Age and clutch size variation in Dusky Flycatcher nest survival

Examination of spatial and temporal factors that influence nest survival can provide insight into habitat selection, reproductive decisions (e.g., clutch size), population dynamics, and conservation requirements for species. We used nest survival data for the Dusky Flycatcher Empidonax oberholseri to examine several factors that may influence nesting success. Our prediction was that the number of nest initiations would be positively associated with period nest survival. We used a model selection framework and found that nesting success was a function of clutch size and a cubic effect of age. Clutches with one, two, three, and four eggs had period survival rates of 0, 0.05, 0.33, and 0.49, respectively. Daily survival rates decreased from the onset of egg-laying and increased during the later stages of incubation before remaining relatively constant through the later portions of the nestling stage. Model-selection criterion provided support for a date effect on daily survival (i.e., daily nest survival declined across the nesting season) although the 95% confidence interval for the estimate included zero. We found that the majority of nest initiations occurred early in the nest season and declined across the season as period nest survival declined. Our prediction concerning nest survival was partially supported. In addition, we found substantial positive associations between clutch size and nest survival. While low daily survival rates for clutches with one or two eggs suggested that individuals may have reduced reproductive effort in response to nest predation risk, we did not find strong evidence that individuals reduced their clutch sizes in subsequent nest attempts. Alternative predictions, including the preferential settlement of higher quality individuals (e.g., those with the ability to lay full clutches to replace depredated nests) into high-quality habitat and differences in behavior patterns (e.g., number of visits to provision nestlings), may provide more consistent explanations for these patterns.     

Long-term familiarity promotes joining in neighbour nest defence

Familiarity plays an important role in the evolution of sociality and cooperation. Familiar individuals may gain a reputation for participating in, or defecting from, cooperative tasks. Previous research suggests that long-term familiarity with territorial neighbours benefits breeders. We tested the hypothesis that great tits (Parus major) are more likely to join in neighbours' nest defence if those neighbours are familiar from the previous year. We show that neighbours that shared a territory boundary the previous year are more likely to join their neighbours' nest defence than neighbours that did not share a boundary before. Closer neighbours did not differ from distant neighbours in their latency to join. For familiar neighbours that joined, there was no difference in call rate in relation to whether one or both members of the focal pair were familiar. First-time breeders (by definition unfamiliar) did not join each other's nest defence. This is the first evidence of a relationship between familiarity and joining in nest defence. Such direct benefits of familiarity may have important implications in the evolution of sociality.     

Overcoming challenges to morphological and molecular identification of Empidonax flycatchers: a case study with a Dusky Flycatcher

Flycatchers in the genus Empidonax are among the most difficult avian taxonomic groups to identify to species. Observers often rely on calls or songs in the field or detailed morphometrics in the hand to identify species. In January and February 2013, we twice captured an Empidonax flycatcher at the Virginia Zoo in Norfolk, Virginia. After being unable to identify the flycatcher to species level using morphometrics and photographs, we extracted DNA from two tail feathers collected during the second encounter to identify the individual genetically. Comparison of cytochrome c oxidase I (COI) with reference sequences in the Barcode of Life Database (BOLD) suggested that the specimen had a >99.8% probability of placement as a Dusky Flycatcher (Empidonax oberholseri). Additional comparisons of NADH dehydrogenase subunit 2 (ND2) to reference sequences in GenBank, however, suggested that the specimen was a Pine Flycatcher (Empidonax affinis), a species not represented in BOLD and confined geographically to a small area in Mexico and Guatemala. After analyzing both COI and ND2 from additional vouchered specimens, the bird caught in Virginia was determined to be a Dusky Flycatcher. We also suspect that some of the sequences in GenBank might derive from incorrectly identified specimens or otherwise could represent overlooked pseudogenes. Because the putative identification, based on GenBank sequences, would have represented the first record of Pine Flycatcher from the United States, our results reinforce the need for carefully vetted and taxonomically comprehensive molecular databases to allow definitive conclusions about sample identity. Further molecular phylogeographic review of this genus is warranted to resolve haplotype ambiguities.     

