Declining egg viability explains higher hatching failure in a suburban population of the threatened Florida scrub‐jay Aphelocoma coerulescens

Hatching failure occurs in approximately 10% of all avian eggs, but varies both within and among species. This reduction in viable offspring can have significant fitness consequences for breeding parents; therefore, it is important to understand which factors influence variation in hatching failure among populations. Previous research suggests that hatching failure is higher in a suburban than in a wildland population in the Florida scrub‐jay. From 2003 to 2007, we performed two experiments to examine whether increased hatching failure in the suburbs resulted from 1) increased length of off‐bouts during incubation (predation risk hypothesis, 2003–2004) or 2) increased exposure to ambient temperature during laying (egg viability hypothesis, 2005–2007). Hatching failure was higher for females that took fewer off‐bouts, but the length of those off‐bouts did not influence hatching failure. Thus, nest predation risk does not appear to explain higher hatching failure in the suburbs. Alternatively, hatching failure increased with increasing exposure of eggs to ambient conditions during the laying period. First‐laid eggs in the suburbs had the greatest pre‐incubation exposure to ambient temperature and the greatest rate of hatching failure, consistent with the egg viability hypothesis. Urbanization influences hatching failure through a series of complex interactions. Access to predictable food sources advances mean laying date in suburban scrub‐jays, leading to larger clutch sizes. Because scrub‐jays begin incubation with the ultimate egg, first‐laid eggs in the suburbs may be exposed to ambient temperatures for longer periods, thus reducing their viability.     

Buildings promote higher incubation temperatures and reduce nest attentiveness in a Neotropical thrush

Incubation is an energetically costly parental task of breeding birds. Incubating parents respond to environmental variation and nest‐site features to adjust the balance between the time spent incubating (i.e. nest attentiveness) and foraging to supply their own needs. Non‐natural nesting substrates such as human buildings impose new environmental contexts that may affect time allocation of incubating birds but this topic remains little studied. Here, we tested whether nesting substrate type (buildings vs. trees) affects the temperature inside the incubation chamber (hereafter ‘nest temperature’) in the Pale‐breasted Thrush Turdus leucomelas, either during ‘day’ (with incubation recesses) or ‘night’ periods (representing uninterrupted female presence at the nest). We also tested whether nesting substrate type affects the incubation time budget using air temperature and the day of the incubation cycle as covariates. Nest temperature, when controlled for microhabitat temperature, was higher at night and in nests in buildings but did not differ between daytime and night for nests in buildings, indicating that buildings partially compensate for incubation recesses by females with regard to nest temperature stability. Females from nests placed in buildings exhibited lower nest attentiveness (the overall percentage of time spent incubating) and had longer bouts off the nest. Higher air temperatures were significantly correlated with shorter bouts on the nest and longer bouts off the nest, but without affecting nest attentiveness. We suggest that the longer bouts off the nest taken by females of nests in buildings is a consequence of higher nest temperatures promoted by man‐made structures around these nests. Use of buildings as nesting substrate may therefore increase parental fitness due to a relaxed incubation budget, and potentially drive the evolution of incubation behaviour in certain urban bird populations.     

Temperature and life history: experimental heating leads female tree swallows to modulate egg temperature and incubation behaviour

1. Life-history decisions are strongly affected by environmental conditions. In birds, incubation is energetically expensive and affected significantly by ambient temperature. We reduced energetic constraints for female tree swallows (Tachycineta bicolor) by experimentally heating nests during incubation by an average of 6.9 degrees C to test for changes in incubation behaviour. 2. Females in heated boxes (hereafter 'heated females') increased time spent incubating and maintained higher on-bout and off-bout egg temperatures. This indicates that female energetic constraints, not maximizing developmental conditions of offspring, determine incubation investment. Furthermore, this result suggests that embryonic developmental conditions in unmanipulated nests are suboptimal. 3. We found individual variation in how females responded to experimental heating. Early-laying (i.e. higher phenotypic quality) females with heated nests increased egg temperatures and maintained incubation constancy, while later-laying (lower quality) heated females increased incubation constancy. Changes in egg temperature were due to changes in female behaviour and not due directly to increases in internal nest-box temperatures. 4. Behaviour during the incubation period affected hatching asynchrony. Decreased variation in egg temperature led to lower levels of hatching asynchrony, which was also generally lower in heated nests. 5. Our study finds strong support for the prediction that intermittent incubators set their incubation investment at levels dictated by energetic constraints. Furthermore, females incubating in heated boxes allocated conserved energy primarily to increased egg temperature and increased incubation attentiveness. These results indicate that studies investigating the role of energetics in driving reproductive investment in intermittent incubators should consider egg temperature and individual variation more explicitly.     

