Adaptive nest clustering and density‐dependent nest survival in dabbling ducks

Density‐dependent population regulation is observed in many taxa, and understanding the mechanisms that generate density dependence is especially important for the conservation of heavily‐managed species. In one such system, North American waterfowl, density dependence is often observed at continental scales, and nest predation has long been implicated as a key factor driving this pattern. However, despite extensive research on this topic, it remains unclear if and how nest density influences predation rates. Part of this confusion may have arisen because previous studies have studied density‐dependent predation at relatively large spatial and temporal scales. Because the spatial distribution of nests changes throughout the season, which potentially influences predator behavior, nest survival may vary through time at relatively small spatial scales. As such, density‐dependent nest predation might be more detectable at a spatially‐ and temporally‐refined scale and this may provide new insights into nest site selection and predator foraging behavior. Here, we used three years of data on nest survival of two species of waterfowl, mallards and gadwall, to more fully explore the relationship between local nest clustering and nest survival. Throughout the season, we found that the distribution of nests was consistently clustered at small spatial scales (?50–400 m), especially for mallard nests, and that this pattern was robust to yearly variation in nest density and the intensity of predation. We demonstrated further that local nest clustering had positive fitness consequences – nests with closer nearest neighbors were more likely to be successful, a result that is counter to the general assumption that nest predation rates increase with nest density.     

Interrupted incubation: How dabbling ducks respond when flushed from the nest

Nesting birds must provide a thermal environment sufficient for egg development while also meeting self‐maintenance needs. Many birds, particularly those with uniparental incubation, achieve this balance through periodic incubation recesses, during which foraging and other self‐maintenance activities can occur. However, incubating birds may experience disturbances such as predator or human activity which interrupt natural incubation patterns by compelling them to leave the nest. We characterized incubating mallard Anas platyrhynchos and gadwall Mareca strepera hens’ responses when flushed by predators and investigators in Suisun Marsh, California, USA. Diurnal incubation recesses initiated by investigators approaching nests were 63% longer than natural diurnal incubation recesses initiated by the hen (geometric mean: 226.77 min versus 142.04 min). Nocturnal incubation recesses, many of which were likely the result of predators flushing hens, were of similar duration regardless of whether the nest was partially depredated during the event (115.33 [101.01;131.68] minutes) or not (119.62 [111.96;127.82] minutes), yet were 16% shorter than natural diurnal incubation recesses. Hens moved further from the nest during natural diurnal recesses or investigator‐initiated recesses than during nocturnal recesses, and the proportion of hen locations recorded in wetland versus upland habitat during recesses varied with recess type (model‐predicted means: natural diurnal recess 0.77; investigator‐initiated recess 0.82; nocturnal recess 0.31). Hens were more likely to take a natural recess following an investigator‐initiated recess earlier that same day than following a natural recess earlier that same day, and natural recesses that followed an investigator‐initiated recess were longer than natural recesses that followed an earlier natural recess, suggesting that hens may not fulfill all of their physiological needs during investigator‐initiated recesses. We found no evidence that the duration of investigator‐initiated recesses was influenced by repeated visits to the nest, whether by predators or by investigators, and trapping and handling the hen did not affect investigator‐initiated recess duration unless the hen was also fitted with a backpack‐harness style GPS–GSM transmitter at the time of capture. Hens that were captured and fitted with GPS–GSM transmitters took recesses that were 26% longer than recesses during which a hen was captured but a GPS–GSM transmitter was not attached. Incubation interruptions had measurable but limited and specific effects on hen behavior.     

