Robert A. Kennamer

Incubation as a Reproductive Cost in Female Wood Ducks

-We investigated the effects of body mass of incubating female Wood Ducks (Aix sponsa) on aspects of their current and future reproduction, and we examined factors that affect length of the incubation period. During three breeding seasons, body mass of female Wood Ducks averaged 578.0 g early and 553.3 g late in the incubation period. Body mass at the start of incubation was not related to either hatching success or length of the incubation period. In one of three years, females that were heavy at the end of incubation survived better to the next breeding season than those that were light. Reduced survival of light females in one year coincided with a greater loss of body mass in that year relative to other years, which indicates that incubation can be an important reproductive cost to female Wood Ducks. There were no relationships between body mass at the end of incubation and date of nesting or clutch size in the next breeding season. Partial correlations between clutch mass and length of incubation that controlled for date of nesting indicated a positive association between clutch mass and incubation length in every year. This relationship was evident only for parasitic nests (i.e. nests in which more than one female was laying eggs). Increased length of the incubation period associated with larger clutch mass represents a potential cost of intraspecific nest parasitism not previously recognized. Received 26 December 1989, accepted 17 June 1990. INCUBATING birds must provide the proper thermal environment for embryonic development. Simultaneously they must maintain their body condition so that survival and subsequent reproduction are not affected adversely. Time for feeding is restricted during incubation, which often makes it difficult for incubating individuals to meet daily metabolic costs (see Drent et al. 1985). Some avian species have adjusted to the demands of incubation by having biparental incubation (Eisner 1960, Feare 1984). In other species, males provide incubating females with food (Lyon and Montgomerie 1985, Nilsson and Smith 1988). In waterfowl (Anatidae), females of large-bodied species generally begin incubation with large energy reserves and are more attentive during incubation than females of small-bodied species, because largebodied females spend less time feeding (review in Afton and Paulus 1990). Small anatids depend on exogenous foods to meet most metabolic demands during incubation and take two 2 Present address: Maryland Department of Natural Resources, P.O. Box. 68, Wye Mills, Maryland 21679 USA. to three recesses each day to forage (Afton 1980, Hohman 1986). Successful development of bird eggs occurs within a relatively narrow range of incubation temperatures (White and Kinney 1974). Cooling of eggs increases as ambient temperature decreases, and as time away from the nest by incubating individuals increases (Caldwell and Cornwell 1975, Afton 1979). Short-term declines in egg temperature, however, apparently have little effect on hatching success (Vleck 1981, Haftorn 1988). Nevertheless, a decrease in average egg temperature may lengthen the incubation period, which exposes the nest to greater risk of predation and increases the energy expended by developing embryos (Vleck et al. 1980, Booth 1987). Greater amounts of energy used by embryos of precocial species before hatching may decrease the size of residual yolk reserves that are important to newly hatched chicks for maintenance and growth (Peach and Thomas 1986). Many species of birds modify activity patterns during incubation in response to variation in weather and food availability (Caldwell and Cornwell 1975, Cartar and Montgomerie 1985, 756 The Auk 107: 756-764. October 1990 This content downloaded from 157.55.39.124 on Mon, 16 Jan 2017 18:11:55 UTC All use subject to http://about.jstor.org/terms October 1990] Incubation Costs of Wood Ducks 757 Drent et al. 1985). Large-bodied species are affected less by environmental factors than smallbodied species (Afton 1980). Flexibility of incubation patterns within and among species suggests there is a tradeoff between maintaining body condition during incubation and providing eggs with a suitable environment for development. If time and energy constraints are important during incubation, then attentiveness at the nest should increase as body condition or food availability increases, assuming that greater attentiveness shortens the incubation period and increases hatching success (Martin 1987). Several studies support this idea. Aldrich and Raveling (1983) reported that female Canada Geese (Branta canadensis moffitti) that began incubation in good condition spent more time on the nest and had shorter incubation periods than females in poor condition. In years when food was abundant, European Starlings (Sturnus vulgaris) spent less time feeding, and some females were able to incubate without assistance from their mates (Drent et al. 1985). Female Blue Tits (Parus caeruleus) that were given a food supplement during incubation had shorter incubation periods and greater hatching success than females that were not provisioned (Nilsson and Smith 1988). Loss of body mass during incubation may reflect the need to provide constant care for developing embryos (Sherry et al. 1980), or it may enable females to reduce wingloading and to conserve energy during brood rearing (Freed 1981). However, a critical body mass certainly exists for individuals during incubation. Below that threshold, birds either spend more time feeding (Aldrich and Raveling 1983) or they abandon the nest (Drent 1975, Ankney and MacInnes 1978, Jones 1987). Body mass of female waterfowl during the annual cycle typically is lowest at the end of incubation (Afton and Paulus 1990). Ability of individuals to balance the conflicting demands of incubation may influence current and future reproductive suc-

