Variation In Nest Success Research Articles

Trends in Black Swift (Cypseloides niger) breeding phenology, productivity, and nest success in southwest Colorado, 1996–2017

This study presents breeding phenology, fledgling production, and nest success for the Black Swift (Cypseloides niger) over a 22 year period at the Box Canyon colony, Ouray, Colorado, USA. Data were collected annually from 1996 to 2017 by direct observation from arrival of the birds at the nesting colony until all adults and fledglings departed. We documented dates of first arrival, laying, incubation onset, hatching, and fledging, and determined the intervals from arrival to laying, laying to incubation, length of incubation, and length of the nestling period in each year. Breeding events followed each other closely and showed little chronological change throughout the study. We identified a negative trend in the number of nest attempts and fledged Black Swift chicks over time but found no relationship between these 2 trends and the environmental factors of summer temperature, summer precipitation, or tourist visitation. The Mayfield estimate of nest success for all nest attempts was 77.5%. There was unexplained annual variation in nest success, but the trend in nest success remained static, with no correlation between nest success and temperature, precipitation, or tourist visitation. This study represents the largest dataset and longest-duration investigation into the breeding phenology and nest success of the Black Swift.

Are Sage‐Grouse Fine‐Scale Specialists or Shrub‐Steppe Generalists?

ABSTRACTSage‐grouse (Centrocercus spp.) are influencing rapidly evolving land management policy in the western United States. Management objectives for fine‐scale vegetation characteristics (e.g., grass height >18 cm) have been adopted by land management agencies based on resource selection or relationships with fitness proxies reported among numerous habitat studies. Some managers, however, have questioned the appropriateness of these objectives. Moreover, it remains untested whether habitat–fitness relationships documented at fine scales (i.e., among individual nests within a study area) also apply at scales of management units (e.g., pastures or grazing allotments), which are many orders of magnitude larger. We employed meta‐analyses of studies published from 1991 to 2019 to help resolve the role of fine‐scale vegetation structure in nest site selection and nest success across the geographic range of greater sage‐grouse (C. urophasianus) and evaluate the validity of established habitat management objectives. Specifically, we incorporated effects of study design and functional responses to resource availability in meta‐regression models linking vegetation structure to nest site selection, and used a novel meta‐analytic approach to simultaneously model vegetation structure and its relationship to nest success. Our approach tested habitat relationships at a range‐wide extent and a grain size closely matching scales at which agencies make management decisions. We found moderate, but context‐dependent, effects of shrub characteristics and weak effects of herbaceous vegetation on nest site selection. None of the tested vegetation characteristics were related to variation in nest success, suggesting nesting habitat–fitness relationships have been inappropriately extrapolated in developing range‐wide habitat management objectives. Our findings reveal surprising flexibility in habitat use for a species often depicted as having very particular fine‐scale habitat requirements, and cast doubt on the practice of adopting precise management objectives for vegetation structure based on findings of individual small‐scale field studies. © 2020 The Authors. The Journal of Wildlife Management published by Wiley Periodicals, Inc. on behalf of The Wildlife Society.

Extreme weather affects Peregrine Falcon (Falco peregrinus tundrius) breeding success in South Greenland

Abstract In order to better understand the potential effects of climate change on the Peregrine Falcon, we investigated the relationship between extreme weather events and Peregrines’ breeding success in South Greenland. We defined three variables – number of days with extremely low temperatures, extreme precipitation, consecutive rainy days – and an additive variable, total days with extreme weather, and tested their relationship with Peregrines’ breeding success (measured as young per site and nest success) over a 33 year study period. Breeding success was negatively influenced by the number of days with extreme weather and extremely low temperature. The strongest relationship found was total days with extreme weather in the entire breeding season, which explained 22% and 27% of the variation in nest success and young per site, respectively. The number of days with extreme weather in our study related to fluctuations in the North Atlantic Oscillation (NAO). Thus, with a strengthening of the NAO, linked to climate change, more extreme weather may occur in the Arctic and induce increased variation in Peregrines’ breeding success. Our data did not allow us to pinpoint when in the breeding cycle inclement weather was particularly harmful, and we recommend finer-scale research (e.g. automated nest cameras) to better monitor the species-specific effects of rapidly changing climate.

