Incubation Bouts Research Articles

The asymmetric incubation game: a prospective model and a house sparrow investigation

Many birds show biparental incubation patterns in which male and female parents alternate their incubation bouts. We constructed prospective sealed bid (rather than negotiation) models of optimal (both cooperative and ESS) incubation bout lengths for each sex (‘the incubation game’) to examine whether the asymmetry in incubation efficiency can generate sex biases in incubation bout lengths. This model may apply generally when two individuals cooperate in alternating bouts to complete a task requiring a fixed total effort, but where one individual accomplishes more per unit time spent on the task than the other, and where bout lengths can be varied strategically. Both the cooperative (suitable for true monogamy) and the (more realistic) ESS versions predict that the less efficient partner should perform shorter bouts, although the ESS model predicts that both partners should perform shorter bouts than the cooperative model. We explored these predictions with field data on house sparrow, Passer domesticus, incubation. Previous work has shown that male bouts are typically shorter than female bouts. Here, we tested the model's prediction that such an asymmetry in parental care may result from differences in the relative competence of each sex at transferring heat during incubation. Females were notably more efficient incubators: they warmed the nest more than males during the first 4 min of their bouts, increased mean nest temperatures more per degree starting temperature and, by the end of their bouts, they had elevated both final nest temperature and mean nest temperature more per minute than males. Thus, even in the absence of sex differences in parentage, asymmetries in parental effectiveness can promote disparities between the sexes in cooperative investment. We discuss the implications for sexual conflict of parental asymmetries in the value of care.

Regulation of breeding behavior: do energy-demanding periods induce a change in prolactin or corticosterone baseline levels in the common tern (Sterna hirundo)?

Hormones are involved in reproductive decisions, linking environmental cues and body condition and adapting behavior. Mass loss is often accompanied by decreased prolactin and increased corticosterone concentrations, influencing incubation and brooding behavior and ultimately triggering nest desertion. Using blood-sucking bugs (Dipetalogaster maxima), we measured baseline prolactin, corticosterone, and ketone body values in incubating common terns (Sterna hirundo) between 2006 and 2009 during energy-demanding periods: 50 pairs were sampled hungry (after an incubation bout) and again fed (after foraging). In a second approach, we sampled 57 other pairs (experienced and inexperienced birds) three times over their individual breeding period, because reproduction, especially chick rearing, is a very energy-demanding process. In line with the common physiological pattern of fasting, we found significantly lower baseline prolactin values in hungry terns, which were negatively related to mass loss over the incubation bout, whereas corticosterone and ketone body levels were marginally increased. Compared to that in the incubation phase, the prolactin level dropped after hatching of chicks in inexperienced birds, perhaps indicating lower parental expenditure. Corticosterone, on the other hand, increased after hatching in males, probably linked to higher foraging activity, as males mainly deliver food during the first days. These energy-demanding periods clearly influenced hormones and ketone bodies, maybe reinforced by the low energy margin of this small seabird species, but energy reserves were not depleted to a level affecting behavior or reproductive success.

First description of nest, eggs, incubation behavior, and nestlings of Trilling Tapaculo (Scytalopus parvirostris)

Scytalopus, the largest genus of tapaculos (Family: Rhinocryptidae) typically build mossy globular nests within bank or tree cavities. Only 23 of the 40 recognized species' nests have been described, and of these only one has incubation patterns described. We provide the first description of the nest architecture, eggs, incubation behavior, and nestlings of the Trilling Tapaculo (Scytalopus parvirostris). The globular nest was mostly built of dark rootlets, and was lined with fern scales, hyphae, and feathers placed within the cavity of a rotting tree 1.5 m off the ground. The nest contained two white eggs. Nest attentiveness averaged 91 ± 0.5% per day, with an average of 12.6 ± 0.3 trips per day. Incubation bouts on the nest averaged 54.6 ± 2.9 mins and foraging trips averaged 8.8 ± 0.7 mins. Combined daily nestling growth rates averaged 18 ± 2% weight, 9 ± 2% tarsus, and 16 ± 3% wing (n = 4 days).

