How Kentish plovers,<Emphasis Type="Italic"> Charadrius alexandrinus</Emphasis>, cope with heat stress during incubation

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

Offspring reproductive value and nest defense in the magpie (Pica pica)

Summary Magpie (Pica pica) brood defense against a human at the nest was studied in a Mediterranean population with low renesting potential. Variations in two defense measures recorded during 106 trials at 41 different nests were positively correlated with brood age. Ineremental effects due to the number of successive visits to nests by us, brood size, and the time in the breeding season were not significant. Partial correlation analyses showed that visit rate was not an important determinant of nest defense, which thus favors an adaptive explanation of nest defense patterns. Two functional hypotheses to account for the increase in defense intensity with brood age were tested: whether (1) increased parental defense serves to compensate the higher predation risk of older nests or (2) increased parental defense reflects the increasing reproductive value of nestlings as they grow older. Daily mortality and incidende of predation (estimated from contribution of whole-brood losses to total mortality) was higher early in the nestling period, hence providing weak evidence for the assumption on which hypothesis (1) is based. The timing of parental defense intensity did not mirror variations in predation risk for the nest but variations in reproductive value of the brood, as can be estimated from daily mortality, thus supporting hypothesis (2). Magpie parents increased defense intensity in response to premature escaping by almost fully-developed nestlings. Since such a response lowers predation risk for the offspring and increases their probability of survival, this finding supports hypothesis (2), but runs contrary to hypothesis (1). Parents also increased defense in response to play-backs of alarm calls uttered by nestlings during escaping episodes. It is argued that parents should continuously monitor the degree of offspring development in order to assess their reproductive value and that, by alarm calling, chicks honestly make their parents aware of the gain in reproductive value that results from enhancement in locomotory abilities that occur at the end of the nestling period.     

Risk taking during parental care: a test of three hypotheses applied to the pied flycatcher

According to life-history theory, there will often be a conflict between investment in current versus future reproduction. If a predator appears during breeding, parents must make a compromise between ensuring the growth and survival of offspring (nest defence, feeding and brooding of young), and reducing the risk of predation to ensure their own survival. We model three hypotheses for the outcome of this conflict which are particularly relevant for altricial birds. They are not mutually exclusive, but focus on different costs and benefits. (1) Parental investment is determined by the parents’ own risk of predation. This hypothesis predicts that a lone parent should take smaller risks than a parent that has a mate. (2) Parental investment is related to the reproductive value of the offspring: Parents are predicted to take greater risks for larger broods, larger-sized or older offspring. (3) Finally, we present the new hypothesis that parental investment is related to the harm that offspring would suffer during a period of no parental care (incubation, brooding, feeding). This hypothesis predicts that parents should take greater risks for younger offspring, or for offspring in poorer condition, because the marginal benefit of parental care is largest in such cases. Hence, one may also expect that lone parents should take greater risks than two parents because their offspring are more in need of care. We tested these hypotheses on the pied flycatcher (Ficedula hypoleuca) by presenting a stuffed predator of the parents (a sparrowhawk, Accipiter nisus) close to the nest when parents were feeding the young. Risk taking was measured as the time that elapsed until the first visit to the nest. Most support was found for the ‘‘harm to offspring’’ hypothesis. Previous studies have usually measured the intensity of nest defence against typical nest predators, and have found evidence for the ‘‘reproductive value of offspring’’ hypothesis. However, our model predicts that the importance of the reproductive value of the offspring should decrease relative to the harm that offspring would suffer if they were not cared for when the predator type changes from a nest predator to a predator of adults, and when conditions for breeding turn from good to bad. Received: 13 April 1995/Accepted after revision: 11 March 1996     

Does egg colour affect predation rate on open passerine nests?

The breeding success of many passerines is strongly reduced by egg predation. The adaptive significance of egg crypsis in open nesters is often taken for granted, but visually searching predators may first detect the nest or adult bird and not the eggs. Götmark predicted that selection should favour egg crypsis in the absence of conspicuous nests, whereas birds with conspicuous nests should have non-cryptic eggs. I compared the effect of egg colour treatment (white, blue, brown-spotted) on nest survival (1) among species characterized by different egg coloration, nest size and nest placement, and (2) between relatively well and poorly concealed nests within species. I used artificial nests (n=1,296) and eggs mimicking (except in egg colour) those of the yellowhammer (Emberiza citrinella), blackcap (Sylvia atricapilla) and song thrush (Turdus philomelos). Concurrently, I monitored survival of real nests (n=1,106). Nest survival differed among species, increased with nest concealment and throughout the breeding season, but was not significantly related to egg colour in any species. Nevertheless, the data for the yellowhammer suggest a trend in survival rates across the colour treatments. Brown eggs survived better than white eggs by 11% and 4% in 2 years, but this study had insufficient power to detect effects of this size. The results thus suggest that egg coloration in the song thrush and blackcap (shrub nesters) may be a neutral trait with regard to nest predation, whereas egg crypsis may be an anti-predation feature for the yellowhammer (ground/near-ground nester). The role of predation in the evolution of eggshell colour may vary not only between cavity and open nesters, but also across nest sites within the latter group.     