Managing Native Predators: Evidence from a Partial Removal of Raccoons (Procyon lotor) on the Outer Banks of North Carolina, USA

Raccoons (Procyon lotor) are important predators of ground-nesting species in coastal systems. They have been identified as a primary cause of nest failure for the American Oystercatcher (Haematopus palliatus) throughout its range. Concerns over the long-term effects of raccoon predation and increased nest success following a hurricane inspired a mark-resight study of the raccoon population on a barrier island off North Carolina, USA. Approximately half of the raccoons were experimentally removed in 2008. Nests (n = 700) were monitored on two adjacent barrier islands during 2004–2013. Daily nest survival estimates were highest for 2004 (0.974 ± 0.005) and lowest for 2007 and 2008 (0.925 ± 0.009 and 0.925 ± 0.010, respectively). The only model in our candidate set that received any support included island and time of season, along with a diminishing effect of the hurricane and a constant, 5-year effect of the raccoon removal. For both hurricane and raccoon removal, however, the support for island-specific effects was weak (β = -0.204 ± 0.116 and 0.146 ± 0.349, respectively). We conclude that either the raccoon reduction was inadequate, or factors other than predation cause more variation in nest success than previously recognized. A multi-faceted approach to management aimed at reducing nest losses to storm overwash, predation, and human disturbance is likely to yield the largest population level benefits.

Effects of common raven and coyote removal and temporal variation in climate on greater sage-grouse nesting success

Predator removal has been simultaneously proposed and criticized as a mitigation measure for low reproductive rates of prey species, including greater sage-grouse (Centrocercus urophasianus; hereafter “sage-grouse”). Depredation of sage-grouse nests can limit their productivity. In Wyoming, lethal removal of common ravens (Corvus corax: hereafter “ravens”) and coyotes (Canis latrans) has been conducted by USDA/APHIS/Wildlife Services (WS) for the protection of livestock. During 2008–2011, we evaluated sage-grouse nest success in study sites (1) where WS initiated a raven removal program, (2) WS removed coyotes, and (3) WS did not manipulate ravens and/or coyotes. Precipitation and temperature were analyzed individually and as interactive effects with coyote removal numbers as sources of annual variation in nest success. Over the course of our study, raven densities decreased at study sites with WS raven removal, while sage-grouse nest success in those study sites was higher during years with reduced raven density. Temperature effects on nest success were dependent on timing with successful nests having cooler temperatures prior to the nesting season (conditions promoting water retention and grass growth) and warmer temperatures the week before nest fate (conducive to degradation of sage-grouse odorants used by mammalian predators). Lower nest success was associated with more lethally removed coyotes interacting with greater precipitation suggesting mesopredator release. Raven removal may have a place in sage-grouse management as an interim mitigation measure when sage-grouse populations are subjected to high densities of ravens. However, long-term solutions are necessary, such as reducing supplemental food sources and perch structures used by ravens.

Do seasonal patterns of rat snake (Pantherophis obsoletus) and black racer (Coluber constrictor) activity predict avian nest predation?

Avian nest success often varies seasonally and because predation is the primary cause of nest failure, seasonal variation in predator activity has been hypothesized to explain seasonal variation in nest success. Despite the fact that nest predator communities are often diverse, recent evidence from studies of snakes that are nest predators has lent some support to the link between snake activity and nest predation. However, the strength of the relationship has varied among studies. Explaining this variation is difficult, because none of these studies directly identified nest predators, the link between predator activity and nest survival was inferred. To address this knowledge gap, we examined seasonal variation in daily survival rates of 463 bird nests (of 17 bird species) and used cameras to document predator identity at 137 nests. We simultaneously quantified seasonal activity patterns of two local snake species (N = 30 individuals) using manual (2136 snake locations) and automated (89,165 movements detected) radiotelemetry. Rat snakes (Pantherophis obsoletus), the dominant snake predator at the site (~28% of observed nest predations), were most active in late May and early June, a pattern reported elsewhere for this species. When analyzing all monitored nests, we found no link between nest predation and seasonal activity of rat snakes. When analyzing only nests with known predator identities (filmed nests), however, we found that rat snakes were more likely to prey on nests during periods when they were moving the greatest distances. Similarly, analyses of all monitored nests indicated that nest survival was not linked to racer activity patterns, but racer‐specific predation (N = 17 nests) of filmed nests was higher when racers were moving the greatest distances. Our results suggest that the activity of predators may be associated with higher predation rates by those predators, but that those effects can be difficult to detect when nest predator communities are diverse and predator identities are not known. Additionally, our results suggest that hand‐tracking of snakes provides a reliable indicator of predator activity that may be more indicative of foraging behavior than movement frequency provided by automated telemetry systems.