Nesting of the White-throated WoodcreeperXiphocolaptes albicollis

We report on the breeding biology of the White-throated Woodcreeper (Xiphocolaptes albicollis), a species endemic to the Atlantic forest of South America, based on 16 nesting attempts in 11 cavities (9 natural tree cavities and 2 nest boxes) in Misiones, northeastern Argentina. Natural cavities were 3.3–8.3 cm in diameter and 46–103 cm in depth, and generated by decay processes (not woodpeckers) at heights of 3–17 m in live trees 29–106 cm in diameter at breast height. White-throated Woodcreepers laid 2–4 eggs on alternate days, on a bed comprised of bark flakes, leaf fragments and seed pods. Incubation bouts (n = 3) were more than 1 hr 40 mins. Incubation lasted 17 days and the nestling period 18–22 days, shorter than the congeneric Great Rufous Woodcreeper (Xiphocolaptes major). Both adults brought nest material, incubated the eggs, fed the nestlings, and removed fecal sacs; however, we suspect that the male contributed more to fecal sac removal. Both adults roosted in the cavity a few nights before eggs were laid, but only one adult did so during the incubation and nestling periods. Nestlings were fed arthropods and small vertebrates at an average rate of 3 visits/hr with no change in delivery rate over the nestling period. Adults defended their nests from four bird species but shared one cavity with a roosting female Helmeted Woodpecker (Dryocopus galeatus) throughout their incubation period. We confirm that the White-throated Woodcreeper exhibits biparental care like the Great Rufous Woodcreeper, the Dendrocolaptes and the Lepidocolaptes, but in contrast to Sittasomus, Dendrocincla and Xiphorhynchus. We recommend studies with banded individuals to determine the relative contribution of each parent.

Notes on the Incubation, Brooding, and Provisioning Behavior of Chuck-will's-widows

Abstract Because of their cryptic plumage and nocturnal habits, little is known about the breeding behavior of Chuck-will's-widows (Antrostomus carolinensis). Using temperature loggers and video-recordings, we quantified the incubation, brooding, and provisioning behavior of male and female Chuck-will's-widows in Adams County, Ohio, in 2011 and 2012. We located six nests, all 2-egg clutches laid on the ground. Four nests were successful, one was depredated, and one was abandoned. We quantified incubation behavior at all nests (mean = 5.5 days; range = 0.4–12.9 days). Adult Chuck-will's-widows incubated an average of 89.2% of the time, with females observed incubating more than males. Most incubation off-bouts (n = 104) occurred at dawn (0400–0659; n = 45) and dusk (1900–2200; n = 48) and averaged 35 mins in duration. After hatching, the mean duration of brooding bouts was 20 mins (age of young = 1–7 days). Both males and females provisioned young, and feeding rates were highest (P = 0.003) d...

Biparental incubation patterns in a high-Arctic breeding shorebird: how do pairs divide their duties?

In biparental species, parents may be in conflict over how much they invest into their offspring. To understand this conflict, parental care needs to be accurately measured, something rarely done. Here, we quantitatively describe the outcome of parental conflict in terms of quality, amount, and timing of incubation throughout the 21-day incubation period in a population of semipalmated sandpipers (Calidris pusilla) breeding under continuous daylight in the high Arctic. Incubation quality, measured by egg temperature and incubation constancy, showed no marked difference between the sexes. The amount of incubation, measured as length of incubation bouts, was on average 51min longer per bout for females (11.5h) than for males (10.7h), at first glance suggesting that females invested more than males. However, this difference may have been offset by sex differences in the timing of incubation; females were more often off nest during the warmer period of the day, when foraging conditions were presumably better. Overall, the daily timing of incubation shifted over the incubation period (e.g., for female incubation from evening-night to night-morning) and over the season, but varied considerably among pairs. At one extreme, pairs shared the amount of incubation equally, but one parent always incubated during the colder part of the day; at the other extreme, pairs shifted the start of incubation bouts between days so that each parent experienced similar conditions across the incubation period. Our results highlight how the simultaneous consideration of different aspects of care across time allows sex-specific investment to be more accurately quantified.