Nest-maintenance effort and health status in chinstrap penguins, Pygoscelis antarctica: the functional significance of stone-provisioning behaviour

Stone provisioning is a nest-maintenance activity accomplished by pygoscelid penguins after reliefs during the incubation/brooding period. The functional significance of this behaviour has been mainly explained as a parental strategy preventing nest flooding under detrimental weather conditions. In addition, and in the light of recent studies, this behaviour could also fit into the sexual selection process. In this study, we tested the first idea, that is, whether stone provisioning is a nest-maintenance behaviour to increase egg/nestling survival by lowering the risk of nest flooding, and can thus be considered a form of parental care. Additionally, we investigated if the effort invested by parents in nest maintenance is constrained by physiologically limiting resources. The effort of stone collection and the perceived risk of nest flooding were experimentally manipulated during the incubation and early brooding phases in a chinstrap penguin, Pygoscelis antarctica, colony. Three groups of nests were established. After weighing, control nests were left unmanipulated. In a second group of nests (reduced group), only one-half of the initial weight of stones was returned to the nests. In a third group of nests (snow-added group), we both reduced nest weight by a half and added snow outside the nest bowl over 6 consecutive days. Ten days after manipulation, the difference in nest weight between initial and final conditions was significantly related to treatment: penguins increased stone provisioning in the reduced group (44% of half-reduced nests), but drastically more in the reduced and snow-added group (123% of half-reduced nests), while the weight of control nests was unchanged compared to premanipulation conditions. The intensity of stone provisioning was affected by nest date, peaking about hatching time and shortly after, and declining with advancing chick age. These results suggest that stone provisioning is a mechanism that has evolved to prevent egg or chick mortality by nest flooding. The haematocrit, but not leukocyte numbers as expressed by the buffy coat layer, varied with the experimental conditions. Penguins investing more time in nest maintenance had a lower haematocrit, suggesting a physiological trade-off probably mediated by competition between the time devoted to nest maintenance versus foraging activities. The amount of stones collected and the haematocrit were positively related to the number of neighbour nests, so those individuals surrounded by more nests seemed to obtain benefits in the availability of nest material and energy savings. This study indicates that stone-provisioning behaviour is a nest-maintenance activity evolved to improve thermal nest characteristics potentially increasing offspring survival, and competing in time and energy with other reproductive activities. Stone provisioning in penguins should therefore be regarded as a form of parental care and an important part of individual reproductive effort in species breeding in harsh environments. Furthermore, nest size and nest-maintenance effort should be considered reproductive traits indicative of parental quality and thus could also be involved in the post-mating sexual selection process.     

Effects of incubation temperature on hatchling pine snakes: implications for survival

Incubation temperature in ectothermic vertebrates affects incubation periods, and in some reptiles it affects sex ratios and behavior. I present evidence that incubation temperature affects emergence and post-hatching behavior of pine snakes (Pituophis melanoleucus) that could influence survival in the weeks before hibernation. Hatchlings incubated at low temperatures remained in the nest longer, had fewer alternate nest openings, and fewer underground tunnels to hide in than did hatchlings from warmer temperatures. These conditions could render hatchlings from low-temperature nests more vulnerable to predation because, if a nest is opened, they are not inside tunnels where they would be protected. Hatchlings from nests incubated at low temperatures took longer to find shade during a thermoregulation test, and were less likely to move about in search of other cover than were those from higher-incubation-temperature artificial nests. Similarly, hatchlings from nests with low incubation temperatures were less responsive to a predatory stimulus and had a longer latency to strike than other hatchlings. Taken together, hatchlings from nests with low incubation temperatures might be less able to avoid predators and find shade than those from nests incubated at higher temperatures, and thus could be expected to have lower survival in nature. Received: 21 July 1997 / Accepted after revision: 15 February 1998     