Snake predation on North American bird nests: culprits, patterns and future directions

Predation is the leading cause of nest failure for most birds. Thus, for ornithologists interested in the causes and consequences of variation in nest success, knowing the identity and understanding the behavior of dominant nest predators is likely to be important. Video documentation of nests has shown that snakes are frequent predators. Here we reviewed 53 North American studies that used nest cameras and used these data to identify broad patterns in snake predation. Snakes accounted for 26% (range: 0–90%) of recorded predation events, with values exceeding 35% in a third of studies. Snakes were more frequent nest predators at lower latitudes and less frequent in forested habitat relative to other nest predators. Although 12 species of snakes have been identified as nest predators, ratsnakes Elaphe obsoleta, corn snakes E. guttata and fox snakes E. vulpina were the most frequent, accounting for > 70% of all recorded nest predation events by snakes and have been documented preying on nests in 30–65% of studies conducted within their geographic ranges. Endotherm‐specialist snakes (Elaphe and Pituophis genera) were more likely to depredate nests in forests and the canopy relative to other snakes, due to their affinity for edge habitat. Predation by only ratsnakes and corn snakes was predominantly nocturnal and only ratsnakes were more likely to prey on nests during the nestling stage. Snakes were not identified to species in over 30% of predation events, underlining the need for more complete reporting of results. A review of research to date suggests the best approach to investigating factors that bring snakes and nests into contact involves combining nesting studies with radio tracking of locally important snake nest predators.

Nest survival of wild turkeys Meleagris gallopavo silvestris in a mixed‐use landscape: influences at nest‐site and patch scales

Nest survival is a critical factor affecting avian demographics, and can be influenced by nesting chronology, fine scale nest‐site selection and broad‐scale landscape characteristics. We modeled the relative influences of nest age, temporal variation in nest success and habitat‐related covariates at two spatial scales (nest‐site and patch scale) on daily nest survival during incubation for eastern wild turkeys Meleagris gallopavo silvestris in a mixed‐use landscape. Daily survival rate of turkey nests during incubation increased as percent understory cover (vegetation < 1 m tall) increased and decreased with increasing density of woody shrubs and saplings and herbaceous stems < 1 m tall (understory vegetation density) around the nest. We suggest that nest survival may be dependent on a balance of sufficient understory cover around nests to provide concealment for hens and nests, but with understory vegetation density below levels that reduce the hen's ability to detect a predator or to escape after detecting a potential threat. The balance between sufficient understory cover and limited density of understory vegetation occurred where understory (< 1 m tall) cover exceeded 50% and understory vegetation density was < 25 stems/m2. Models that included variables related to the patch scale (e.g. fragmentation, edge and dominant land‐cover class in a patch) did not receive strong support, demonstrating the relative importance of finer scale nest‐site variables over patch‐scale variables in determining survival of wild turkey nests in our highly variable mixed‐use landscape.