Nest desertion by Grey Fantails during nest building in response to perceived predation risk

ABSTRACT Grey Fantails (Rhipidura albiscapa), a common Australian flycatcher, commonly desert their nests before egg‐laying. We tested the hypothesis that Grey Fantails desert incomplete nests in response to the attention of predators by placing a mounted Pied Currawong (Strepera graculina), a common nest predator, near fantail nests that were under construction. As a control, we placed a mounted King Parrot (Alisteris scapularis), a nonpredatory bird similar in size to Pied Currawongs, near other fantail nests. Four of six female fantails (67%) deserted incomplete nests in response to the presentation of the Pied Currawong. In contrast, none of the seven females presented with a mounted King Parrot deserted. Female Grey Fantails may use the attention of a predator at the nest during the building stage as a cue to desert. Such desertion may be adaptive for Grey Fantails because currawongs are large predators, making successful nest defense unlikely, and they also present considerable risk to adults. In addition, fantails may raise multiple broods during a breeding season and, therefore, have a high renesting potential.     

Increased nest defence of upland‐nesting ducks in response to experimentally reduced risk of nest predation

Parent birds should take greater risks defending nests that have a higher probability of success. Given high rates of mammalian nest predation, therefore, parents should risk more for nests in areas with a lower risk of mammalian predation. We tested this hypothesis using nest defence data from over 1300 nests of six species of dabbling ducks studied in an area where predation risk had been reduced through removal of mammalian predators. When predator removal reduced nest predation, the ducks increased risk taking as predicted. Also as predicted, risk taking varied inversely with body size, an index of annual survival, among species. For ducks to vary nest defence in response to variation in predation risk they must be able to assess the risk of nest predation. Because ducks modified nest defence in the breeding season immediately following predator removal, ducks may be able to assess predator abundance indirectly (e.g. by UV reflection from urine) rather than by seeing or interacting directly with the predators.     

Interspecific interactions and co-existence in dabbling ducks: observations and an experiment

We studied the possible role of resource limitation and interspecific competition in assemblages of dabbling ducks on breeding lakes in Finland and Sweden with observational and experimental data. After initial vegetation mapping and yearly censuses of ducks in 1985–1990, we collected observational data in 1991–1994 from 28 lakes with natural populations of mallard Anas platyrhynchos and teal A. crecca. Mallard and teal co-occur over vast areas in the Holarctic and they are the only breeding dabbling ducks on many oligotrophic lakes. Both species are migratory in our study regions, teal arriving later in spring than mallards. Log-linear model analysis of observational presence/absence data revealed a positive, not a negative, association between the species. This association was independent of habitat diversity as well as of lake size. Mallard-teal interaction was also studied in a cross-over introduction experiment in 32 other lakes in two years. Wing-clipped mallards were introduced to breeding lakes before the arrival of teal to induce resource limitation and interspecific competition, hypothesized to reduce lake use by teal. The density of mallard pairs on experimental lakes was 2.9–8.0 times higher than on controls, but there was no negative response by teal to the treatment. This is the first combined observational-experimental demonstration of lack of interspecific competition in waterfowl. Our results indicate that heterospecific attraction may affect species co-existence in dabbling ducks. Received: 28 October 1996 / Accepted: 13 January 1997     

Breeding success of sympatric dabbling ducks in relation to population density and food resources

Breeding success in sympatric mallard Anas platyrhynchos , teal A. crecca and wigeon A. penelope in a boreal watershed in Finland was studied for 12 years. Benthic and surface-emerging prey animals were trapped to obtain annual indices of food abundance. Mallard and teal were equally abundant over the years, being roughly twice as numerous as wigeon.
Pair density, brood:pair ratio and duckling:pair ratio were used to test the hypothesis that per capita breeding success decreases in a density-dependent fashion as either pair density or the number of nesting pairs per available food unit increases. In mallard we found no density-dependent patterns at all. In teal per capita brood production decreased as prey animals became relatively scarcer, but this interpretation may not be robust. In wigeon, however, there were two independent significant patterns of direct density-dependence in a temporal succession, i.e. between pair density and per capita brood production in the early part of the breeding season, and then between per capita abundance of surface-emerging insect prey and the number of ducklings per pair. Despite wide dietary overlap and frequent co-occurrence on single lakes among species in the guild, we found no evidence for interspecific density-dependent effects. We hypothesize that there is no or infrequent food limitation for breeding dabblers in this system, and that behavior may be the process behind the pattern of density-dependence in wigeon.     