Individual birds advance offspring hatching in response to increased temperature after the start of laying

In seasonally reproducing organisms, timing reproduction to match food availability is key to individual fitness. Ambient temperature functions as an important cue for the timing of the food peak in temperate-zone birds. After laying start, individual birds may still improve synchrony between offspring hatching and food availability by adjusting the onset of incubation to most up-to-date cues about the development of the food source. However, it is unknown whether individuals respond to changes in temperature after the onset of laying, and whether individuals adjust incubation onset independent of clutch size. Here, I show in free-living blue tits (Cyanistes caeruleus) that experimental heating of nestboxes in the laying phase resulted in increased duration of nocturnal incubation bouts prior to clutch completion, leading to earlier hatching of eggs and increased hatching asynchrony. Experimental heating did not affect the number of laying gaps, egg volume and clutch size, nor were any carry-over effects on offspring detected. These results are best explained as a response to increased temperature acting as a cue for an advanced food-peak, rather than a relief of energetic constraints, because improved energetic conditions would not favour more hatching asynchrony. Other benefits cannot be excluded, since increased laying-phase incubation under warmer conditions may also help maintain egg viability. This study is the first to show that temperature has a causal effect on the time between clutch completion and hatching of the first offspring, indicating that behavioural adjustment to climate change can continue after laying start.

Factors Affecting Incubation Patterns And Sex Roles of Black Oystercatchers in Alaska

Studies examining the effects of human disturbance on avian parental behavior and reproductive success are fundamental to bird conservation. However, many such studies fail to also consider the influence of natural threats, a variable environment, and parental roles. Our work examines interactive relationships of cyclical (time of day, tide, temperature, seasonality) and stochastic (natural/human disturbance) processes with incubation patterns (attendance, bout lengths, recess rates) of the Black Oystercatcher (Haematopus bachmani), a shorebird of conservation concern. We used 24-hr-per-day video monitoring of 13 molecularly sexed breeding pairs to systematically examine incubation, revealing previously undocumented information that may inform conservation practices for the genus. Seven of 22 video-monitored nests failed, primarily from egg depredation by nocturnal mammals. Analyses of 3177 hr of video footage indicated a near doubling of incubation-bout lengths at night, corresponding to the increased risk of nighttime egg predation. Females had higher overall nest attendance (54% vs. 42%) and longer mean incubation bouts than did males (88 min vs. 73 min). Uninterrupted incubation bouts were over twice as long as bouts interrupted by disturbance. Incubating males departed nests substantially more frequently because of nest-area disturbances than did females in one but not both years of our study. Our findings suggest that the sexes incubate in different but complementary patterns, facilitating efficient egg care in a dynamic environment with several nest threats. We emphasize the importance of considering natural influences when human threats to shorebird reproductive behavior and success are evaluated.

Nesting of the Fulvous-breasted Flatbill (Rhynchocyclus fulvipectus) in Southeastern Perú

Abstract The Fulvous-breasted Flatbill (Rhynchocyclus fulvipectus) has an Andean distribution from Colombia and Venezuela to northeastern Bolivia between 750 and 2,300 m elevation. We describe the nesting behavior, nest, eggs, and nestlings of this species in the buffer zone of Manu National Park at Cock of the Rock Field Station, Cusco, Peru, from August through December 2009. We monitored seven nests using data loggers to describe incubation patterns and conducted direct observations of provisioning behavior. The two-egg clutch size and pear-shaped nest structure were consistent with previous reports. Incubation lasted 24 days (n = 1) and nestlings were in the nest for at least 29 days. We only observed one parent incubating (presumably the female) with average nest attentiveness of 64%, which decreased as the incubation period progressed. The adult made 10 to 15 foraging trips per day (n = 21) during incubation, when it spent on average (±SD) 32.9 ± 2.8 min during incubation bouts and 23.1 ± 6.3 mi...