Testing common assumptions in studies of songbird nest success

We studied Ovenbird Seiurus aurocapilla and Golden-winged Warbler Vermivorachrysoptera populations in northern Minnesota, USA, to test two common assumptions instudies of songbird nest success: (1) that the condition of an empty nest on or near itsexpected fledge date is an indicator of nest fate; and (2) that the presence of a fledgling orfamily group within a territory confirms a successful nest in that territory. We monitoredthe condition of nests and used radiotelemetry to monitor juveniles through the expectedfledging date and early post-fledging period. Of nests that contained nestlings 1–2 daysbefore the expected fledge date, fates were misidentified using nest condition alone for9.5% of Ovenbird nests, but those misidentifications were made in both directions (suc-ceeded or failed), yielding only a small bias in estimated nest success. However, 20% ofGolden-winged Warbler nests were misidentified as successful using nest condition duringthe final visit interval, biasing the nest success estimate upward by 21–28% depending onthe treatment of uncertain nest fates. Fledgling Ovenbirds from 58% of nests travelledbeyond their natal territory within 24 h, rising to 98% after 5 days, and those fledglingstravelled up to 390 m from nests within 10 days of fledging. Fledgling Golden-wingedWarblers from 13% of nests travelled beyond their natal territory within 24 h, rising to85% after 5 days, and those fledglings travelled up to 510 m from nests within 10 days offledging. We conclude that nest condition and fledgling presence can be misleading indica-tors of nest fate, probably commonly biasing nest success estimates upward, and we rec-ommend that these assumptions should be tested in additional species.Keywords: fledgling, Golden-winged Warbler, Ovenbird, Seiurus aurocapilla, telemetry,Vermivora chrysoptera.Estimates of songbird reproductive success, typi-cally limited to nest data, are used to assess habitatquality (e.g. Weinberg & Roth 1998), model pop-ulation dynamics (e.g. Podolski et al. 2007), iden-tify source and sink populations (e.g. Donovan etal. 1995), and inform conservation and manage-ment plans (e.g. Woodworth 1999). Althoughsongbird population growth may be generally moresensitive to adult annual survival and fledgling sur-vival (Donovan & Thompson 2001, Streby &Andersen 2011), population growth is also sensi-tive to variation in nest success (Donovan et al.1995), and nest success is the only directly esti-mated parameter in most studies of songbirdreproductive success (Anders et al. 1997). Manypopulation models account for re-nesting (birdsnesting again after initial failure) and estimates ofnest productivity (number of young produced persuccessful nest). All such studies require accuratefield identification of whether each monitored nestsucceeded or failed in producing young. However,observational studies of songbird nests often

Snake behavior and seasonal variation in nest survival of northern cardinals Cardinalis cardinalis

Seasonal variation in nest success is well documented for many bird species. Predator behavior has been suggested as a mechanism behind these seasonal patterns, but this hypothesis has received little attention. Here we test the hypothesis that predator behavior produces seasonal patterns of nest success by relating nest success of northern cardinals Cardinalis cardinalis to the activity of Texas rat snakes Elaphe obsoleta. Cardinal nest survival varied over the season and was lower when rat snakes were more active. The probability that a nest survived was associated both with when cardinals nested and with nest height, indicating that both temporal and habitat factors affected predation risk. The increased success of higher nests could be associated with some aspect of rat snakes’ climbing ability. In combination with results for two other species studied previously at the same location, our results for cardinals suggest that the specific seasonal pattern of nest success expected for a given bird species will depend on how its nesting season coincides with predator activity. Determining the generality of seasonal variation in predator behavior as a mechanism for producing seasonal patterns of avian nest success will require additional studies that investigate birds and their nest predators simultaneously.

Nest success and hatchling survival of American alligators within inland wetlands of east Texas

AbstractBecause of liberalization of American alligator (Alligator mississippiensis) harvest management in Texas, estimates of nest success and hatchling survival for inland populations are essential for long‐term, sustainable population and harvest management. To date, few studies have examined American alligator nest success and hatchling survival. We initiated a 3‐year study from 2006 to 2008 to document alligator nest success and hatchling survival within several wetlands in east Texas. From June 2006 to August 2008, we located 30 nests from 3 wetlands within east Texas, where overall nest success was 44.2% (95% CI = 25.1–63.1%), irrespective of year. Nest circumference and day during the nesting season exerted the greatest influence on nest success. Additionally, from August 2006 to August 2008 we captured, marked, and released 271 hatchling alligators at Little Sandy National Wildlife Refuge, and recaptured an additional 192 hatchling alligators during this time. We estimated yearly apparent survival at 6.0% (95% CI = 2.0–14.6%) for hatchling alligators born in 2006 and 43.0% (95% CI = 28.4–57.8%) for those hatched in 2007. Variation in nest success and hatchling survival was likely attributed to fluctuating water levels and habitat management practices. Alligator harvest regulations need to account for variability in nest success and hatchling survival by including site‐specific estimates of these metrics into harvest models. Failing to account for spatial and temporal variation in nest success and hatchling survival may result in unsustainable harvest and/or overharvest. © 2012 The Wildlife Society.