Predator-induced female behavior in the absence of male incubation feeding: an experimental study

Parental care plasticity is critical to understanding the ecological and evolutionary influence of nest predation on life history strategies. In birds, incubation imposes a trade-off between the requirements of females (i.e., food) and egg requirements (i.e., heat and protection from predators). However, studies on this topic are rare and usually restricted to species where the male feeds the incubating female, relaxing her incubation costs. Males and females can reduce their activity at the nest to avoid detection by predators. However, females could follow two alternative antipredator strategies: to delay their return to the nest to avoid attracting attention from the potential predator or to return to the nest as soon as possible to enhance nest concealment. In this study, we manipulated the perceived risk of nest predation of incubating common blackbirds (Turdus merula), a species without incubation feeding, to study female behavioral changes induced by nest predation risk. We show experimentally that female blackbirds can reduce their nest visits in the situation with higher nest predation risk. In addition, we confirm that females significantly delay their return to the nest in the presence of a nest predator, contradicting the nest concealment hypothesis. However, our results could be interpreted as a passive antipredator response (to minimize clues given to predators) or as an active antipredator response (to search for predators to expel them from their territories).     

Song rate as a signal for nest site quality in blackcaps (Sylvia atricapilla)

In this study we examine male song output as a measure of nest site quality in blackcaps (Sylvia atricapilla). Song rate, breeding success, predation on nests and reaction to playbacks were investigated in individual males. Habitat features determining nest site and song post quality in terms of vegetation cover were compared between successful nests and nests that had suffered predation. We then related song rate of unmated males to habitat factors in territories and nesting sites in order to examine a possible predictor function of blackcap song for habitat quality. Several habitat features are responsible for variation in nesting success. These features also correlate with song rate of unmated males. The study indicates a potential role of song rate in the advertisement of territory quality. Furthermore, the data suggest that females use song rates rather than territory quality in mating decisions. The information females may gain about male quality in relation to territory quality are also discussed.     

A nesting association between semi-colonial Bullock's orioles and yellow-billed magpies: evidence for the predator protection hypothesis

Mixed-species nesting associations have been reported for many birds. However, few studies have shown that one species specifically chooses to nest close to another in order to obtain a direct benefit and only two studies have managed to adequately test the predator protection hypothesis for such associations. Here we demonstrate a nesting association between Bullock's orioles (Icterus galbula bullockii) and yellow-billed magpies (Pica nuttalli). The evidence indicates that this is an active association in which, given a choice of suitable habitat areas in which to nest, orioles choose those areas that contain magpie nests. Oriole nests associated with magpie nests have a significantly reduced predation rate compared to those without associated magpie nests. This suggests that orioles prefer to nest near to magpies and that this preference has evolved due to the protection provided by neighbouring magpies. The nesting association results in the clumping of oriole nests around magpie nests, and therefore provides a possible explanation for the semi-colonial nature of Bullock's orioles. Received: 23 November 1998 / Received in revised form: 25 May 1999 / Accepted: 30 May 1999     

Sex-biased costs in nest defence behaviours by lesser snow geese (Chen caerulescens): consequences of parental roles?

Nest defence behaviours, such as attacking predators and defending against predator attacks, expose birds to risk of injury and death. However, direct costs of such behaviours are poorly documented. To evaluate potential costs of nest defence behaviours in lesser snow geese (Chen caerulescens), we (1) estimated the proportion of interactions between arctic foxes (Alopex lagopus) and geese that resulted in physical contact and (2) examined how nest defence behaviours varied between male and female geese. We separated interactions into attacks initiated by foxes (attacks by foxes) and attacks initiated by geese (attacks by geese). Risks associated with attacks by geese were considerably lower than the risks associated with attacks by foxes; only 1 of 1,179 attacks by geese resulted in physical contact between foxes and geese, whereas 26 of 89 attacks by foxes involved such contact (two female geese were killed during these attacks). Attacks by geese were made almost exclusively by male geese (>97%), whereas female geese were involved in 75% of all attacks by foxes that resulted in physical contact with geese. There was, thus, a considerable difference in risks associated with male and female nest defence behaviours. We suggest that parental roles during nesting (i.e., females incubate and males guard) expose female geese to greater risk of injury and death. Male geese may, however, reduce the risk of injury or death to their mates with pre-emptive attacks on foraging foxes.     

Is reduced clutch size a cost of parental care in Eastern Phoebes (Sayornis phoebe)?