Avian Incubation Patterns Reflect Temporal Changes in Developing Clutches

Incubation conditions for eggs influence offspring quality and reproductive success. One way in which parents regulate brooding conditions is by balancing the thermal requirements of embryos with time spent away from the nest for self-maintenance. Age related changes in embryo thermal tolerance would thus be expected to shape parental incubation behavior. We use data from unmanipulated Black-capped Chickadee (Poecile atricapillus) nests to examine the temporal dynamics of incubation, testing the prediction that increased heat flux from eggs as embryos age influences female incubation behavior and/or physiology to minimize temperature fluctuations. We found that the rate of heat loss from eggs increased with embryo age. Females responded to increased egg cooling rates by altering incubation rhythms (more frequent, shorter on- and off- bouts), but not brood patch temperature. Consequently, as embryos aged, females were able to increase mean egg temperature and decrease variation in temperature. Our findings highlight the need to view full incubation as more than a static rhythm; rather, it is a temporally dynamic and finely adjustable parental behavior. Furthermore, from a methodological perspective, intra- and inter-specific comparisons of incubation rhythms and average egg temperatures should control for the stage of incubation.     

Advancing breeding phenology does not affect incubation schedules in chestnut‐crowned babblers: Opposing effects of temperature and wind

Projecting population responses to climate change requires an understanding of climatic impacts on key components of reproduction. Here, we investigate the associations among breeding phenology, climate and incubation schedules in the chestnut‐crowned babbler (Pomatostomus ruficeps), a 50 g passerine with female‐only, intermittent incubation that typically breeds from late winter (July) to early summer (November). During daylight hours, breeding females spent an average of 33 min on the nest incubating (hereafter on‐bouts) followed by 24‐min foraging (hereafter off‐bouts), leading to an average daytime nest attentiveness of 60%. Nest attentiveness was 25% shorter than expected from allometric calculations, largely because off‐bout durations were double the expected value for a species with 16 g clutches (4 eggs × 4 g/egg). On‐bout durations and daily attentiveness were both negatively related to ambient temperature, presumably because increasing temperatures allowed more time to be allocated to foraging with reduced detriment to egg cooling. By contrast, on‐bout durations were positively associated with wind speed, in this case because increasing wind speed exacerbated egg cooling during off‐bouts. Despite an average temperature change of 12°C across the breeding season, breeding phenology had no effect on incubation schedules. This surprising result arose because of a positive relationship between temperature and wind speed across the breeding season: Any benefit of increasing temperatures was canceled by apparently detrimental consequences of increasing wind speed on egg cooling. Our results indicate that a greater appreciation for the associations among climatic variables and their independent effects on reproductive investment are necessary to understand the effects of changing climates on breeding phenology.     

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.     

Nest attendance by tropical and temperate passerine birds: Same constancy,different strategy

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Incubation rhythm in the great snipe Gallinago media

The incubation rhythm of four female great snipe was monitored with telemetric equipment. The mean daily incubation constancy was 90.3 ± 2.0 (SD) per cent, and the mean daily time off the nest amounted to 139.8 ± 28.8 min. The number of recesses per day averaged 8.7 ± 1.9, with a mean duration of 15.7 ± 6.1 min. Generally, the birds incubated for long bouts during the night, and left the nest frequently during the daylight hours.
Recesses were concentrated in the warmest part of the day in cool periods, but were more evenly distributed throughout the day in warmer periods. Recess duration decreased with decreasing temperature. These adjustments minimize egg cooling when the bird is off the nest, and thus allow the incubating bird more time to feed without lowering the mean egg temperature.
Calculations of the cooling rates of eggs indicate that the bird minimizes incubation energy expenditure as far as possible, but without letting the eggs cool beyond the temperature of no embryonic development.     