Sources of variation in the nesting success of understory tropical birds

Survival of offspring is a key fitness component and, for birds, the threat of predation on nests is especially influential. Data on rates of nest success from tropical regions are comparatively few, conservation‐relevant, and essential for assessing the validity of models comparing the life histories and behavior or birds across latitudinal gradients. We monitored over 2 000 nests in the lowland forests of central Panama and, using the logistic exposure to model the fate of nests, explored the importance of variation in rate of nest success according to type of nest, height of nests, among years, in early versus late nests, and at different stages of the nest cycle. Analyses of over 1 400 nests for 18 species revealed considerable variation among species in the daily survival rate of nests (range among 18 species=0.91 to 0.98), but nest type and stage of the nesting cycle were generally influential on the probability of nest success. Cavity or enclosed nesters experienced greater nest success than open cup nesters and rates of nest loss were generally greatest in the nestling stage. We found limited evidence that height of nests affected probability of success, but no indication that timing of nesting effort was influential. Despite the occurrence of a severe ENSO event during our sampling, annual variation in nest success was not consistent among species. Interspecific variation in the rates and patterns of nest predation in our study, coupled with reports of high rates of nest loss at temperate latitudes, lead us to question long standing assumptions about latitudinal trends in rates of nest loss. We urge further work to understand the implications of nest predation on the evolutionary ecology of tropical birds.

Investigating the effects of predator removal and habitat management on nest success and breeding population size of a farmland passerine: a case study

Passerines are especially vulnerable to predation at the pre‐independence stage. Although the role of nest success in British farmland passerine declines is contentious, improvement in nest success through sympathetic management could play a role in their reversal. Because habitat is known to interact with predation, management options for mitigation will need to consider effects of nest predation. We present results from an observational study of a population of Common Blackbird Turdus merula on a farm which has experienced a range of agri‐environment and game‐management options, including a period with nest predator control, as a case study to address some of these issues. We used an information theoretic model comparison procedure to look for evidence of interactions between habitat and nest predation, and then asked whether habitat management and nest predator abundances could explain population trends at the site through their effects on nest success. Interactions were detected between measures of predator abundance and habitat variables, and these varied with nest stage – habitat within the vicinity of the nest appeared to be important at the egg stage, and nest‐placement characteristics were important at the nestling stage. Although predator control appeared to have a positive influence on Blackbird breeding population size, the non‐experimental set‐up meant we could not eliminate other potential explanations. Variation in breeding population size did not appear to be influenced by variation in nest success alone. Our study demonstrates that observational data can only go so far in detection of such effects, and we discuss how it might be taken further. Agri‐environment and game‐management techniques are likely to influence nest predation pressure on farmland passerines, but the patterns, mechanisms and importance to population processes remain not wholly understood.

Snake activity affects seasonal variation in nest predation risk for birds

Variation in predator behavior has been proposed, but not tested, as a mechanism producing seasonal declines in avian nest success. We test this hypothesis by documenting seasonal activity of Texas ratsnakes Elaphe obsoleta and nest failure of endangered black‐capped vireos Vireo atricapilla and golden‐cheeked warblers Dendroica chrysoparia on which the snakes prey. Nest survival analysis was based on 880 vireo and 228 warbler nests and 3,060 snake locations from 62 radio‐tracked snakes. Although nest success varied with snake activity for both birds, specific patterns differed substantially. Vireo daily nest survival was negatively correlated with snake activity over the three‐year study, despite substantial variation among years in weather, and the fact that these birds are almost certainly a minor prey species of the ratsnakes. Warblers exhibited less clear‐cut seasonal variation in nest success, and the association between nest success and snake activity was less pronounced than for vireos. Increased activity at warmer temperatures explained some of the seasonal change in snake movements, although mating may have accounted for a mid‐season peak in activity. These results indicate that variation in predator behavior can be associated with and potentially cause seasonal changes in nest success, but also that these relationships are species specific even within the same community and may depend on aspects of the nesting ecology of the prey such as nest site selection.