What is the cost of parental care in birds? Previous studies using observational and experimental techniques on nest building and clutch sizes in a small migrant flycatcher, the Eastern Phoebe (Sayornis phoebe), led to contradictory results that did not show a consistent cost of current reproductive effort on residual reproductive output. The data presented here indicate that different elements of parental behaviors are indeed costly because they reduce various aspects of phoebes' subsequent reproductive performance. Experimental removal of old nesting structures at previously used breeding sites reduced but did not eliminate the chance of phoebes' settlement in the subsequent year. Comparing sites at which phoebes did and did not build new nests showed that nest builders completed their first clutches later, had lower probabilities of second breeding attempts, and more often lost their nesting attempt due to fallen nest structures than nest reusers. There was, however, no significant effect of nest building on the clutch sizes and rates of cowbird parasitism of first nesting attempts. Overall, sites with newly built nests had lower seasonal reproductive effort than sites with reused nests. I also examined phoebes' relative residual reproductive output in a separate breeding season when nest building was not experimentally manipulated. When controlled for confounding variables this analysis indicated that in those phoebes that did breed for a second time, the relative decrease of the sizes of first to presumed second clutches was greater at sites where first breeding attempts consisted of more total nestlings. These data are consistent with the hypothesis that parental care is costly in Eastern Phoebes and support predictions of trade-offs between the nest building, brood care, and residual egg-investment components of reproduction.     

Evidence of an Edge Effect on Avian Nest Success

Abstract:  Habitat fragmentation may modify ecological patterns by increasing the importance of edge effects, including elevating rates of predation on avian nests. Conventional wisdom suggests an increased rate of predation along habitat edges, and previous reviews support this view. These reviews did not apply recent statistical approaches, however, and some were based on a small number of studies. In our meta-analysis of 64 nest-predation experiments, our results supported prior reviews of the general pattern of increased nest predation along habitat edges (  p < 0.01). We separated studies into ecologically relevant categories and found the following patterns: (1) Edge effects were more pronounced in North America and northwestern Europe than in central Europe or Central America. This result may be biased, however, by the different habitats studied in the regions. (2) Marshes and deciduous forests had significant edge effects, whereas edge effects were not apparent in coniferous forests, tropical forests, or fields. (3) Ground and natural nest studies were more likely to exhibit edge effects. (4) Edge effects were detected in studies that used quail eggs and real eggs. (5) Edge effects were not significant when artificial nests were exposed for typical incubation periods, but were significant for shorter exposures. Three alternative hypotheses may explain increased nest predation along edges. The edge-effects hypothesis states that increased nest losses along edges are the result of the habitat discontinuity. The landscape-structure hypothesis states that more fragmented landscapes are more heavily depredated by nest predators. The human-disturbance hypothesis states that near anthropogenic edges increased nest predation is related to human activities. Nest-predation experiments should be placed in a landscape context to reveal differences between the hypotheses.     

Does the great spotted cuckoo choose magpie hosts according to their parenting ability?

When brood parasites are about to lay an egg, they have to decide which nest to parasitize. The best nest in which to lay will depend on the parenting ability of the host. We have studied selection of magpie (Pica pica) hosts by great spotted cuckoos (Clamator glandarius). Great spotted cuckoos preferentially parasitize large host nests. Nest volume in magpies is a good indicator of territory quality, since there is a negative relationship between magpie nest size and breeding date, and timing of breeding in magpies is known to be positively related to territory quality. Moreover, magpies occupying high-quality territories have high breeding success. Therefore, nest size is positively related to the quality of magpies. Parasitized magpie nests were of greater volume than the nearest neighbouring nest not parasitized by the great spotted cuckoo. In order to test whether the great spotted cuckoos might select high-quality magpie hosts, we manipulated pairs of parasitized and non-parasitized nests with identical laying dates and habitats, introducing into each of the nests the same number of parasitic and non-parasitic eggs. The number of fledglings reared (magpie plus great spotted cuckoo chicks) in naturally parasitized nests was higher than in experimentally parasitized nests. Thus, the probability of survival of the parasite chicks increased if cuckoo eggs were laid in the nests of high-quality hosts originally chosen by the parasite.     