How avian incubation behaviour influences egg surface temperatures: relationships with egg position,development and clutch size

While understanding heat exchange between incubating adults and their eggs is central to the study of avian incubation energetics, current theory based on thermal measurements from dummy eggs reveals little about the mechanisms of this heat exchange or behavioural implications for the incubating bird. For example, we know little about how birds distribute their eggs based on temperature differences among egg positions within the nest cup. We studied the great tit Parus major, a species with a large clutch size, to investigate surface cooling rates of individual eggs within the nest cup across a range of ambient temperatures in a field situation. Using state‐of‐the‐art portable infrared imaging and digital photography we tested for associations between egg surface temperature (and rate of cooling) and a combination of egg specific (mass, shape, laying order, position within clutch) and incubation specific (clutch size, ambient temperature, day of incubation) variables. Egg surface temperature and cooling rates were related to the position of the eggs within the nest cup, with outer eggs being initially colder and cooling quicker than central eggs. Between foraging bouts, females moved outer eggs significantly more than centrally positioned eggs. Our results demonstrate that females are capable of responding to individual egg temperature by moving eggs around the nest cup, and that the energy cost to the female may increase as incubation proceeds. In addition, our results showing that smaller clutches experience lower initial incubation temperatures and cool quicker than larger clutches warrant further attention for optimal clutch size theory and studies of energetic constraints during incubation. Finally, researchers using dummy eggs to record egg temperature have ignored important elements of contact‐incubation, namely the complexity of how eggs cool and how females respond to these changes.     

Great tits provided with ad libitum food lay larger eggs when exposed to colder temperatures

The amount of nutrients deposited into a bird egg varies both between and within clutches of the same female. Larger eggs enhance offspring traits, but as a tradeoff, laying large eggs also infers energetic costs to the female. Income breeders usually lay larger eggs later in the season, when temperatures and food availability are higher. Egg size is thus affected by the daily amount of energy available to produce an egg under cold conditions, but it is less well known in how far temperature exerts direct effects on egg size. We show that great tit females Parus major with access to ad libitum food and breeding in climate‐controlled aviaries varied their egg investments. The size of an individual egg was best predicted by mean temperatures one week pre‐laying, with females laying larger, rather than smaller, eggs under colder conditions. Eggs increased in size over the season, but not significantly over the laying sequence. The degree of daily temperature fluctuation did not influence egg size. In addition to a substantial between‐female variation, sisters were more similar to each other than unrelated females, showing that egg size does also reflect heritable intrinsic female properties. Natural variation in egg size is thus not only determined by energy‐limitation, but also due to females allocating more resources to eggs laid in colder environments, thus increasing early survival of the chicks. That the positive correlation between temperature and egg investments that is found in a natural population is reversed under ad libitum food conditions demonstrates that wild great tits tradeoff own condition with survival prospects of their chicks as a function of available food, not ambient temperature.     

Off‐nest behaviour in a biparentally incubating shorebird varies with sex,time of day and weather

Biparental incubation is a form of cooperation between parents, but it is not conflict‐free because parents trade off incubation against other activities (e.g. self‐maintenance, mating opportunities). How parents resolve such conflict and achieve cooperation remains unknown. To understand better the potential for conflict, cooperation and the constraints on incubation behaviour, investigation of the parents' behaviour, both during incubation and when they are off incubation‐duty, is necessary. Using a combination of automated incubation‐monitoring and radiotelemetry we simultaneously investigated the behaviours of both parents in the biparentally incubating Semipalmated Sandpiper Calidris pusilla, a shorebird breeding under continuous daylight in the high Arctic. Here, we describe the off‐nest behaviour of 32 off‐duty parents from 17 nests. Off‐duty parents roamed on average 224 m from their nest, implying that direct communication with the incubating partner is unlikely. On average, off‐duty parents spent only 59% of their time feeding. Off‐nest distance and behaviour (like previously reported incubation behaviour) differed between the sexes, and varied with time and weather. Males roamed less far from the nest and spent less time feeding than did females. At night, parents stayed closer to the nest and tended to spend less time feeding than during the day. Further exploratory analyses revealed that the time spent feeding increased over the incubation period, and that at night, but not during the day, off‐duty parents spent more time feeding under relatively windy conditions. Hence, under energetically stressful conditions, parents may be forced to feed more. Our results suggest that parents are likely to conflict over the favourable feeding times, i.e. over when to incubate (within a day or incubation period). Our study also indicates that Semipalmated Sandpiper parents do not continuously keep track of each other to optimize incubation scheduling and, hence, that the off‐duty parent's decision to remain closer to the nest drives the length of incubation bouts.     