Recruitment and Natal Philopatry of Wood Ducks

We tested whether hatch date and body mass at hatching affected recruitment of female Wood Ducks (Aix sponsa) to a breeding population in South Carolina. During six breeding seasons, day—old ducklings (n = 2945) were individually marked before leaving the nest. Females were recaptured later while nesting, and recruitment averaged 5.2% with no yearly differences. Most (82%) of the returning females nested as yearlings, but drought conditions in one year postponed the breeding of some females until they were 2 and 3 yr old. Most (60%) females did not return to nest on natal areas, but dispersed to sites a short distance away (mean = 1.6 km). Probability of dispersal was not related to competition for nesting space, and dispersing females did not begin nesting later than females that remained on natal areas. There was no evidence that early—hatched females had greater probabilities of being recruited than late—hatched females. In fact, late—hatched females had greater recruitment in two of six years than females that hatched early in the season. There also was no relationship between hatch date and the time that female Wood Ducks initiated their first nests. Heavy ducklings at hatching had higher probabilities of being recruited in one of six years, but composite statistics showed no overall relationship. We suggest that hatch date affects recruitment more at northern latitudes where time for producing and rearing offspring is short. Individuals hatching late at high latitudes may have insufficient time to mature before they must begin autumn migration.

Characteristics and consequences of nest-site fidelity in wood ducks

-We used nine years of nesting data from a population of Wood Ducks (Aix sponsa) using nest boxes to test predictions regarding proximate controls of nest-site fidelity and the consequences of returning to the same nest site. Overall, 41.9% of females returned to the same nest box in year t + 1, 37.5% nested in a different box on the same wetland, and 20.5% of females moved to a different wetland to nest. There were no yearly differences in the degree of nest-site fidelity, and females using different wetlands between years travelled a median of two wetlands and moved an average distance of 1.3 km. Females nesting successfully used the same nest box to a greater extent in year t + 1 than females that were not successful. The positive association between nest success and nest-site fidelity also occurred within breeding seasons. Degree of nest-site fidelity exhibited by females was similar within and between breeding seasons. After controlling for variation in nest success, nest-site fidelity of yearlings did not differ from that of adults. Proportion of females returning to the same box in year t + 1 was not correlated with estimated population size of breeding females in that year. Females returning to the same box nested earlier than females using different boxes, but clutch size did not differ. Overall, females nesting in the same box did not have greater nest success in year t + 1, were not more likely to have nests parasitized, and did not survive better to year t + 2 than females nesting in different boxes. However, females that nested unsuccessfully tended to improve nest success by moving to a different nest box. Received 1 August 1991, accepted 18 February 1992. MANY SPECIES of migratory birds show a high degree of fidelity to previous breeding sites both within and between seasons (Greenwood and Harvey 1982). Males of most species have greater breeding-site fidelity than do females (Greenwood 1980). In waterfowl, however, female-biased natal philopatry and nest-site fidelity is the rule (Rohwer and Anderson 1988). Pair bonds are formed on winter areas or during spring migration, and the pair returns to the females natal area or previous nesting location (Dow and Fredga 1983, Hepp et al. 1989, Gauthier 1990). Widespread occurrence of breeding-site fidelity in birds suggests that the behavior is beneficial. Familiarity with an area may increase foraging efficiency, predator avoidance, dominance status, and the likelihood of pairing with a familiar partner, all of which may enhance reproductive success (Greenwood and Harvey 1982). Common Goldeneyes (Bucephala clangula), for example, that changed nest boxes nested later, produced smaller clutches, and had lower nesting success than females that did not change nest sites (Dow and Fredga 1983). Female Willow Ptarmigans (Lagopus lagopus) pairing with previous mates nested earlier and produced heavier chicks than females that switched partners (Schieck and Hannon 1989). The decision to return to a previous breeding site may involve several factors. Competition for nest sites may influence whether individuals return and may be especially important in cavity-nesting species (Dow and Fredga 1983). Previous experience is another factor influencing nest-site fidelity in birds. Individuals nesting successfully are more likely to return to the same nest site than individuals that are not successful (Gavin and Bollinger 1988, Gauthier 1990, Beletsky and Orians 1991; but see Haig and Oring 1988). Site quality may interact with nest success to influence subsequent nesting decisions (Weatherhead and Boak 1986, Bollinger and Gavin 1989). Nest-site fidelity also may be influenced by age, with yearlings having a greater probability of changing nest sites than adults (Newton and Marquiss 1982). Causes for age-specific differences in fidelity are unclear (Greenwood and Harvey 1982). It is possible that young individuals use poor-quality sites during initial nest attempts and move to betterquality sites when these sites become available.