Factors Affecting Nest Survival of Greater Sage‐Grouse in Northcentral Montana

Abstract: We studied greater sage‐grouse (Centrocercus urophasianus) in northcentral Montana, USA, to examine the relationship between nest success and habitat conditions, environmental variables, and female sage‐grouse characteristics. During 2001‐2003, we radiomarked 243 female greater sage‐grouse, monitored 287 nests, and measured 426 vegetation plots at 4 sites in a 3,200‐km2 landscape. Nest survival varied with year, grass canopy cover, daily precipitation with a 1‐day lag effect, and nesting attempt. In all years, daily survival rate increased on the day of a rain event and decreased the next day. There was temporal variation in nest success both within and among years: success of early (first 28 d of nesting season) nests ranged from 0.238 (SE = 0.080) in 2001 to 0.316 (SE = 0.055) in 2003, whereas survival of late (last 28 d of nesting season) nests ranged from 0.276 (SE = 0.090) in 2001 to 0.418 (SE = 0.055) in 2003. Renests experienced higher survival than first nests. Grass cover was the only important model term that could be managed, but direction and magnitude of the grass effect varied. Site, shrub and forb canopy cover, and Robel pole reading were less useful predictors of nest success; however, temporal and spatial variation in these habitat covariates was low during our study. We note a marked difference between both values and interpretations of apparent nest success, which have been used almost exclusively in the past, and maximum‐likelihood estimates used in our study. Annual apparent nest success (0.46) was, on average, 53% higher than maximum‐likelihood estimates that incorporate individual, environmental, and habitat covariates. The difference between estimates was variable (range = +8% to +91%). Management of habitats for nesting sage‐grouse should focus on increasing grass cover to increase survival of first nests and contribute to favorable conditions for renesting, which should be less likely if survival of first nests increases.

Effects of Nest Habitat, Food, and Parental Behavior on Shorebird Nest Success

AbstractAbstractIn environments such as arctic tundra, where bird densities are low and habitats are comparatively homogeneous, suitable nest sites likely are not limited. Under these conditions, reproductive success of birds may be determined by factors other than the habitat characteristics of nest sites. We studied the relative influence of nest habitat, food, nest distribution, and parental behavior on the reproductive success of tundra-breeding shorebirds at East Bay, Southampton Island, Nunavut, Canada. From 2000 to 2002, we monitored the nests of five species: Black-bellied Plover (Pluvialis squatarola), Semipalmated Plover (Charadrius semipalmatus), Ruddy Turnstone (Arenaria interpres), White-rumped Sandpiper (Calidris fuscicollis), and Red Phalarope (Phalaropus fulicarius). For each species, habitat differed between nest sites and random sites. In contrast, habitat differed between successful and failed nest sites only for White-rumped Sandpipers. Shorebirds did not prefer to nest in habitats where food was most abundant. Although nest success varied among species in all years, artificial nest experiments suggested that interspecific variation in predation rate was not related to habitat type. Instead, the marked interspecific variation in nest success may have been related to incubation behavior. Species taking fewer incubation recesses had higher nest success, although these results should be viewed as preliminary. The factor with the greatest interannual influence on nest success was fluctuating predation pressure, apparently related to the abundance of predators and lemmings.

Sensitivity Analyses of the Life Cycle of Midcontinent Mallards

Relationships between vital rates and population growth rate (λ) are critical to understanding and managing population dynamics. Considerable study of the midcontinent mallard (Anas platyrhynchos) population has been directed to understanding how vital rates respond to environmental fluctuations and management, but inference to the relative importance of specific vital rates to λ remains weak. We used analytic and simulation-based sensitivity analyses of a stage-based matrix model of female midcontinent mallards to compare the relative importance of vital rates to λ. For each vital rate, we estimated mean values and process variation (biological variation across space and time) for females breeding on sites of approximately 70 km 2 in the Prairie Pothole Region (PPR) of the United States (Montana, North Dakota, South Dakota, Minnesota) and Canada (Saskatchewan, Manitoba, Alberta). We conducted perturbation analyses (i.e., analytic sensitivities and elasticities) to predict the relative influence of changes in vital rates on λ. We conducted variance decomposition analyses to assess the proportion of spatial and temporal variation in λ explained by process variation in each vital rate. At mean values of vital rates, analytic sensitivity of λ was highest to nest success and survival of adult females during the breeding season and non-breeding season; hence, equal absolute changes in these vital rates would be predicted to result in the largest Δλ, relative to other vital rates. Variation in sensitivities and elasticities across process variation in vital rates was primarily explained by variation in nest success and survival of ducklings. Process variation in breeding parameters was driving variation in λ: vital rates explaining the most variation were nest success (43%), survival of adult females during the breeding season (19%), and survival of ducklings (14%). Survival of adult females outside the breeding season accounted for only 9% of variation in λ. Our analyses suggested that predation processes on the breeding grounds were the primary proximate factors limiting population growth.