Sex-Specific Effects of Increased Incubation Demand on Innate Immunity in Black Guillemots

Life-history theory predicts that there should be negative fitness consequences, in terms of future reproduction and survival, for parents with increased reproductive effort. We examined whether increased incubation demand affected innate immunity and body condition by performing a clutch-size manipulation experiment in black guillemots (Cepphus grylle). We found that plasma from males incubating experimentally enlarged clutches exhibited significantly reduced lysis titers compared with plasma from males incubating control clutches, while this was not observed in females. The increased incubation demand also impacted agglutination titers differently in males and females, although the effect of treatment was not significant in either sex. Among all birds, lysis titers increased and haptoglobin concentrations decreased from mid- to late incubation. Natural antibody-mediated agglutination titers and body condition were highly repeatable within the incubation bout and between years. This suggests that agglutination titers may serve as a reliable and resilient index of the immunological character of individuals in future studies. Overall, this study demonstrates that increased incubation demand impacts indices of innate immunity differently in males and females. The potential for different components of the immune system to be impacted sex-specifically should be considered in future studies linking immune function and life-history trade-offs.

Behavioral responses of nesting female dark‐eyed juncos Junco hyemalis to hetero‐ and conspecific passerine preen oils

Several studies have suggested a greater role for olfactory cues in avian social interactions than previously recognized, but few have explicitly investigated the effect of odor on parental behavior. We present results from a preliminary study in which we applied hetero‐ and conspecific preen gland secretions, which are known to contain volatile compounds, to the nests and eggs of incubating female dark‐eyed juncos Junco hyemalis. The responses to these two conditions were compared to the responses of females whose nests were treated with their own preen oil as a control condition, and to females whose nests were treated with the vehicle only. We found that females significantly reduced incubation bout length, a form of parental care, in response to alien secretions, more so if they came from a heterospecific than a conspecific. Females did not reduce incubation bout length in response to their own preen oil or to a vehicle‐only control. These results suggest that odors in the nest may influence avian parental care. However, the behavioral change was only temporary and had no effect on later hatching success. In our study population, brood parasitism by brown‐headed cowbirds is common, but resulting nest abandonment is rare; juncos are frequently able to successfully breed even with cowbird nestlings in their nests. Thus, we suggest that more extreme behavioral responses to alien odor, such as nest abandonment or egg ejection, may not be adaptive and should not be expected.

Negotiation of parental care when the stakes are high: experimental handicapping of one partner during incubation leads to short-term generosity

1. Most game theoretical models of biparental care predict that a reduction in care by one partner should not be fully compensated by increased work of its mate but this may not be true for incubating birds because a reduction in care could cause the entire brood to fail. 2. I performed the first handicapping experiment of both males and females during incubation, by placing small lead weights on the tails of male and female northern flickers Colaptes auratus, a woodpecker in which males do most of the incubation. 3. Females responded to the acute stressor (handling and handicapping) by tending to abandon more readily than males and staying away from the nest longer in the first incubation bout. Among pairs that persisted, both males and females compensated fully for a handicapped partner, keeping the eggs covered nearly 100% of the time. 4. Partners did not retaliate by forcing their handicapped mate to sit on the eggs with a long incubation bout length subsequent to having a long bout length themselves. Instead, during the 24 h immediately after handicapping, males behaved generously by relieving handicapped females early. 5. Such generosity was probably not energetically sustainable as these male partners took on less incubation in the 72 h following handicapping compared to female partners of handicapped males. Males and females are probably generous in the short-term because of the high cost of nest failure during incubation but maintaining increased work loads in the longer term is probably limited by body condition and abandonment thresholds consistent with game theory models.

Influence of food and nest predation on the life histories of two large honeyeaters