Inter- and intraspecific competition for nest holes in a population of the Collared Flycatcher Ficedula albicollis

Field experiments with nest boxes show that nest holes are a limiting factor on the local population size of the Collared Flycatcher Ficedula albicollis. When this limiting resource occurs with different types that are quality related there is intraspecific competition among males for the best nest boxes, with bigger males being the better competitors. Population dynamics of Great Tit Parus major and Blue Tit P. caeruleus when compared with Collared Flycatchers suggests that there is strong interspecific Competition for available nest holes. This is also shown by removal experiments.     

Intraseasonal patterns in shorebird nest survival are related to nest age and defence behaviour

Nest survival may vary throughout the breeding season for many bird species, and the nature of this temporal variation can reveal the links between birds, their predators, and other components of the ecosystem. We used program Mark to model patterns in nest survival within the breeding season for shorebirds nesting on arctic tundra. From 2000 to 2007, we monitored 521 nests of five shorebird species and found strong evidence for variation in nest survival within a nesting season. Daily nest survival was lowest in the mid-season in 5 of 8 years, but the timing and magnitude of the lows varied. We found no evidence that this quadratic time effect was driven by seasonal changes in weather or the abundance of predators. Contrary to our prediction, the risk of predation was not greatest when the number of active shorebird nests was highest. Although nest abundance reached a maximum near the middle of the breeding season, a daily index of shorebird nest activity was not supported as a predictor of nest survival in the models. Predators’ access to other diet items, in addition to shorebird nests, may instead determine the temporal patterns of nest predation. Nest survival also displayed a positive, linear relationship with nest age; however, this effect was most pronounced among species with biparental incubation. Among biparental species, parents defended older nests with greater intensity. We did not detect a similar relationship among uniparental species, and conclude that the stronger relationship between nest age and both nest defence and nest survival for biparental species reflects that their nest defence is more effective.     

Comparison of nest defence behaviour between two associate passerines

Nest predation is one of the most important factors limiting reproductive success, and antipredator behaviour can significantly reduce the loss of avian broods. I carried out field experiments on two sympatric passerines: the barred warbler and the red-backed shrike. Many authors have described the protective nature of nesting association between these species. However, we have little knowledge about the true nature of the relationships between associates. I examined (1) whether barred warblers and red-backed shrikes respond differently to an avian predator, and (2) whether males and females differ in the intensity of nest defence. Decoys of a known nest predator and a non-predatory control species were used to examine the types and relative intensity of parental response. I measured behavioural responsiveness by recording aggressive behaviour toward each model during the nestling period. Barred warblers and red-backed shrikes showed considerable variation in their response. Warblers more vigorously defended their own territories than shrikes. No differences between the sexes in antipredator behaviour in red-backed shrike were found. By contrast, in barred warbler, male was more involved in nest defence. The experimental tests provide evidence that these two species are able to differentiate between a predator and non-predator species.     

Factors affecting nest defence in female Canada Geese Branta canadensis

The effect of various characteristics of Canada Goose Branta canadensis females and clutch size on their nest defence intensity during incubation was studied in the field during five nesting seasons. Nests were approached by a human, and the leaving distance was measured. Risk-taking in post-encounter behaviour was categorized at three levels. Nesting success was significantly associated with nest tenacity, a variable derived from leaving distance, but not with post-encounter behaviour. Nest tenacity, as well as post-encounter behaviour, differed significantly between individuals. Intensity of post-encounter behaviour, but not nest tenacity, increased with increasing age of the female. Nest tenacity, standardized over the incubation period, increased with increasing clutch size, but post-encounter behaviour intensity did not. None of the nest defence variables was correlated with the structural size of the female. Nest tenacity, but not post-encounter behaviour intensity, increased during incubation. Nest tenacity did not differ between early and late breeders, but post-encounter behaviour was more intense in late breeders. There was no evidence of positive reinforcement of nest defence through repeated visits. Nest defence did not differ between lakes, lake types or nest site types, nor was it affected by the distance to the nearest neighbouring nest. The results supported several of the predictions extracted from parental investment theory.     

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.     

