Robert G. Clark

Avian habitat selection : Pattern from process in nest-site use by ducks?

Patterns of habitat use in animals presumably have evolved in response to diverse selective processes, so we first examined whether the theory of natural selection formed the conceptual framework for published studies (N = 270) of nest-site selection by birds. Most (61%) studies of nest-site selection tested for pattern arising from natural selection (whether used nest habitat differed from available nesting habitat), many (54%) tested for evidence of the process of natural selection (whether unsuccessful nests differed from successful nests), some (10%) tested whether the process of natural selection caused subsequent adaptation, but remarkably few conceptually linked these elements or used the theory of natural selection as the rationale for their questions. We then tested for evidence of natural (phenotypic) selection with data for six species of ducks. At nests, we used six variables to describe vegetation structure/nest position and categorized patch types (pond edge, native grass, planted cover, shrubs, or trees) in which nests were found; nest fates (abandoned, depredated, or successful) were also determined. For Blue-winged Teal (Anas discors), Northern Shoveler (A. clypeata), and Mallard (A. platyrhynchos), there were significant patterns of nonrandom nest-site placement within a gradient of vegetation structure/nest position. For Blue-winged Teal and Gadwall (A. strep- era), nest success varied within these gradients in a way that could exert directional se- lection. Several tests for adaptive nest-site choice were conducted. Nest fate did not influence fidelity of females to patch types. However, Mallards with previously unsuccessful nests dispersed farther than females with previously successful nests. Nonetheless, neither fidelity to patch type nor dispersal distance influenced subsequent nest success. In the long term (over 8 yr), there was a weak tendency within species for nest density to be higher among patch types where relative nest success was higher. In the short term (from year t to year t + 1), this pattern was not observed in a vegetation-structure/nest-position gradient for any species. The strongest evidence of adaptive response to nest fate was higher nest density on an island where nest success was relatively high.

VISUAL AND OLFACTORY ATTRIBUTES OF ARTIFICIAL NESTS

Abstract Artificial nests are commonly used to investigate relative rates of nest predation in birds, but several methodological considerations need to be addressed before results from natural and artificial nests can be compared. Using field and laboratory experiments, we examined responses of predators to visual and olfactory cues that were associated with wicker nests and their contents. Avian predators did not discriminate between wicker nests dipped in mud and those covered by a camouflage fabric, whereas mammalian predators showed a weak tendency to depredate camouflaged nests. Nests containing plasticine eggs were depredated more often than nests containing only quail eggs and finch eggs, although no response to number of plasticine eggs in nests was found. The higher predation of nests with plasticine eggs may have resulted because small mammals, relying on olfactory cues, comprised a large portion of the predator assemblage. Field results were supported in tests where captive deer mice (Peromyscus maniculatus) were attracted to assortments of egg types that included plasticine. Time required by captive deer mice to penetrate quail eggs and finch eggs versus plasticine eggs varied as a function of egg size and shell thickness and strength. Overall, domestic finch eggs provided a better alternative to quail eggs because they were small enough to allow detection of predation events by small mammals and did not have an unnatural odor like plasticine. Potential problems with nest concealment, egg visibility, egg odors, and other factors must be resolved to enhance the design and reliability of artificial nest experiments.

Timing of nest searches for upland nesting waterfowl

Searches for duck nests should maximize the probability of locating nests, while minimizing the possibility of causing nest abandonment. Thus, we collected nest attendance data for 44 mallard (Anas platyrhynchos), 13 blue-winged teal (A. discors), 12 lesser scaup (Aythya affinis), 8 northern shoveler (Anas clypeata), 6 gadwall (A. strepera), and 4 green-winged teal (A. crecca) females to determine the optimal times of day to search for active nests. Females of most species attended nests from 0800 to 1400 central standard time (CST). The overall probability of nest attendance during this time was 91.4 ± 2.6 (SE) %. Consequently, nest searches for upland nesting waterfowl should be initiated after 0800 and should be terminated by 1400 CST