Nests of Red Wattlebirds (Anthochaera carunculata) and Noisy Friarbirds (Philemon corniculatus) were watched during four breeding seasons near Armidale, New South Wales (NSW). Friarbirds have larger clutches than Wattlebirds (mode of three v. two) and a shorter breeding season. Two hypotheses (food and nest predation) could explain these differences in life history between the species. We predict that if the food hypothesis is correct Friarbirds would experience a greater seasonal range in food abundance, and hence more abundant food during the breeding season. We also predict that Friarbirds should spend longer incubating, brooding and guarding chicks, because they need to spend less time feeding. If the nest predation hypothesis is correct, Wattlebirds would suffer higher nest predation, and so should spend more time incubating, brooding and guarding their nests, and should visit the nest more synchronously and attack more nest predators. Synchronous visits to the nest reduce the likelihood that predators will be attracted to the nest. We found that Friarbirds spent a greater percentage of time incubating and brooding in the first week after nestlings hatched, and had longer incubation and brooding bouts than Wattlebirds. Wattlebirds left their nests unattended for a greater percentage of the time during incubation, but there was no difference during the nestling stage. There were no differences in the number of incubation bouts per hour, nor in the number of brooding bouts per hour between species. Friarbirds more often visited the nest synchronously during the nestling stage. There was no difference in nest success between the species, with most failures resulting from predation. These results support more of the predictions of the food hypothesis than the nest predation hypothesis. Friarbirds feed themselves and their young on large insects, such as cicadas and scarab beetles, which are abundant for only a short period in the study area, when Friarbird young are dependent. Wattlebirds feed their young on a wide range of insects, including many small ones, which are available over a longer period. The range of clutch-sizes and breeding seasons shown by honeyeaters generally, and friarbirds specifically, provide the opportunity for further testing of these and other hypotheses.

Prolactin, body condition and the cost of good parenting: an interyear study in a long‐lived seabird, Gould's Petrel (Pterodroma leucoptera)

Summary The pituitary hormone prolactin is thought to play an important role in the promotion of parental care in birds and mammals. The level of care parents provide is, however, likely to be influenced by additional factors, such as their physiological condition at the time of breeding. We examined relationships between parental body condition, plasma prolactin levels and reproductive performance in Gould's Petrels (Pterodroma leucoptera), a long‐lived seabird. We predicted that parental body condition would correlate positively with both prolactin level and parenting intensity, as measured by the quality of the chick they produced. We also examined the effects of parenting intensity on parental body condition and reproductive success in the subsequent breeding season. Body condition of male parents positively correlated with prolactin levels at the start of their second protracted incubation bout. The body condition of both parents correlated positively with the body condition of their chick at its peak mass. However, producing a good‐quality chick did not negatively affect parental body condition or reproductive success the following year. These results suggest that prolactin reinforces parental behaviour in parents in good body condition, which facilitates production of good‐quality chicks. Moreover, good‐quality parents consistently produce good‐quality chicks with no apparent trade‐off to their physical condition.

Time allocation between feeding and incubation in uniparental arctic‐breeding shorebirds: energy reserves provide leeway in a tight schedule

Birds with uniparental incubation may face a time allocation problem between incubation and feeding. Eggs need regular warming to hatch successfully, but the parent must leave the nest to feed and safeguard its own survival. Time allocation during incubation is likely to depend on factors influencing egg cooling rates, parental energy requirements and feeding intake rate. How this allocation problem is resolved was subject of this study on arctic‐breeding shorebirds. We compared incubation rhythms between four uniparental shorebird species differing in size and expected to find both species differences and weather effects on the organisation of incubation.Attentive behaviour and responses to variation in weather showed a remarkable consistency across species. All species alternated feeding bouts (recesses) with brooding bouts throughout the day. Recesses were concentrated in the warmer parts of the day, while recess duration showed little diurnal variation. Despite continuous daylight, a pronounced day‐night rhythmicity was apparent. The four species in this study spent a similar proportion (13–19%) of the time off their nest. After correction for weather effects, the number of recesses was largest in the smallest species, while recess duration was longest in the largest species. Total recess time per day increased on cold days through an increase of mean recess length, while the number of recesses decreased.Comparing our observations to predictions derived from criteria that birds might use to organise their attentive behaviour, showed that the limits are set by parental requirements, while the energy stores of adults provide some leeway for short‐term adjustments to environmental variability.If breeding birds trade off feeding time against incubation time, energy stores are expected to be influenced by weather. We expected uniparental species to be more likely to show weather effects on condition than biparentals, as in the latter ‘off duty’ time is much larger and independent of weather. This prediction was tested by comparing energy stores in two uniparental species and a biparental congener. While body mass of uniparental incubators decreased after a period with low temperatures, body mass of the biparental species did not.