Effects of clutch sizes and incubation stage on nest desertion in the female Common Eider Somateria mollissima nesting in the high Arctic

In bird species, one of the trade-offs between reproduction and survival appears in the parental decision to desert the nest. Nest desertion is modulated by several factors including clutch size. However, the incubation stage at which predation occurs is also an important factor. In this study, we examined whether nest desertion was linked to initial clutch size, partial clutch predation (final clutch size) and the incubation stage at which it happened in a capital breeder: the female common eider (Somateria mollissima) nesting in the high Arctic. The study was performed in Kongsfjorden in 2002 on the western coast of the Svalbard Archipelago (78°55′N, 20°07′E). We observed that nest desertion was higher when the initial clutch size was small. Also, females deserted their nests more during the first third of incubation than later. Thus, as incubation proceeded, nest desertion was less likely to occur even after egg reduction. Our results pointed out that this parental decision in female eiders seemed to depend on initial clutch size and on the date into incubation of clutch reduction. Sophie Bourgeon and Francois Criscuolo contributed equally to the work     

Switching to a feeding method that obstructs vision increases head-up vigilance in dabbling ducks

A major assumption in most models of foraging is that feeding and vigilance are mutually exclusive. A recent experimental study challenged this hypothesis and demonstrated that birds are able to detect predators while pecking seeds on the ground (head-down vigilance). Experimental obstruction of head-down vigilance makes birds increase head-up vigilance (i.e. the classical overt vigilance posture). For many foragers in the wild, visibility varies between habitats and foraging methods. We compared the vigilance of Teal Anas crecca and Shoveler Anas clypeata when foraging with their eyes above the water surface (shallow feeding, only the bill submerged) and when foraging with their eyes underwater (deep feeding, head and neck underwater, or upending), at three wintering sites in western France. Birds of both species spent less time in head-up vigilance during shallow foraging than during deep foraging, with no significant difference between sites, which suggests that they are capable of some vigilance during shallow foraging. During deep foraging, the time spent vigilant increased because the frequency of scans was much higher, while scan length decreased. However, these differences could have resulted from variations in the availability of food at different depths. In an experiment where the food availability was constant, we observed the same pattern, with a higher frequency of scans during deep foraging. This study therefore provides strong support for the idea that vigilance and foraging are not always mutually exclusive and shows that switching between searching methods can cause vigilance time – and, as a consequence, loss of feeding time – to vary. This should be taken into account in future field and experimental studies of the trade-off animals make between vigilance and feeding.     

Females compensate for moult‐associated male nest desertion in Hooded Warblers

Uniparental offspring desertion occurs in a wide variety of avian taxa and usually reflects sexual conflict over parental care. In many species, desertion yields immediate reproductive benefits for deserters if they can re‐mate and breed again during the same nesting season; in such cases desertion may be selectively advantageous even if it significantly reduces the fitness of the current brood. However, in many other species, parents desert late‐season offspring when opportunities to re‐nest are absent. In these cases, any reproductive benefits of desertion are delayed, and desertion is unlikely to be advantageous unless the deserted parent can compensate for the loss of its partner and minimize costs to the current brood. We tested this parental compensation hypothesis in Hooded Warblers Setophaga citrina, a species in which males regularly desert late‐season nestlings and fledglings during moult. Females from deserted nests effectively doubled their provisioning efforts, and nestlings from deserted nests received just as much food, gained mass at the same rate, and were no more likely to die from either complete nest predation or brood reduction as young from biparental nests. The female provisioning response, however, was significantly related to nestling age; females undercompensated for male desertion when the nestlings were young, but overcompensated as nestlings approached fledging age, probably because of time constraints that brooding imposed on females with young nestlings. Overall, our results indicate that female Hooded Warblers completely compensate for male moult‐associated nest desertion, and that deserting males pay no reproductive cost for desertion, at least up to the point of fledging. Along with other studies, our findings support the general conclusion that late‐season offspring desertion is likely to evolve only when parental compensation by the deserted partner can minimize costs to the current brood.     