BROOD SURVIVAL AND RECRUITMENT OF MALLARDS IN RELATION TO WETLAND DENSITY AND HATCHING DATE

Reproductive success in birds often is measured in terms of the proportion of nests that hatch or fledge young. We assessed survival of 35 Mallard (Anas platyrhynchos) broods near Yorkton in east-central Saskatchewan, Canada, from 1990 to 1993. Brood sur- vival was higher for offspring that hatched early in the breeding season in three of four years when wetland abundance declined over the summer. Seasonal patterns of wetland abun- dance were reversed in 1993, and early hatching broods suffered higher mortality. A more accurate measure of reproductive success is the number of young that survive to breed, i.e. recruitment. Ducklings were tagged at Yorkton and on the St. Denis National Wildlife Area (1985 to 1995), enabling us to identify individuals that returned to breed in subsequent years. Female recruits were more likely to originate from early hatched nests at both sites. With respect to recruitment, we also tested for an additional advantage of hatching early, over and above that associated with duckling survival. There was no statistical difference in the seasonal distribution of nests producing recruits compared with broods that successfully fledged at least one young. Thus, the selective advantage to nesting early for Mallards likely is driven by a seasonal decline in duckling survival, which in turn may be associated with decreasing wetland density. Received 3 September 1996, accepted 6 August 1997. TIMING OF BREEDING is considered an adap- tive trait that evolved to maximize reproduc- tive output (Lack 1968). Survival of offspring born later in the breeding season often is lower than that of young born earlier in the season (e.g. mammals, Iason 1989, Virgl and Messier 1997; fish, Schultz 1993). Similarly, among birds, young that hatch early in the breeding season have higher survival to independence than young that hatch late in the season (e.g. Perrins 1965, Murton and Westwood 1977, Dow

A method for investigating population declines of migratory birds using stable isotopes: origins of harvested lesser scaup in North America.

Background Elucidating geographic locations from where migratory birds are recruited into adult breeding populations is a fundamental but largely elusive goal in conservation biology. This is especially true for species that breed in remote northern areas where field-based demographic assessments are logistically challenging. Methodology/Findings Here we used hydrogen isotopes (δD) to determine natal origins of migrating hatch-year lesser scaup (Aythya affinis) harvested by hunters in the United States from all North American flyways during the hunting seasons of 1999–2000 (n = 412) and 2000–2001 (n = 455). We combined geospatial, observational, and analytical data sources, including known scaup breeding range, δD values of feathers from juveniles at natal sites, models of δD for growing-season precipitation, and scaup band-recovery data to generate probabilistic natal origin landscapes for individual scaup. We then used Monte Carlo integration to model assignment uncertainty from among individual δD variance estimates from birds of known molt origin and also from band-return data summarized at the flyway level. We compared the distribution of scaup natal origin with the distribution of breeding population counts obtained from systematic long-term surveys. Conclusions/Significance Our analysis revealed that the proportion of young scaup produced in the northern (above 60°N) versus the southern boreal and Prairie-Parkland region was inversely related to the proportions of breeding adults using these regions, suggesting that despite having a higher relative abundance of breeding adults, the northern boreal region was less productive for scaup recruitment into the harvest than more southern biomes. Our approach for evaluating population declines of migratory birds (particularly game birds) synthesizes all available distributional data and exploits the advantages of intrinsic isotopic markers that link individuals to geography.

Long-term declines in nest success of prairie ducks

Increased predation on nests of ducks in prairie uplands, as a result of habitat alteration, has been hypothesized to cause decreased nest success and population sizes. We tested whether, and by how much, nest success declined using data compiled from 37 studies conducted between 1935 and 1992 at 67 sites in the Prairie Pothole Region of Canada and the United States. Nest success declined (P = 0.0002) over time, but time explained only 10% of the variation ; precipitation (P = 0.79) did not account for additional variation in nest success. Nest success declined at similar (P = 0.13) rates among 5 species, but late nesters (gadwall [Anas strepera], blue-winged teal [A. discors], and northern shoveler [A. clypeata]) had higher success (P = 0.004) than early nesters (mallard [A. platyrhynchos], and northern pintail [A. acuta]). Populations of gadwalls and northern shovelers, however, have not declined, indicating that declines in nest success may not be related causally to population change. Long-term population declines in blue-winged teal, northern pintails, and mallards coincide with large-scale temporal declines in nest success. Declines in nest success were parallel in parkland and grassland regions, suggesting a causal agent (or agents) that act(s) at a broad scale, despite inherent differences in the composition of the predator communities and habitats between regions.

