Debora Arlt

Nonideal breeding habitat selection: a mismatch between preference and fitness.

The selection of breeding sites in heterogeneous habitats should ideally be based on cues closely reflecting habitat quality and thus predicting realized individual fitness. Using long-term population data and data on territory establishment of male Northern Wheatears (Oenanthe oenanthe), we examined whether territory characteristics linked to individual fitness (reproductive performance and survival) also were linked to territory preference. Breeding territories varied in their physical characteristics and their potential effects on reproductive performance, and this variation among territories was correlated from one year to the next. Of all measured territory characteristics (from the focal and the previous year) only territory field layer height predicted individual fitness, i.e., reproductive performance was higher in territories with permanently short rather than growing field layers. Territory preference, instead, was only linked to the size of territory aggregations, i.e., males settled earlier at territory sites sharing borders with several adjacent sites than at those with few or no adjacent sites. This mismatch between territory characteristics linked to fitness and those linked to territory preference was not explained by site fidelity or compensated for by the different fitness components measured. Because the results were not in agreement with an ecological trap scenario, where poor habitats are preferred over high-quality habitats, our results suggest a more general case of nonideal habitat selection. Whereas nonideal selection with respect to territory field layer height may be explained by its poor temporal predictability within the breeding season, the preference for territory aggregations is still open to alternative adaptive explanations. Our study suggests that nonideal habitat selection should be investigated by direct estimates of preferences (e.g., order of territory establishment) and their links to habitat characteristics and fitness components. Furthermore, we suggest that the probability of establishing a territory needs to be included as a factor influencing patterns of habitat selection.

Habitat-specific differences in adult survival rates and its links to parental workload and on-nest predation

1. Adult survival rates strongly affect population growth, but few studies have quantified if and why adult survival differs between breeding habitats. We investigated potential causes of habitat-specific adult survival rates for male and female northern wheatears (Oenanthe oenanthe L.) breeding in Swedish farmland. 2. We used multistate mark-recapture models based on 1263 breeding records between 1993 and 2007 to estimate survival rates based on habitat-type (SHORT vs. TALL ground vegetation) and breeding-success state parameters. We also used breeding-season observations from 2002 to 2007 and an experimental manipulation of ground vegetation height to identify factors influencing adult mortality. 3. Females had lower annual survival than males (0.42 +/- 0.02 vs. 0.50 +/- 0.02); this difference largely resulted from low female survival in TALL habitats because of higher nest-predation risk and the large proportion of adult females being killed on the nest (>20%) during nest predation events. 4. Among successful breeders, both sexes displayed similar survival rates, but survival was lower for breeders in TALL as compared to SHORT habitats (0.43 +/- 0.03 vs. 0.51 +/- 0.02). Experimental manipulation of ground vegetation height, controlling for individual and territory quality (n = 132), suggested the cost of rearing young to be higher in TALL habitats (survival of successful breeders in TALL vs. SHORT; 0.43 +/- 0.11 vs. 0.57 +/- 0.05). 5. Detailed observations of food provisioning behaviour during chick rearing revealed a habitat-related difference in parental workload corresponding to the observed habitat differences in adult survival for successful breeders. Adults breeding in TALL habitats were forced to forage further from the nest relative to SHORT-habitat breeders (mean +/- SE; 69 +/- 10 vs. 21 +/- 2 m), which increased the estimated daily workload for adults in TALL vs. SHORT habitats by c. 20%. 6. On-nest predation and parental workload during chick rearing combine to largely explain habitat-specific adult survival rates. The results have implications for our understanding of adult sex ratios, causes of source-sink demography and habitat-specific growth rates. Furthermore, it suggests SHORT field margins and other residual habitat elements to be important for the conservation of farmland passerines breeding in cropland plains.

Habitat-specific population growth of a farmland bird.

Background To assess population persistence of species living in heterogeneous landscapes, the effects of habitat on reproduction and survival have to be investigated. Methodology/Principal Findings We used a matrix population model to estimate habitat-specific population growth rates for a population of northern wheatears Oenanthe oenanthe breeding in farmland consisting of a mosaic of distinct habitat (land use) types. Based on extensive long-term data on reproduction and survival, habitats characterised by tall field layers (spring- and autumn-sown crop fields, ungrazed grasslands) displayed negative stochastic population growth rates (log λs: −0.332, −0.429, −0.168, respectively), that were markedly lower than growth rates of habitats characterised by permanently short field layers (pastures grazed by cattle or horses, and farmyards, log λs: −0.056, +0.081, −0.059). Although habitats differed with respect to reproductive performance, differences in habitat-specific population growth were largely due to differences in adult and first-year survival rates, as shown by a life table response experiment (LTRE). Conclusions/Significance Our results show that estimation of survival rates is important for realistic assessments of habitat quality. Results also indicate that grazed grasslands and farmyards may act as source habitats, whereas crop fields and ungrazed grasslands with tall field layers may act as sink habitats. We suggest that the strong decline of northern wheatears in Swedish farmland may be linked to the corresponding observed loss of high quality breeding habitat, i.e. grazed semi-natural grasslands.