SNAIL KITE NEST SUCCESS AND WATER LEVELS: A REPLY TO BEISSINGER AND SNYDER

Abstract Beissinger and Snyder present a commentary on our recent paper on spatial and temporal variability in nest success of Snail Kites (Rostrhamus sociabilis) in Florida (Dreitz et al. 2001). Beissinger and Snyder reanalyze a subset of data presented in our original paper to show that water levels have a significant influence on nest success. To make their argument, the authors conduct separate analyses for 5 of the original 11 wetlands; including only those having the most data. We agree with Beissinger and Snyder that water levels can affect nest success in some areas or years, as we stated in Dreitz et al. (2001). However, the purpose of our original paper was to examine the influences of nest success over broad spatial and temporal scales. When viewed in this context, using a meta-analysis, water levels alone explain only a small amount of the observed variation in nest success. One of the advantages of using a meta-analysis is that it uses all of the available data to provide an indication of the...

BREEDING SYNCHRONY AND NEST PREDATION IN RED-WINGED BLACKBIRDS

Three previous studies have disagreed on whether breeding synchrony increases nest success through reduced nest predation in Red-winged Blackbirds (Agelaius phoeniceus) in eastern North America. We used data from an 11-yr study of redwings in eastern Ontario, Canada, to duplicate the analyses of the one study (by D. F. Westneat) that did show that nesting synchrony reduced predation. We found no evidence that nest predation and nest success were affected by nesting synchrony. We did find significant effects of female age, clutch size, and clutch initiation date on nest success, but overall, our independent variables explained very little variation in nest success. We hypothesize that the difference between our results and Westneat's may be attributable to differences in the principal nest predators. Mammals appeared to be important nest predators in our study, as is typical for eastern populations of Red-winged Blackbirds. Mammals appeared to be less important nest predators in Westneat's study, probably because that study area was fenced and the grass between ponds was mowed. Red-winged Blackbirds mob avian predators, but have limited ability to defend nests from mammals. Group mobbing of avian predators could result in a positive relationship between nesting synchrony and nest success when birds are the most important nest predators. The greater importance of avian nest predators in western than in eastern populations of Red-winged Blackbirds could account for some differences in social polygyny between those populations. However, our results indicate that nesting decisions by female redwings in eastern populations should not be affected by the nesting activity of other females. Thus, social polygyny in this population does not arise through an advantage of group living.

Using the presence and condition of eggshells to determine nest success in the magpie goose, Anseranas semipalmata

In a sample of 104 closely monitored magpie goose nests, all of 66 successful nests contained substantial numbers of eggshell fragments (52–1000+), many embedded deeply in the nest matrix. Detached shell membranes were also found in all successful nests. All but one of 38 unsuccessful nests contained no shell fragments. The exception contained about 60% of the shell from one egg, but fragments were clearly distinguishable from shells of hatched eggs because they were contaminated with egg yolk. Weight of eggshell in individual nests could be related to the number of hatchlings (excluding failed nests: r2 = 0.67, P < 0.0001, n = 39), but the time required to retrieve and process shell fragments is perhaps too long to justify routine use to estimate hatchling production for individual nests. Rather, we suggest that the presence of substantial quantities of eggshell fragments (>12 g or the equivalent of more than 1 egg) embedded in the nest matrix be regarded as diagnostic of hatching success. We interpret the clear discrimination between successful and failed nests as a direct result of consistent features of magpie goose biology and the breeding environment. The most important of these are: (i) the prolonged hatching and on-nest brooding period, during which eggshells from hatched eggs are crushed and their movement into the nest matrix promoted; (ii) the continuous addition of vegetation to the nest, often covering shells; (iii) the removal of shells from damaged (but not hatched) eggs by attending adults; and (iv) the dominant egg predator being a large snake (the water python, Liasis fuscus) which consumes eggs whole and hence leaves no shells at the nest site. We suggest that post-hatch surveys of nest structures should be used to improve understanding of spatial variation in nest success within colonies, and to compare success between colonies of different size.