Risk-taking by female ducks: intra- and interspecific tests of nest defense theory

We tested several predictions of nest defense theory by observing variation in flushing distance and probability of nest abandonment within and between six species of waterfowl. In these species, only the females incubate eggs and attend offspring. First, we examined whether flushing distance by females varied in relation to clutch size, stage of incubation, and time of season, after controlling for the number of visits made to nests by observers. Revisits by observers appeared to affect flushing distance by females for reasons unrelated to the relative value of the current clutch. We found that as incubation progressed, females allowed observers to approach more closely before flushing from the nest. In some species, females with larger clutches allowed closer approaches to nests before flushing which was also consistent with nest defense theory. In contrast, time of season (Julian date) did not relate to flushing distance for any species. When species were compared, we found that species with moderate to high yearly mortality and high reproductive output per breeding attempt (e.g., northern shoveler and blue-winged teal) were less likely to abandon nesting attempts and exhibited “riskier” behavior (remained at nests when approached closely by observers) than species that had lower yearly mortality (e.g., mallard). Our results show that flushing distance and patterns of nest abandonment by female ducks conform to several predictions of nest defense theory.

EFFECTS OF HATCHING DATE AND EGG SIZE ON GROWTH, RECRUITMENT, AND ADULT SIZE OF LESSER SCAUP

Abstract Evidence suggests that birds breeding early in the season or laying larger eggs are at a selective advantage because quality and survival of their offspring are higher. We tested whether wild Lesser Scaup (Aythya affinis) hatching early in the season or from larger eggs had enhanced growth as ducklings and larger size or higher probability of recruiting to the local breeding population in years after hatching. After correcting for age at capture, body mass, head length, and culmen of ducklings were inversely related to hatching date, but were unrelated to egg volume. As adults, late-hatched birds had shorter wing lengths (second-year birds only) and tended to have smaller head lengths (all after-hatch year birds) than early hatching birds. We suggest that later-hatching birds are smaller due to increased competition for food during brood rearing. Recruitment probability increased as natal egg volume increased and decreased among birds with later natal hatching dates. We speculate these results are due to higher mortality of ducklings hatched from small eggs, and because early-hatched birds have more time to acquire nutrient reserves that would reduce vulnerability to costs of migration.

Factors affecting survival of northern pintail ducklings in Alberta

We determined brood and duckling survival from 57 radio-marked Northern Pintail (Anas acuta) females in southern Alberta during 1994-1996, and related duckling survival to maternal and environmental attributes. Annual brood survival estimates ranged from 72.2% to 88.2%. Brood survival declined with hatch date in all years. Duckling survival was highest in 1994 at 65.2%, but fell to 42.4% and 43.8% in 1995 and 1996, respectively. Duckling mortality was highest during the first 10 days post-hatch in all years. Duckling survival did not vary with female age, or distance from nest to nearest wetland, but did decline throughout the breeding season. Duckling survival was higher for ducklings from larger broods in 1994, but the opposite trend was found in 1995 and 1996.

TIME AND RECRUITMENT COSTS AS CURRENCIES IN MANIPULATION STUDIES ON THE COSTS OF REPRODUCTION

Life history theory predicts that parents will have lower Darwinian fitness if they tend clutches that are above or below the size they naturally produce. We experimentally tested for relationships between fitness and clutch size in Tree Swallow (Tachycineta bicolor) offspring and parents. Over 130 trios of nests initiated on the same day were randomly divided among reduce (-3 eggs), control (3 eggs picked up and replaced), or add (+3 eggs) manipulations. Pre-manipulation modal clutch size was six eggs (range before manipulations was 1-10; afterwards, it was 1-11). Hatching took longer in larger clutches, but the proportion of eggs hatching and fledging was similar for clutches from 4 to 10, so that clutches of 10 produced the maximum number of fledgling. Parental feeding rates were higher for larger broods, but per capita feeds to nestlings were fewer, and nestlings were smaller. Nonetheless, survival of both young and adults, based on recaptures in subsequent years, was not significantly affected by manipulations. Manipulations also had no significant effect on subsequent reproduction, including the number of fledglings produced by either local recruits or returning breeders. Collectively, our results failed to detect fitness costs associated with tending larger clutches for either parents or the offspring reared and suggested directional selection for larger clutch size. However, because clutches that hatch later produce fewer recruits, the extra days required to lay more eggs and to fledge extra young may eliminate a large part of the advantage that would accrue to parents producing enlarged clutches. For example, our data suggest that there may be less than a 16% benefit to producing nine instead of six eggs, rather than 50%, as is suggested by experimentally manipulated egg numbers alone. Thus, time, rather than costs of reproduction, may be the crucial constraint selecting against Tree Swallows laying larger clutches.