining their body temperatures during the acute challenge. Developmentally cold-exposed birds also had higher thyroxine (T4) levels when sampled at lower ambient temperatures, whereas control birds showed the opposite relationship. Among nestlings that failed to maintain their body temperature during the acute cold challenge, developmentally cold-exposed nestlings secreted more corticosterone during the acute cold challenge than control nestlings. Despite these changes, cold-exposed birds did not differ from controls in cold-induced metabolic rates, body mass, or bacteria killing ability. These results are, to our knowledge, the first to show that developmental cold exposure increases the capacity to maintain body temperature during future cold challenges in a songbird. These findings suggest that experiencing even a mild decline in ambient temperatures during embryonic development could prime birds to cope more effectively with harsh or variable environmental conditions later in life.

ecology↗

bioRxiv2025

Organismal robustness and resilience influence the ability to cope with cold snaps

Coping with challenges often requires maintaining performance during poor conditions (robustness) and reestablishing normal performance following disturbance (resilience). Trade-offs between performance under normal and challenging conditions may generate variation in stress coping capacity, but there is little empirical data from wild populations. We studied robustness and resilience to temperature in tree swallows (Tachycineta bicolor) using a 39-year dataset that included >75,000 hours of behavioral data. Cold snaps had profound and sustained impacts on fitness; effects on nestlings were mediated by the behavioral robustness and resilience of parents. Parental robustness was predicted by developmental cold exposure and current endocrine state. As expected, trade-offs between peak performance and behavioral robustness influenced susceptibility. Selection on trade-offs may generate population differences in vulnerability to changing temperature regimes.

ecology↗

bioRxiv2025

Divergent climate impacts despite similar response to temperature in a widespread aerial insectivore

Climate change is shifting when animals breed1,2, but it is still not clear why some populations keep pace with warming while others fall behind3,4. Differences could arise from variation in sensitivity to temperature3 or constraints on the ability to respond to temperature. Without knowing whether populations differ in sensitivity--or in their ability to act on that sensitivity--we cannot identify which are most at risk. Using 1,555 population-years from 123 populations of tree swallows (Tachycineta bicolor), we show that populations have similar sensitivity to local temperature, advancing breeding by about one day per degree of warming. However, northern populations face tighter time constraints and greater exposure to recent warming. Northern populations have advanced laying dates the most, but still experience stronger selection for earlier breeding, especially in warm years; they have also declined most in breeding abundance. These findings show that vulnerability to climate change can arise not just from different sensitivity to warming, but from when and where populations can respond effectively. By disentangling sensitivity from timing constraints, our results support a general mechanism by which even uniformly responsive species can show uneven impacts of climate change across their ranges.

ecology↗

bioRxiv2025

Ambient temperature, clutch size, and daylight are the main drivers of incubation behavior in two neotropical swallows breeding 8,000 km apart

The central dilemma in incubation behaviour is how to allocate time between warming up the clutch and self-preservation activities outside the nest. It is assumed that incubating birds follow a classical framework in which the environmental temperature, the main variable influencing incubation behaviour, and bout duration show a non-linear relationship. In this context, incubating birds would maximize bout duration for most of the temperature range in which they breed. To determine whether this theoretical framework applies to species breeding at different latitudes under extremely different environmental conditions, we collected incubation data from two distant but breeding populations of Tachycineta swallows. T. leucopyga (Chillean swallow) breeds in Ushuaia, Argentina, and T. meyeni (mangrove swallow) breeds in Hill Bank, Belize. These populations do not overlap in terms of ambient temperature; they are exposed to different daylight conditions and the latitudinal differences influence life-history traits such as clutch size. Tachycineta swallows showed similar incubation consistency over 24 hours ([~]75% of nest attendance) but followed different behavioral strategies to achieve this consistency. Daylight duration limited incubation activity in mangrove swallows, but longer nocturnal incubation compensated for shorter days. Larger clutches only took their toll on the Chilean swallows, by shortening their incubation bouts and lengthening their foraging bouts, which affected the length of their daily activity. Overall, both swallow species showed incubation patterns adapted to the local conditions. They responded differently to increasing temperatures, but both species reduced their incubation effort whenever possible. Our results contrast with previous work where maximizing incubation effort was the rule. In our populations, females reduced their incubation effort as soon as weather conditions improved (i.e., increasing temperatures), changing the paradigm of incubation behaviour. Lay summary- In species where only the male or female incubates, they need to find a balance between the time they spend incubating in the nest and the time they spend foraging out of the nest. - The ambient temperature is the most important factor influencing the duration of both behaviours, and birds are thought to respond similarly to the same range of temperatures. - We studied the incubation behaviour of two species of swallows breeding in Belize and Argentina, respectively. - Both species incubated for a similar length of time in 24 hours, but each followed a different strategy depending on clutch size and daylight duration. - Their incubation behaviour was also related to ambient temperature, but differed between populations, suggesting adaptations to local conditions. - We cannot expect different species to show similar responses to ambient temperature under different conditions.