The Effect of Male Incubation Feeding,Food and Temperature on the Incubation Behaviour of New Zealand Robins

Because of finite resources, organisms face conflict between their own self‐care and reproduction. This conflict is especially apparent in avian species with female‐only incubation, where females face a trade‐off between time allocated to their own self‐maintenance and the thermal requirements of developing embryos. We recorded incubation behaviour of the New Zealand robin (Petroica longipes), a species with female‐only incubation, male incubation feeding and high nest predation rates. We examined how male incubation feeding, ambient temperature and food availability (invertebrate biomass) affected the different components of females’ incubation behaviour and whether incubation behaviour explained variation in nest survival. Our results suggest that male incubation feeding rates of 2.8 per hour affect the female’s incubation rhythm by reducing both on‐ and off‐bout duration, resulting in no effect on female nest attentiveness, thus no support for the female‐nutritional hypothesis. The incubation behaviours that we measured did not explain nest survival, despite high nest predation rates. Increased ambient temperature caused an increase in off‐bout duration, whereas increased food availability increased on‐bout duration. While males play a vital role in influencing incubation behaviour, female robins attempt to resolve the trade‐off between their own foraging needs and the thermal requirements of their developing embryos via alternating their incubation rhythm in relation to both food and temperature.     

Variation in incubation effort during egg laying in the Mountain Bluebird and its association with hatching asynchrony

Females in many bird species reportedly begin incubation prior to clutch completion, but the nature of such incubation and the degree to which it varies among females remains undescribed for almost all species. We used continuous recording of nest‐cup temperatures to document incubation effort during egg laying at 57 Mountain Bluebird (Sialia currucoides) nests in a high‐elevation Wyoming population. We then asked whether such effort predicted the degree to which eggs hatch asynchronously. Although substantial egg heating could begin abruptly late in laying (previously reported as the norm for this species) or even after clutch completion, we found that most (>90%) females began incubation gradually, engaging in a few (usually 1–8), brief (<10 min) bouts of heating on the day they laid their first or second egg. Thereafter, females varied markedly in when they increased incubation effort and by how much. The onset of nocturnal incubation also varied, with females beginning to incubate at night after laying their prepenultimate, penultimate, or last egg and not always initially incubating through the night. As an index of the total amount of heat applied to eggs during laying, we calculated the cumulative number of degrees by which nest‐cup temperatures exceeded the threshold temperature required for embryonic development. This value varied by more than 150‐fold between nests and explained >50% of the variation in hatching asynchrony. Our results thus provide strong support for the widely held, but rarely tested, assumption that parent birds can have substantial control over the degree of hatching asynchrony by varying the amount of incubation done prior to clutch completion.     

Reproductive ecology of the jacky dragon (Amphibolurus muricatus): an agamid lizard with temperature‐dependent sex determination

Abstract The jacky dragon, Amphibolurus muricatus (White, ex Shaw 1790) is a medium sized agamid lizard from the southeast of Australia. Laboratory incubation trials show that this species possesses temperature‐dependent sex determination. Both high and low incubation temperatures produced all female offspring, while varying proportions of males hatched at intermediate temperatures. Females may lay several clutches containing from three to nine eggs during the spring and summer. We report the first field nest temperature recordings for a squamate reptile with temperature‐dependent sex determination. Hatchling sex is determined by nest temperatures that are due to the combination of daily and seasonal weather conditions, together with maternal nest site selection. Over the prolonged egg‐laying season, mean nest temperatures steadily increase. This suggests that hatchling sex is best predicted by the date of egg laying, and that sex ratios from field nests will vary over the course of the breeding season. Lizards hatching from eggs laid in the spring (October) experience a longer growing season and should reach a larger body size by the beginning of their first reproductive season, compared to lizards from eggs laid in late summer (February). Adult male A. muricatus attain a greater maximum body size and have relatively larger heads than females, possibly as a consequence of sexual selection due to male‐male competition for territories and mates. If reproductive success in males increases with larger body size, then early hatching males may obtain a greater fitness benefit as adults, compared to males that hatch in late summer. We hypothesize that early season nests should produce male‐biased sex ratios, and that this provides an adaptive explanation for temperature‐dependent sex determination in A. muricatus.     