Population parameters and philopatry of breeding female wood ducks

Capture-recapture methods were used to estimate population size, survival rate, and recruitment to a breeding population of female wood ducks (Aix sponsa) in 1979-86. A total of 181 females was captured in nest boxes and banded during the 8-year period. Population size averaged 44 individuals. Mean annual survival rate was 0.55, and annual recruitment averaged 22 females. Data were used to illustrate the philopatric behavior of female wood ducks during the breeding season. J. WILDL. MANAGE. 51(2):401-404 Estimation of waterfowl survival rates has been improved by developing and using a series of models for analyzing band-recovery data (Brownie et al. 1978). Studies using these analytical methods have examined the relationship between survival and harvest rate (reviewed in Nichols et al. [1985]), population density (Anderson 1975, Conroy and Eberhardt 1983), and habitat conditions on breeding (Nichols et al. 1982) and wintering (Nichols and Hines, in press) areas. Capture-recapture methods allow estimation of population size and recruitment as well as survival, but are used infrequently to study avian population dynamics (Nichols et al. 1981, Pollock 1981). Open population models that allow gains and/or losses to occur between sampling periods seem particularly well-suited for long-term studies of waterfowl populations (Anderson and Sterling 1974, Sulzbach and Cooke 1979). In this paper we use 8 years of capture-recapture data from a breeding population of female wood ducks to estimate population size, recruitment, and survival and to determine whether wood duck hens are philopatric. We thank J. D. Nichols and J. E. Hines for helping with data analysis. J. D. Nichols provided an especially helpful review of an early draft of the paper. We also are grateful to the many people who helped in various ways durIPresent address: Indiana Department of Natural Resources, R.R. 4, Box 47, Peru, IN 46970. This content downloaded from 157.55.39.238 on Sat, 02 Jul 2016 05:33:15 UTC All use subject to http://about.jstor.org/terms 402 WOOD DUCK HEN SURVIVAL * Hepp et al. J. Wildl. Manage. 51(2):1987 ing the study, particularly L. D. Vangilder. This study was supported by a U.S. Dep. Energy Contract, DE-AC09-76SR00-819, with the Univ. Georgia (SREL).