animal behavior and cognition↗

bioRxiv2022

An Experimental Analysis of the Fine-Scale Effects of Nest Ectoparasites on Incubation Behavior

Avian incubation is a highly complex, adjustable behavior essential to embryo development and survival. When incubating, parents face a tradeoff between investing in incubation to maintain optimal temperatures for egg development or in self-maintenance behaviors to ensure their own survival and future reproduction. Because nest ectoparasites are costly and can reduce nestling quality and survival, infections could shift parental investment in current vs. future offspring. However, it is not well understood whether birds change investment in incubation in response to nest ectoparasitism, particularly in the context of other factors that are known to influence incubation behavior, such as ambient temperature, clutch size, and embryo development. We hypothesized that parents could respond to nest parasites by 1) investing more in incubation to promote the development of high-quality offspring to help offset the cost of parasites, 2) investing less in incubation or even abandoning their nest during incubation to save resources for future reproduction, or 3) being unresponsive to parasite infections, as incubation is more constrained by other factors. We tested these hypotheses by experimentally removing and adding mites in barn swallow nests at the start of incubation and deploying thermocouple eggs to measure egg temperatures at one-minute intervals until hatching. We found that while ambient temperature, clutch initiation date, embryo age, and clutch size were the main factors driving variation in egg conditions and parental incubation behavior, parasitized nests had higher mean egg temperatures, which could aid in nestling immune development. However, despite more optimal developmental temperatures, eggs in parasitized nests also had lower hatching success. Our results indicate that incubation is a dynamic behavior that is largely driven by the constraints of maintaining appropriate egg temperatures for development. Although quite costly upon hatching, ectoparasites appear to play a relatively minor role in driving variation in parental incubation investment. LAY SUMMARYO_LIIncubation is a complex behavior shaped by both internal and external factors. C_LIO_LIEctoparasites often decrease quality and survival of nestlings and could influence investment in incubation because of tradeoffs in how parents spend energy. C_LIO_LIWe manipulated parasites in barn swallow nests and used fake eggs with temperature sensors to collect data on egg temperatures and parental behavior throughout incubation. C_LIO_LIWe found that other factors were the main drivers of variation in incubation behavior, but that eggs in parasite nests were warmer on average, which could help nestlings better cope with parasites upon hatching. C_LIO_LIDespite more optimal developmental temperatures, eggs in parasitized nests had lower hatching success. C_LIO_LIAlthough quite costly to nestlings, ectoparasites played a minor role in shaping parental incubation behavior. C_LI

animal behavior and cognition↗

bioRxiv2019

Environmental unpredictability shapes glucocorticoid regulation across populations of tree swallows

Responding appropriately to challenges is an important contributor to fitness. Variation in the regulation of glucocorticoid hormones, which mediate the phenotypic response to challenges, can therefore influence the ability to persist in a given environment. We compared stress responsiveness in four populations of tree swallows (Tachycineta bicolor) along an environmental gradient to evaluate support for different selective pressures in driving the evolution of glucocorticoid regulation. In accordance with the environmental unpredictability hypothesis, stronger stress responses were seen in more unpredictable environments. Contrary to the reproductive value hypothesis, the stress response was not lower in populations engaging in more valuable reproductive attempts. Populations with stronger stress responses also had stronger negative feedback, which supports a "mitigating" rather than a "magnifying" effect of negative feedback on stress responses. These results suggest that combining a robust stress response with strong negative feedback may be important for persisting in unpredictable or rapidly changing environments.

evolutionary biology↗