Hatching asynchrony is constrained by parental nest attendance during laying

Hatching asynchrony is widespread amongst animals, but no consensus has yet emerged as to why asynchronous hatching has evolved. It is generally thought to have adaptive benefits during the raising of dependent young. However, here, we considered an alternative view of hatching asynchrony in birds as a consequence of factors acting at the onset of incubation. We recorded parental nest attendance behaviour during laying using continuous records of nest temperature in herring gulls, Larus argentatus. We tested whether nest attendance during laying was related to individual factors (clutch size and diet) and whether it had consequences on fitness outcomes (hatching spread, incubation period, hatching success and chick survival). Low nest attendance was associated with small clutch size, and independent of clutch size, pairs on a more marine diet had lower nest attendance than pairs on a lower trophic level terrestrial diet, possibly due to higher foraging effort for marine food. Broods hatched more asynchronous where pairs had a lower nest attendance during laying or took longer to complete a clutch and where the last egg took longer to hatch. Low nest attendance was also related to shorter incubation periods, possibly representing a strategy of birds in poor condition to reduce the demand of incubation by reducing the length of incubation. We found that low nest attendance during laying and increasing hatching asynchrony had detrimental effects on hatching success for small eggs laid early in the laying sequence. Increasing hatching asynchrony also had a detrimental effect on the survival of the youngest sibling. In our study population, hatching asynchrony was influenced by a more complex set of factors than simply onset of incubation and appears to be constrained by circumstances at the onset of incubation rather than to be an adaptive strategy. Thus, factors acting both during offspring rearing and at the onset of incubation need to be considered for a better understanding of hatching asynchrony.     

Antipredation role of clumped nesting by marsh-nesting red-winged blackbirds

Summary Red-winged blackbirds, Agelaius phoeniceus, breed in marshes in high densities and their nests are frequently clumped. Because predation is consistently the most important cause of redwing nesting mortality, high densities of breeding individuals could be an anti-predation adaptation. In our study site predation by marsh wrens, Cistothorus palustris, was the main cause of redwing nesting losses. In situations when marsh wrens were near, predation rates on redwing nests decreased with increasing female density. Group life could reduce predation because of improved nest defense, selfish herd effects, or predator dilution effects. We differentiated between these possibilities by introducing experimental colonies consisting of 3, 6, and 9 artificial nests near and away from active redwing nests. The experimental colonies near active nests suffered less predation, but predation rates were not correlated with colony size or a nest's location within the colony. Therefore, the advantage of group life in this population is probably mutual nest protection.     

Egg adoption can explain joint egg-laying in common eiders

Hypotheses regarding the evolution and maintenance of intraspecific nest parasitism were tested with data collected during a 3-year study of common eiders (Somateria mollissima) breeding near Churchill, Manitoba. The nest parasitism rate was highest (42.4% of nests) during the year with the highest nest density and the best environmental conditions, and lowest (20.2% of nests) in the year with the lowest nest density and the poorest environmental conditions. Over the nesting season, parasitic eggs were laid at the same time as normally laid eggs. Most parasitic eggs (>75%) were laid before the host female laid her third egg. The majority of the parasitic eggs were the first or second egg produced by the parasitic female. When a parasitic egg was laid before or on the same day as the host female initiated her clutch, the probability of her first egg being depredated before incubation was significantly lowered. First- and second-laid eggs suffered a high rate of predation probably because nesting females do not attend their clutch until their second or third egg is laid. Hypotheses that some females use intraspecific nest parasitism to parasitize the parental care of other females were inconsistent with these data. Egg adoption is a likely explanation for the prevalence of females incubating parasitic eggs in this population. Received: 30 September 1997 / Accepted after revision: 6 May 1998     

Brood parasitism disproportionately increases nest provisioning and helper recruitment in a cooperatively breeding bird

Obligate avian brood parasites lay their eggs in nests of other species (hosts), which raise parasitic young. Parasitic nestlings are likely to influence host’s parental behaviours as they typically beg for food more vigorously than young host for a given hunger level. However, few studies have tested this idea, with conflicting results. These prior studies were largely limited to biparental hosts, but little is known about the effect of brood parasitism on parental behaviours in hosts that breed cooperatively. We followed a multimodel approach to examine the effect of brood parasitism on nest provisioning and helper recruitment in the baywing (Agelaioides badius), a cooperative breeder parasitised by screaming (Molothrus rufoaxillaris) and shiny (Molothrus bonariensis) cowbirds. Multimodel inference results indicated that feeding visits increased with nestling age, cooperative group size and number of cowbird nestlings in the brood. Brood size had little influence on feeding visits, which further suggests that baywings adjusted their provisioning effort in response to cowbird parasitism. In addition, nests parasitised artificially with shiny cowbird eggs or hatchlings recruited more helpers than unmanipulated nests having only host or screaming cowbird young. Our results provide novel evidence that brood parasitism and cooperative breeding interact in determining the levels of nest provisioning.     