Maternal and environmental effects on offspring phenotypes in an oviparous lizard: do field data corroborate laboratory data?

A vast literature suggests that maternal factors and egg incubation conditions have substantial effects on offspring phenotypes in oviparous species. However, many studies that evaluate these effects have relied on experimental conditions that are rarely, if ever, encountered under natural conditions. To address this issue, we evaluated relationships among maternal factors, natural nest conditions, egg development in the field, and the resultant offspring phenotypes in a lizard with temperature-dependent sex determination, the jacky dragon (Amphibolurus muricatus, Agamidae). Many, but not all, of the relationships shown in our field-based study corroborate results from laboratory-based experiments. Offspring body size was affected primarily by egg size at oviposition, as well as by water uptake by eggs, rather than by environmental variables measured within the nest. Date of oviposition was related to offspring growth rate and body size prior to the onset of winter; this relationship is likely mediated through an influence on the timing of hatching. Nest temperature generated substantial variation in egg survival; nests that experienced higher temperatures and higher thermal fluctuations suffered relatively high egg mortality. Contrary to results from laboratory incubation, however, nest temperature did not predict offspring sex ratios. Hence, although many results from this field study corroborate those from the laboratory, caution is needed when extrapolating laboratory-incubation results to field conditions. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Gold climates and the evolution of viviparity in reptiles: cold incubation temperatures produce poor-quality offspring in the lizard, Sceloporus virgatus

Evolutionary origins of viviparity among the squamate reptiles are strongly associated with cold climates, and cold environmental temperatures are thought to be an important selective force behind the transition from egg-laying to live-bearing. In particular, the low nest temperatures associated with cold climate habitats are thought to be detrimental to the developing embryos or hatchlings of oviparous squamates, providing a selective advantage for the retention of developing eggs in utero, where the mother can provide warmer incubation temperatures for her eggs (by actively thermoregulating) than they would experience in a nest. However, it is not entirely clear what detrimental effects cold incubation temperatures may have on eggs and hatchlings, and what role these effects may play in favouring the evolution of viviparity. Previous workers have suggested that viviparity may be favoured in cold climates because cold incubation temperatures slow cmbryogenesis and delay hatching of the eggs, or because cold nest temperatures are lethal to developing eggs and reduce hatching success. However, incubation temperature has also been shown to have other, potentially long-term, effects on hatchling phcnotypcs, suggesting that cold climates may favour viviparity because cold incubation temperatures produce offspring of poor quality or low fitness. We experimentally incubated eggs of the oviparous phrynosomatid lizard, Sceloporus virgatus, at temperatures simulating nests in a warm (low elevation) habitat, as is typical for this species, and nests in a colder (high elevation) habitat, to determine the effects of cold incubation temperatures on embryonic development and hatchling phenotypes. Incubation at cold nest temperatures slowed embryonic development and reduced hatching success, but also affected many aspects of the hatchlings' phenotypes. Overall, the directions of these plastic responses indicated that cold-incubated hatchlings did indeed exhibit poorer quality phenotypes; they were smaller at hatching (in body length) and at 20 days of age (in length and mass), grew more slowly (in length and mass), had lower survival rates, and showed greater fluctuating asymmetry than their conspecifics that were incubated at warmer temperatures. Our findings suggest that cold nest temperatures are detrimental to S. virgatus, by delaying hatching of their eggs, reducing their hatching success, and by producing poorer quality offspring. These negative effects would likely provide a selective advantage for any mechanism through which these lizards could maintain warmer incubation temperatures in cold climates, including the evolution of prolonged egg retention and viviparity.     