The role of melanin- and carotenoid-based plumage coloration in nest defence in the Great Tit

Although plumage coloration is recognized to convey valuable information about the bearer's parental abilities, few studies have explored the relationship between coloration and nest defence. In this study in Great Tit Parus major, we analysed the relationship between nest defence and melanin‐ as well as carotenoid‐based plumage coloration, after controlling for ecological variables known to influence nest defence. A principal components analysis was applied to classify birds according to how vigorously they defended the nest, and the intensity of nest defence was tested against plumage coloration. Males with a large black tie defended their nests more vigorously, but no such effect was found for yellow breast coloration. This suggests that melanin‐based coloration in the Great Tit is associated with aggression, including both dominance‐aggression and nest defence, whereas carotenoid‐based coloration is not. The challenge in future studies will be to demonstrate whether females use this trait as an ornament to assess male quality and whether they trade off between the different ornaments a male may exhibit.     

Nest intruders, nest defence and foraging behaviour in the Black-and-white Casqued Hornbill Bycanistes subcylindricus

The female Black-and-white Casqued Hornbill Bycanistes subcylindricus uses mud to seal herself into a nest cavity and remains there until the nesting attempt has ended. The male hornbill is solely responsible for food provisioning and external defence of the nest. Data on hornbill nests in a Ugandan rain forest were used to test Martindale's (1982) model of nest defence and central place foraging. As predicted, nest guarding by the resident male hornbill during intrusions by conspecifics altered foraging patterns; the resident male foraged closer to the nest, made a larger number of shorter visits, and brought smaller food loads. There was a significant change in size composition of fruits brought before, during, and after intrusions. For short intrusions, the volumes of food brought per unit time did not change. However, if intrusions lasted for days or weeks, food delivery rates declined. Nest-sealing by the Black-and-white Casqued Hornbill appears to function primarily to protect the nest from conspecifics rather than from interspecific predators.     

Parental investment in nest defence by stonechats (Saxicola torquata)

Breeding stonechats (Saxicola torquata) made mixed sequences of two calls when a human intruder entered territories. ‘Whits’ are modulated notes with a small frequency range, and in laboratory tests caused nestlings to stop begging. ‘Chacks’ cover a wide range of frequencies, and in the field were combined with flights made so as to distract an intruder from the nest. On average male and female call-rates were similar, but varied greatly according to the intruder's distance from the nest, and at different stages of the nesting cycle. Rates increased rapidly after hatching, and this correlated most closely with the cumulative total of parents' visits to feed nestlings. This suggests that the level of defence may be adjusted to the value of the offspring to their parents. Call-rates declined about one week after fledging. A smaller peak by some pairs at the start of incubation was apparently related to probable poor condition after a previous breeding attempt, and after laying large clutches. Rates of Whits were higher at nests with larger broods, up to an asymptote, but rates of Chacks were independent of brood size. Birds suffering nest-predation showed lower call-rates before the event than equivalent successful birds, suggesting that the calls do reduce the risk of predation.     

Intensity of nest defence is related to offspring sex ratio in the great tit Parus major

Nest-defence behaviour of passerines is a form of parental investment. Parents are selected, therefore, to vary the intensity of their nest defence with respect to the value of their offspring. Great tit, Parus major, males were tested for their defence response to both a nest predator and playback of a great tit chick distress call. The results from the two trials were similar; males gave more alarm calls and made more perch changes if they had larger broods and if they had a greater proportion of sons in their brood. This is the first evidence for a relationship between nest-defence intensity and offspring sex ratio. Paternal quality, size, age and condition, lay date and chick condition did not significantly influence any of the measured nest-defence parameters.     

Intensity of nest defence is not related to degree of paternity in the willow tit Parus montanus

We asked whether willow tit Parus montanus males adjust their parental care according to their paternity in current brood. The origin of the nestlings was determined by using molecular technique, and the studied broods were assigned into extra-pair paternity (EPP) broods, if at least one nestling was fathered by another male, and truly monogamous broods. Over 3  years, 14 of 40  broods (35%) included EP-offspring, and 29 of 273  nestlings (11%) were EP-young. Intensity of parental care was measured with risk-taking against a potential predator, mounted stoat Mustela erminea . The results showed that risk-taking by EPP males did not differ from that by monogamous males. Neither was the sexual difference in risk-taking different at EPP and monogamous broods. Our results are consistent with the hypothesis that males do not adjust their level of care to paternity, perhaps because they have no reliable cues for assessing their paternity. This may be related to the success of mate-guarding in their breeding environment, closed forests. Guarding is seemingly successful as the EPP levels are rather low, but it is not totally sure making the potential costs, rejection of own young, too high. We also discuss other population characteristics which may further prevent the evolution of paternity assessment in northern willow tits.