Geographical origin of dabbling ducks wintering in Iberia: sex differences and implications for pair formation

Natural and anthropogenic Iberian wetlands in southern Europe are well known for supporting large numbers of migratory Palaearctic waterbirds each winter. However, information on the geographical origin of dabbling ducks overwintering in these wetlands is scarce and mostly limited to data from ringing recoveries. Here, we used intrinsic isotopic markers to determine the geographical origin of male and female Northern Pintails Anas acuta and Eurasian Teal Anas crecca in Extremadura, inland Iberia, a key site for overwintering dabbling ducks. Additionally, we fitted six Northern Pintails with GPS‐GSM tags to complement the data derived from stable isotope analysis. Most (> 70%) first calendar‐year Northern Pintails were assigned to regions above 55°N, flying 2600–5600 km from their main natal regions to Extremadura. Mean values of δ²H_f varied significantly between male and female Northern Pintails, suggesting that the sexes had different geographical origins. Data from tagged adult Northern Pintails supported the isotopic data, one male flying more than 5000 km to the coast of the Pechora Sea (Russia). Most (> 70%) first calendar‐year Eurasian Teal were assigned to the region between 48° and 60°N, travelling 1500–4500 km to arrive in Extremadura. Male and female Eurasian Teal showed marginal differences in mean values of δ²H_f. In migratory dabbling ducks, pairing typically occurs on the wintering grounds, and ducks in their first winter can breed the following spring. For Northern Pintails, pair formation in Extremadura could occur between individuals with different geographical origins, which could contribute to the genetic variability of their offspring.     

Clutch desertion and re-nesting in pied flycatchers: an experiment with progressive clutch removal

Clutch desertion and re-nesting are important components of fitness when predation is frequent. In nestbox populations however, nest predation and desertion are rare but can be studied by experimental manipulations. We experimentally reduced clutches of pied flycatcher,Ficedula hypoleucaby removing one egg per day until desertion occurred. The size of the clutch at desertion and whether females re-nested or not were used as measures of the female response. Of the deserting females, 74% re-nested in our study area. Re-nesting frequency was correlated with date but not with the size of the clutch laid. The majority of the non-re-nesting females deserted empty nests, while the majority of re-nesting females deserted one egg. Clutch size at desertion was not correlated with the size of the clutch laid nor with laying date; it was smaller than the size predicted by an optimality analysis of the value of both the current (deserted) and the replacement clutch. For the re-nesting females, there was a negative correlation between fledging rate of the replacement clutch and the size of the clutch at desertion. Our predictions, made under the hypothesis that desertion and re-nesting are adaptive behaviours, were partly supported by the data; we explain the discrepancy by the constraint of searching for a new nest site or mate for re-nesting.     

Variation in nest defense in ducks: methodological and biological insights

Studies of avian nest defense generally explain only a small proportion of the total variation in defense behavior. We explored two potential methodological sources of variation in nest defense in three species of ducks. One common method of quantifying nest defense rests on the assumption that different components of nest defense (e.g. flushing distance, distraction displays) are highly positively correlated. Defense behaviors we observed in this study were weakly related or unrelated to each other. Thus, the assumption of strong positive covariance between components of nest defense was not supported. We also considered the effect of repeated visits to the same nests on nest defense. Females of all three species took less risk defending their nests with repeated visits, and the effect of visit number on nest defense was greater than the effect of increasing value of nests associated with advancing incubation. Ducks appear to be different from other birds in the consistency with which they alter their nest defense in response to repeated nest visits. We propose that this could be a consequence of having nest predators that return to the vicinity of a nest if they were previously unsuccessful finding the nest, thus making repeated nest visits more dangerous to the ducks. By testing this or other hypotheses it should be possible to go beyond understanding the methodological implications of the effect of repeated visits on nest defense, and use this phenomenon to gain insight into the predator-prey interactions that underlie nest defense.     