Embryonic Development and Nest Attentiveness of Wood Ducks during Egg Laying

Waterfowl often begin incubation during egg laying, thus creating developmental asynchrony within clutches. We investigated levels of intraclutch developmental asynchrony (IDA) during early incubation within naturally incubated clutches of Wood Ducks (Aix sponsa) in South Carolina. IDA averaged 2.2 ? 1.0 SD days (n = 59) during a 2-year study. A higher average level of IDA in 1988 corresponded with larger average clutch size during that year. Date of nesting did not affect levels of IDA, but larger clutches had increased levels of IDA. In 1988, reduced hatching success was observed in clutches with greater than 3 days of developmental asynchrony, suggesting a cost associated with the early initiation of incubation while still laying eggs. Nest attendance data collected during the later stages of egg laying indicated that females spent approximately 50% of the day at the nest. Female Wood Ducks engaged in nocturnal nest attendance during egg laying thereby allowing time to meet the nutrient demands of egg synthesis during the diurnal period.

Warm is better: incubation temperature influences apparent survival and recruitment of wood ducks (Aix sponsa).

Avian parents that physically incubate their eggs must balance demands of self-maintenance with providing the proper thermal environment for egg development. Low incubation temperatures can lengthen the incubation period and produce changes in neonate phenotype that may influence subsequent survival and reproduction. We artificially incubated wood duck (Aix sponsa) eggs at three temperature regimes (low, 35.0; mid, 35.9; and high, 37.3°C) that are within the range of temperatures of naturally-incubated nests. We tested the effect of incubation temperature on duckling body composition, fledging success, the probability that females were recruited to the breeding population, and their subsequent reproductive success. Incubation period was inversely related to incubation temperature, and body mass and lipid mass for newly-hatched ducklings incubated at the lowest temperature were lower than for ducklings produced at higher temperatures. In 2008, ducklings (n = 412) were individually marked and broods (n = 38) containing ducklings from each temperature treatment were placed with wild foster mothers within 24 hrs of hatching. Ducklings incubated at the lowest temperature were less likely to fledge from nest sites than ducklings incubated at the higher temperatures. We recaptured female ducklings as adults when they were either prospecting for nest sites (n = 171; 2009–2011) or nesting (n = 527; 2009–2012). Female ducklings incubated at the lowest temperature were less likely to survive and be recruited to the breeding population than females incubated at higher temperatures. Reproductive success of surviving females also was greater for females that had been incubated at warmer temperatures. To our knowledge, this is the first avian study to link developmental conditions experienced by neonates during incubation with their survival and recruitment to the breeding population, and subsequent reproductive success. These results advance our understanding of incubation as an important reproductive cost in birds and support the potential significance of incubation in influencing the evolution of avian life histories.

Date of Nest Initiation Mediates Incubation Costs of Wood Ducks (Aix sponsa)

ABSTRACT. Incubation has a significant reproductive cost in birds that can limit both current and future reproductive success. We manipulated the incubation period of Wood Ducks (Aix sponsa) to examine how female body mass, incubation behavior, and nest temperature responded to changes in incubation costs. We found no relationships between the length of the incubation period and either the percent loss of body mass or the body mass at the end of incubation. However, females that initiated nests early in the season lost more body mass than females that nested later. The number of daily recesses increased slightly with longer incubation periods, but incubation constancy over the full incubation period and during the last week of incubation was not affected by incubation-period length. Variation in incubation constancy was explained best by nest initiation date. Incubation constancy was greatest for early-nesting females and declined for females that nested later. There also was a weak positive relationship between body mass and incubation constancy. Average nest temperature was not associated with the length of the incubation period but increased with increasing incubation constancy and as the breeding season progressed. Evidence from this study is consistent with the idea that incubation is an important reproductive cost that may constrain the timing of nest initiation in Wood Ducks. Poor-quality females, by delaying nest initiation, experienced reduced incubation costs and were able to maintain nest temperatures at levels similar to those of early-nesting females.