Differences in Predators of Artificial and Real Songbird Nests: Evidence of Bias in Artificial Nest Studies

Abstract:  In the past two decades, many researchers have used artificial nests to measure relative rates of nest predation. Recent comparisons show that real and artificial nests may not be depredated at the same rates, but no one has examined the mechanisms underlying these patterns. We determined differences in predator-specific predation rates of real and artificial nests. We used video cameras to monitor artificial nests baited with quail and plasticine eggs and Field Sparrow ( Spizella pusilla ) and Indigo Bunting ( Passerina cyanea ) nests in field habitats in central Missouri (U.S.A.). Although daily predation estimates (all predators pooled) were similar between artificial and real nests, predators differed substantially in their depredation of artificial versus real nests. Snakes were the major predator at real nests, and raccoons ( Procyon lotor) were the major predator at artificial nests. We found strong support for models that distinguished predation between two or among three predator groups and between artificial and real nests. There was no snake predation of artificial nests, and the odds of predation of artificial nests was 115–551% (95% confidence interval) and 2–154% of the odds of predation of real nests by mammals and birds, respectively. Artificial nests with plasticine eggs could not be used reliably to identify predators. In several cases plasticine eggs were marked by mice, and raccoons were recorded on video removing the quail egg. Because biases for artificial nests were positive for some predators and negative for other predators (and could be compensating), and potentially existed for all predator groups, conclusions based on artificial nest studies should be suspect even when there is evidence that overall predation rates are similar among real and artificial nests.     

The effects of nesting stage,sex, and type of predator on parental defense by killdeer (Charadrius vociferous): testing models of avian parental defense

Summary Two models predicting the temporal patterns of parental investment in offspring defense over the nesting cycle were tested. The first is based on offspring age, the other on the vulnerability of offspring to predation. Both models make very similar predictions for altricial species after eggs have hatched, i.e., increases in intensity of parental defense until fledging. For precocial species, however, the post-hatching predictions of each model are different: the offspring age model predicts a continued increase in defense intensity, while the vulnerability model predicts a decline. I examined the temporal patterns of parental defense of a precocial shorebird, the killdeer (Charadrius vociferus), and determined which model was supported. Killdeer responses to human and natural predators were observed. Killdeer were less willing to leave the nest, responded most intensely, and displayed closest to a potential predator around hatching. Defense intensity increased from early to late incubation as predicted by the offspring age model. However, after hatching killdeer parental defense declined for both males and females, thus supporting the vulnerability model for this stage. Males and females responded significantly differently to all types of predators. Males took greater risks, remained on the nest longer, defended offspring more intensely, and displayed closer to the predator than females at the approach of a potential predator. Responses to natural predators depended on the type of predator and the approach made by the predator; a greater range of defense behavior was used for predators approaching on the ground compared to aerial predators. In general, killdeer responses to humans were more intense and less variable than their responses to natural predators. This was most likely because the human intruder approached nests and chicks more directly and closer than natural predators.     

Patterns of Nest Predation on Artificial and Natural Nests in Forests

Abstract:  Artificial nest experiments have been used in an attempt to understand patterns of predation affecting natural nests. A growing body of literature suggests that neither relative rates nor patterns of predation are the same for artificial and natural nests. We studied nest predation and daily mortality rates and patterns at real and artificial ground and shrub nests to test the validity of artificial nest experiments. We monitored 1667 artificial and 344 natural nests, over seven trials, in three regions, across 58 sites in Ontario. We controlled for many of the factors thought to be responsible for previously reported differences between predation rates on natural and artificial nests. Although artificial nests in our study resembled natural nests, contained eggs of appropriate size, shape, and color of target bird species, and were placed in similar microhabitats as natural nests, the rates of predation on these nests did not parallel rates on natural nests for any region in terms of absolute rate or pattern. Predation rates on artificial nests did not vary between years, as they tended to for natural nests, and the magnitude of predation pressure on artificial ground nests compared with shrub nests did not show the same pattern as that on natural nests. In general, rates of predation on artificial nests were significantly higher than on natural nests. Our results suggest that conclusions derived from artificial nest studies may be unfounded. Given that many influential ideas in predation theory are based on results of artificial nest experiments, it may be time to redo these experiments with natural nests.