Nest temperature, incubation period, and investment decisions of incubating wood ducks Aix sponsa

Incubating birds must balance their energetic demands with the time needed to provide care to developing embryos. Reduced care by incubating parents can result in longer incubation periods that increase predation risk and potentially influence neonate phenotype. We measured nest temperature, incubation period, and body mass dynamics of female wood ducks Aix sponsa , and used an information-theoretic approach to investigate effects of several explanatory variables on incubation period and thermal characteristics of nests. A model that included clutch size and standard deviation of nest temperature best explained the variation in incubation period. Parameter estimates indicated that incubation period increased with increases in clutch size and standard deviation of nest temperature. Next, we examined relationships between maternal effects and the standard deviation of the nest temperature. The best fitting model included initiation date of incubation. There was little support for including early incubation body mass of females, incubation constancy, and percent change in female body mass in the model. The parameter estimate showed that standard deviation of nest temperature declined as initiation date of incubation advanced. Female body mass at the start of incubation was not related to structural size suggesting that heavy females were in better physical condition than were light females. Heavy females nested earlier and lost more body mass during incubation than light females, but heavy females did not reduce variation in nest temperature to decrease the incubation period. The fact that early nesting females in good physical condition did not shorten incubation periods by keeping nest temperatures less variable could have been due to either energetic limitations or restraints. Experimental manipulations of incubation costs will be needed to distinguish between these hypotheses.     

Geographic variation in avian incubation periods and parental influences on embryonic temperature

Theory predicts shorter embryonic periods in species with greater embryo mortality risk and smaller body size. Field studies of 80 passerine species on three continents yielded data that largely conflicted with theory; incubation (embryonic) periods were longer rather than shorter in smaller species, and egg (embryo) mortality risk explained some variation within regions, but did not explain larger differences in incubation periods among geographic regions. Incubation behavior of parents seems to explain these discrepancies. Bird embryos are effectively ectothermic and depend on warmth provided by parents sitting on the eggs to attain proper temperatures for development. Parents of smaller species, plus tropical and southern hemisphere species, commonly exhibited lower nest attentiveness (percent of time spent on the nest incubating) than larger and northern hemisphere species. Lower nest attentiveness produced cooler minimum and average embryonic temperatures that were correlated with longer incubation periods independent of nest predation risk or body size. We experimentally tested this correlation by swapping eggs of species with cool incubation temperatures with eggs of species with warm incubation temperatures and similar egg mass. Incubation periods changed (shortened or lengthened) as expected and verified the importance of egg temperature on development rate. Slower development resulting from cooler temperatures may simply be a cost imposed on embryos by parents and may not enhance offspring quality. At the same time, incubation periods of transferred eggs did not match host species and reflect intrinsic differences among species that may result from nest predation and other selection pressures. Thus, geographic variation in embryonic development may reflect more complex interactions than previously recognized.     

Nest Predation Avoidance: an Alternative Explanation for Male Incubation in Acrocephalus melanopogon

In the monogamous moustached warbler, male incubation changes from predictably variable (it is dependent on ambient temperature and time of day in April) to high average levels across the day (with no predictor variables in May) as the season progresses. In contrast, females contribute the constant incubation component from April to May. This paper investigates possible explanations for the change in male incubation effort involving changing risks to either (1) embryonic survival within the egg, and/or (2) egg predation. Using egg temperature readouts, the probability of reaching the 25 °C thermal threshold (below which embryonic development ceases) across the season against the probability of predator sightings 0–15 m from the nest was calculated. The results show an inverse relationship between these two risks. During April, male incubation correlates with egg cold stress and changeovers between males and females occur prior to egg cooling below the thermal stress line. During May, the risk of predation increases. The results show increased predator encounter rates from April to May and active nest defense by the incubating parent. Furthermore, high male incubation reduces brood predation. Selection for reduction of the costs of laying replacement clutches (after predation) is suggested given high male incubation and infrequent male-female changeovers during midday, when egg temperatures are highest, with direct benefits to females of increased foraging. Thus, the shift in male incubation across the season may be explained by minimization of changing risk to offspring survival.