Plasticity of incubation behaviors helps Wood Ducks (Aix sponsa) maintain an optimal thermal environment for developing embryos

ABSTRACT Optimal development of avian embryos occurs within a narrow range of incubation temperatures. Most parents that physically incubate their eggs through direct contact are challenged to balance their time on the nest with taking foraging recesses to satisfy their energetic requirements. To explore the costs and investment strategies of incubating female Wood Ducks (Aix sponsa), we manipulated the microclimate of nests by reducing down insulation from the typical 4.0 g to 0.5 g. Cooling rates of clutches during morning recesses increased when down insulation was reduced, especially at low ambient temperatures. Females with reduced down responded to increased cooling rates by shortening morning recesses and increasing daily incubation constancy, and these behavioral changes were independent of their body mass at the start of incubation. Females in both treatment groups responded similarly to changes in ambient temperature and spent less time incubating as ambient temperatures increased. Clutch temperatures at the end of morning recesses were similar for females with reduced and normal insulation. Average clutch temperatures for the full incubation period did not differ between treatments, and, correspondingly, there were no differences in length of the incubation period, hatching success, or duckling phenotype. Our results show that female Wood Ducks were sensitive to changes in both clutch temperature and ambient temperature and that they modified their time on the nest to provide developing eggs with an optimal thermal environment without negatively affecting their body mass at the end of incubation. Further examination of the limits of behavioral plasticity in incubating birds will be essential, particularly in light of future challenges presented by climate change.

Effects of current reproductive success and individual heterogeneity on survival and future reproductive success of female Wood Ducks

ABSTRACT Estimates of vital rates and their sources of variation are necessary to understand the population dynamics of any organism. These data have been used to test predictions of life history theory as well as to guide decisions of wildlife managers and conservation biologists. Life history theory predicts tradeoffs among life history traits, such that current reproductive effort will be negatively correlated with survival and/or future reproduction. Many studies support this prediction, but others report positive covariation between fitness traits, and attribute positive correlations to differences in individual quality. In this study, we used 11 yr of capture–mark–recapture data of breeding female Wood Ducks (Aix sponsa), along with their breeding histories, to examine sources of variation in annual survival rates and to assess the impact of current reproductive success on probabilities of survival and future reproductive success. Cormack-Jolly-Seber models indicated that apparent survival of female Wood Ducks did not vary annually and was only weakly affected by age class and breeding habitat conditions, but that there was a strong positive relationship between survival and the number of successful nests (0, 1, or 2). Next, we used a multistate analysis to examine the importance of female nest fate (successful or failed) on the probability of surviving and of nesting successfully the next year. Early incubation body mass was used to assess the nutritional status and quality of females. Females that nested successfully in year t were not less likely to nest successfully in year t + 1 than females that had nested unsuccessfully in year t. We also found strong positive covariation between nest success in year t and the probability of surviving. However, being in relatively good or poor condition had no effect on these relationships. Our results are consistent with the idea that female quality is heterogeneous, but body mass was not a good proxy of quality. Therefore, the existence of tradeoffs between female reproductive success and survival or future reproduction was less clear because of our inability to identify and control for differences in female quality.

Wood duck hatch date: relationship to pairing chronology, plasma luteinizing hormone, and steroid hormones during autumn and winter.

We examined variation in courtship activity and hormone levels of male and female wood ducks (Aix sponsa) in relation to hatch date. Young wood ducks were assigned in groups of 8 (4 males and 4 females) to 4 experimental pens; 2 pens contained early-hatched ducks (3-12 April) and 2 pens contained late-hatched ducks (7-16 June). Courtship behaviors occurred less frequently in October and November than in December and January-February for both early- and late-hatched groups. Early-hatched wood ducks participated in courtship more frequently and formed pair bonds sooner than late-hatched individuals. Testosterone (T) and androstenedione (AD) levels of males did not differ between treatment groups; however, average levels of luteinizing hormone (LH) were greater for early-hatched males. Differences in LH occurred in 2 of 12 samples (6 October and 3 November); otherwise, LH did not vary by treatment. Levels of T and LH in males varied temporally, but there was no significant temporal variation in AE levels. Temporal variation in hormone levels was similar for early- and late-hatched males. Estradiol (E), progesterone (P), and LH levels of females did not differ between treatment groups. There was temporal variation in levels of E and LH, but not of P; this variation was similar for early- and late-hatched females. These data indicate that behavioral differences occurring temporally and between early- and late-hatched wood ducks were not related to corresponding differences in hormone levels.