Breeding biology of the Grey Shrike-thrush (Colluricincla harmonica)

Grey Shrike-thrushes (Colluricincla harmonica) occur in a wide range of habitats over most of Australia but their life history and breeding behaviour have not been fully documented. Over eight seasons, from 1996 to 2004, we studied the reproductive efforts of Grey Shrike-thrushes using primarily property homesteads as nest sites in the Warrumbungle Mountains near Coonabarabran, New South Wales, Australia. Twenty-four nests were observed at three sites, involving six male and six female birds. Eggs were laid at intervals of ∼24 h and the clutch size was usually three (range 2–4). Parents contributed almost equally to incubation, brooding and feeding nestlings and removing faecal sacs during the day, whereas the female alone incubated and brooded at night. After the laying period, the birds combined to incubate the eggs for 93–97% of time. Incubation bouts averaged 41.5 min during laying, 50.7 min during early incubation, and 42.6 min (males) and 50.2 min (females) during late incubation. Mean length of bouts decreased to 10.5 min during the hatchling period and 13.9 min during the early nestling period. The mean incubation period was 17.4 days, the nestling period 15.8 days and the breeding cycle (incubation plus nestling) 32.5 days. Successful fledging of one brood was often followed by re-nesting, with up to three attempts in a single breeding season. The median period to re-nesting (last young fledged to first egg of new clutch) was 16 days, and median length of the breeding season was 18.3 weeks (4.2 months). There was a significant decrease in the length of the nestling period and the breeding cycle for later (second or third) attempts compared with first attempts. Habituated Shrike-thrushes (those nesting near houses; n = 22 attempts) fledged on average 2.7 young per nest (89% success rate). Experienced pairs of habituated Shrike-thrushes fledged an average of 3.1 young per nest compared with inexperienced pairs, which averaged 1.3 young per nest.

EFFECTS OF FOOD SUPPLEMENTATION ON FEMALE NEST ATTENTIVENESS AND INCUBATION MATE FEEDING IN TWO SYMPATRIC WREN SPECIES

We examined effects of incubation mate feeding on female incubation behavior and correlates of fitness by providing female Bewick's Wrens (Thryomanes bewickii) and House Wrens (Troglodytes aedon) with food supplements. Males of these species vary in their rates of feeding; Bewick's Wrens feed their incubating mates frequently, whereas House Wrens seldom engage in this behavior. Average length of incubation bout and nest attentiveness (proportion of time spent on the nest) were higher for supplemented female Bewick's Wrens and House Wrens compared to controls. Furthermore, mates of supplemented Bewick's Wrens provisioned females at lower rates than controls, and their rate of feeding was inversely correlated with ambient temperature. Incubation length and hatching success were not significantly different between treatments for either species. These results suggest that incubation mate feeding can increase female nest attentiveness and perhaps enhance fitness of both males and females. In House Wrens, potential tradeoffs between the benefits of parental care and opportunities to obtain additional mates may explain why males rarely feed incubating females.

How Kentish plovers, Charadrius alexandrinus , cope with heat stress during incubation