Sitting ducklings: Timing of hatch,nest departure,and predation risk for dabbling duck broods

For ground‐nesting waterfowl, the timing of egg hatch and duckling departure from the nest may be influenced by the risk of predation at the nest and en route to wetlands and constrained by the time required for ducklings to imprint on the hen and be physically able to leave the nest. We determined the timing of hatch, nest departure, and predation on dabbling duck broods using small video cameras placed at the nests of mallard (Anas platyrhynchos; n = 26), gadwall (Mareca strepera; n = 24), and cinnamon teal (Anas cyanoptera; n = 5). Mallard eggs began to hatch throughout the day and night, whereas gadwall eggs generally started to hatch during daylight hours (mean 7.5 hr after dawn). Among all species, duckling departure from the nest occurred during daylight (98%), and 53% of hens typically left the nest with their broods 1–4 hr after dawn. For mallard and gadwall, we identified three strategies for the timing of nest departure: (a) 9% of broods left the nest the same day that eggs began to hatch (6–12 hr later), (b) 81% of broods left the nest the day after eggs began to hatch, and (c) 10% of broods waited 2 days to depart the nest after eggs began to hatch, leaving the nest just after the second dawn (27–42 hr later). Overall, eggs were depredated at 10% of nests with cameras in the 2 days prior to hatch and ducklings were depredated at 15% of nests with cameras before leaving the nest. Our results suggest that broods prefer to depart the nest early in the morning, which may best balance developmental constraints with predation risk both at the nest and en route to wetlands.     

The evolution of postcopulatory displays in dabbling ducks (Anatini): a phylogenetic perspective

Although postcopulatory displays are widespread in animals, little is known about their function or the intended receiver. The postcopulatory displays of dabbling ducks are among the best described for any animal group. We documented the presence of initial and additional postcopulatory displays in nearly all dabbling duck species. We then reconstructed the evolution of postcopulatory displays in dabbling ducks using a phylogeny derived from mitochondrial DNA sequences. The display immediately following copulation (the initial display) is highly stereotyped in most species and shows extreme phylogenetic conservation. In contrast, the performance of additional displays is less stereotyped and less phylogenetically conservative. We review the possible functions of postcopulatory displays. Using evidence from display orientation, display form and phylogenetic reconstruction, we suggest that the most likely functions of postcopulatory displays in dabbling ducks are pair bond maintenance, individual identification, or signalling a successful copulation. Copyright 2000 The Association for the Study of Animal Behaviour.     

Sexual conflict over parental care in Penduline Tits Remiz pendulinus: the process of clutch desertion

Do the two parents at a nest make simultaneous decisions whether to care for their offspring or to desert? If a single parent is sufficient for rearing young, one parent (typically, the male) may desert and reproduce with a new mate within the same breeding season, leaving the other parent with the brunt of care. As each parent is expected to maximize its own reproductive success, the interests of the two parents do not necessarily coincide, and a sexual conflict over care may emerge. Here we investigate the process of clutch desertion in a small passerine bird, the Penduline Tit Remiz pendulinus. Among birds, this species has a remarkably variable breeding system, because a single parent (either the male or the female) may provide the full care of the young, whereas about 30% of clutches are abandoned by both parents. First, we show that biparental desertion occurs within a single day in 73.7% of the clutches (n = 14), whereas desertion decisions are sequential in 26.3% of the clutches (n = 5) (male first: 10.5% (n = 2); female first: 15.8% (n = 3); n = 19 clutches in total). Secondly, we observed the behaviour of both parents before desertion, and investigated whether desertion can be predicted from their behaviour. However, neither singing nor nest‐building behaviour predicted whether the male or the female would desert. We therefore suggest that biparental desertion may be simultaneous by male and female in our population of Penduline Tits. Furthermore, the parents do not appear to signal their intention to desert their mate. We argue that the parents’ interest may be actually to disguise their intention to desert.