Biparental incubation is frequent among shorebirds and is expected when the survival prospects of offspring increase relative to uniparental incubation. To understand why this occurs, it is important to identify the factors that constrain uniparental incubation. It is assumed that birds choose nesting sites that provide an appropriate microclimate for incubation. Many shorebirds nest in sites with no or little cover, where ambient temperatures at ground level might be >50°C during very hot days. Shorebirds nest in exposed sites because predation risk on incubating adults is higher in covered sites. In hot environments, incubating shorebirds might experience heat stress in exposed sites, and this may compromise nesting success if adults are unable to attend their nests continuously, limiting the possibilities of uniparental incubation and thus the expression of a sexual conflict over incubation. The operative temperatures of Kentish plovers (Charadrius alexandrinus) were recorded in exposed and covered sites, and the thermal behaviour and incubating tactics of pair members were studied in a hot environment. During the hottest part of the day, there was a difference of 10–15°C in the operative temperatures of plovers between covered and exposed sites. Plovers in covered sites did not exhibit any thermoregulatory behaviour indicative of thermal stress, probably because the thermal range encountered by them in such places during most of the daytime was close to the thermo-neutral zone. The frequency with which plovers in exposed sites exhibited thermoregulatory behaviour was related to ambient temperature. Under very hot conditions, incubating birds were probably unable to maintain homeostasis for long periods and pair members resorted to shortening incubation bouts. Female Kentish plovers mainly incubate in the daytime and males during the night. However, the probability of diurnal incubation by males increased with ambient temperature in exposed nests, but not in covered ones. In fact, the frequency of participation in diurnal incubation by males was greater in exposed than in covered sites, suggesting that the participation of males in diurnal incubation may be related to the inability of females to stay at the nest during long periods when the ambient temperature is high. Even after resorting to shortened incubation bouts, the plovers may be unable to attend their nests continuously during heat waves, and the nests may be deserted. The propensity of plovers to desert their nests was affected by proximity to water, with nests located close to water being deserted less frequently. It seems likely that susceptibility to thermal stress changed in relation to proximity to water because in sites close to water it was possible to belly-soak, which would allow a more continuous nest attendance. Therefore, despite the adoption of behavioural solutions to face heavy heat loads, nesting success was vulnerable to these solutions because heat stress during extended periods may constrain parental nest attendance, and this may limit the opportunities for sexual conflicts over incubation.

EARLY ONSET OF INCUBATION BY WOOD DUCKS

Abstract I examined onset of incubation in Wood Ducks (Aix sponsa) and evaluated the hypotheses that early onset improves hatchability, reduces brood parasitism, and shortens incubation periods. Most (21 of 22) females began incubating at night, a median of 4 days before egg laying ended. Nocturnal incubation bouts began 18 min before sunset, ended 15 min before sunrise, lasted 732 min each night, and totaled 47.2 hr before egg-laying ended (means). Nocturnal incubation did not begin earlier in the egg-laying period as the breeding season progressed, as would be expected if it improved hatchability of first-laid eggs. Early onset of incubation did not reduce brood parasitism. Females ended nocturnal incubation 35 min before egg laying began, the number of nights of incubation was not related to the number of parasitic eggs laid, and most (83%) nests were parasitized. In support of the third hypothesis, egg-laying females spending more nights incubating had somewhat shorter incubation periods. Inicio Tempr...

Breeding biology of the Rainbow Pitta, Pitta iris, a species endemic to Australian monsoon-tropical rainforests

The Rainbow Pitta, Pitta iris, is endemic to the monsoon rainforests of the wet—dry tropics of northwestern Australia. In the Darwin region, Northern Territory, the breeding season lasted 3–4 months, eggs being laid from mid October to late February over three years (1992–95). This period coincided with the wet season, and the start and length of the breeding season varied annually with rainfall. The number of nests was correlated with both rainfall and the abundance of earthworms, the major food item of the species during the wet season. During one season two out of four intensively studied pairs attempted second broods, building new nests only 3–5 days after the first brood fledged. The large domed nests were built in a variety of plant species and sites, but half were built on the tops of stumps or in tree-trunk forks. Heights of the 111 nests varied from ground level (9% only) to 20 m, averaging 5.4 m. Both sexes built the nest, incubated eggs and fed the young. Clutches consisted of 3–5 eggs, the average being 3.9 eggs (n = 21). The incubation and nestling periods each averaged only 14 days. Nests were rarely left unattended, and six incubation bouts averaged 87 min (s.d. = 27 min). Fledglings remained in their natal territory for 15–20 days before dispersing to other forest patches. Fledglings had significantly shorter primary and tail feathers than adults, but tarsus length was similar. The relatively large clutch, and short breeding season, nest cycle and period of dependency of this species are atypical for Australian and tropical passerines in general, but may be typical for this predominantly south-east Asian family. In the Rainbow Pitta, these life-history traits may be facilitated by the super-abundance of earthworms during the wet season.