Prospectors combine social and environmental information to improve habitat selection and breeding success in the subsequent year

1. Because habitats have profound effects on individual fitness, there is strong selection for improving the choice of breeding habitat. One possible mechanism is for individuals to use public information when prospecting future breeding sites; however, to our knowledge, no study has shown prospecting behaviour to be directly linked to subsequent choice of breeding site and future reproductive success. 2. We collected long-term data on territory-specific prospecting behaviour and subsequent breeding in the short-lived northern wheatear (Oenanthe oenanthe). Non-breeders established prospecting territories (<2 ha) that overlapped the breeding territories of conspecifics. We tested whether: (i) prospectors used social and environmental cues that predicted territory-specific breeding success in the following year, and (ii) the prospecting territory was tightly linked to the subsequent breeding territory of the prospector, and whether this link would be weakened by intraspecific competition with original territory owners if they also survived. 3. As expected, prospectors were attracted to a combination of site-specific cues that predicted future breeding success, i.e. short ground vegetation, a successfully breeding focal pair and successful close neighbours. 4. Prospecting behaviour was directly linked to the choice of the following years breeding territory: 79% of surviving prospectors established a breeding territory at their prospecting site in the following year, with their breeding success being higher than other individuals of the same age. As predicted, fidelity to the prospected site was strongly dependent on whether the original territory owner of the same sex had died or moved. 5. Our findings suggest that the use of multiple cues reduces the negative impact of stochasticity on the reliability of social cues at small spatial scales (e.g. territories) and hence increases the probability of breeding success in the next year. Also, the use of conspecific attraction (i.e. the preference for breeding aggregations) is selectively advantageous because individuals are more likely to find a vacancy in an aggregation as compared to a solitary site. By extension, we hypothesize that species life-history traits may influence the spatial scale of prospecting behaviour and habitat selection strategies.

Effect of Geolocators on Migration and Subsequent Breeding Performance of a Long-Distance Passerine Migrant

Geolocators are small light-weight data loggers used to track individual migratory routes, and their use has increased exponentially in birds. However, the effects of geolocators on individual performance are still poorly known. We studied geolocator effects on a long-distance migrating passerine bird, the northern wheatear (Oenanthe oenanthe L.). We asked the general question of whether geolocators affect migratory behaviour and subsequent reproductive performance of small passerines by comparing arrival time, breeding time, breeding success and survival of geolocator versus control birds of known identity and breeding history. During two years geolocator birds (n=37) displayed a lower apparent survival (30%) as compared to controls (45%, n=164). Furthermore, returning geolocator birds (n=12) arrived on average 3.5 days later, started laying eggs 6.3 days later, and had lower nest success (25%) than control birds (78%). Our results suggest that geolocators affect migratory performance with carry-over effects to the timing of breeding and reproductive success in the subsequent breeding season. We discuss the implications of such geolocator effects for the study of migratory strategies of small passerines in general and suggest how to identify and investigate such effects in the future.

Sex-biased dispersal: a result of a sex difference in breeding site availability.

Sex‐biased dispersal is often explained by assuming that the resource‐defending sex pays greater costs of moving from a familiar area. We hypothesize that sex‐biased dispersal may also be caused by a sex bias in breeding site availability. In avian resource‐defense mating systems, site availability is often more constrained for females: males can choose from all vacant sites, whereas females are restricted to sites defended by males. Using data on breeding dispersal of a migratory passerine, we show that average number of available breeding options and availability of the previous year’s territory was greater for males than females. The female bias in site unavailability may explain the female bias in probability of breeding dispersal because there was no sex bias in dispersal among birds with their previous year’s territory available. We suggest that sex biases in the availability of breeding options may be an important factor contributing to observed variation in sex‐biased dispersal patterns.

Empirical evidence for ecological traps: a two-step model focusing on individual decisions

Ecological traps are poor quality habitats that are preferred by individuals and that cause standard indicators of habitat quality (e.g., density, age structure, order of settlement) to be misleading. A maladaptive choice of habitats may have detrimental effects on individual fitness and long-term population persistence. For conservation purposes, it is therefore critical to identify ecological traps. We discuss a recent review on the relationships between density, reproductive success and productivity at the patch level. We also discuss the concept of ecological traps and definitions of habitat quality and habitat preference. Based on recent literature on ecological traps, we argue that no study shows evidence of an ecological trap in its strict sense, i.e., a preference for sink and an avoidance of source habitat. This lack of evidence is partly due to poor measures of preference and individual fitness, and to an absence of studies at the individual level. We suggest a two-step protocol at the level of individuals to investigate ecological traps by linking: (1) habitat characteristics, i.e., potential cues used by individuals, to fitness, and (2) preference for specific habitat characteristics. This protocol is illustrated by a long-term study on breeding territory selection in Northern Wheatears (Oenanthe oenanthe) in Sweden. Using our protocol, we could determine at least in part why Wheatears show a mismatch between habitat preference and fitness. We conclude that there is an urgent need for more studies on individual habitat-selection decisions before we can find less labour-intensive ways to identify potential trap situations.

The short- and long-term fitness consequences of natal dispersal in a wild bird population

Dispersal is a key process in population and evolutionary ecology. Individual decisions are affected by fitness consequences of dispersal, but these are difficult to measure in wild populations. A long-term dataset on a geographically closed bird population, the Mauritius kestrel, offers a rare opportunity to explore fitness consequences. Females dispersed further when the availability of local breeding sites was limited, whereas male dispersal correlated with phenotypic traits. Female but not male fitness was lower when they dispersed longer distances compared to settling close to home. These results suggest a cost of dispersal in females. We found evidence of both short- and long-term fitness consequences of natal dispersal in females, including reduced fecundity in early life and more rapid aging in later life. Taken together, our results indicate that dispersal in early life might shape life history strategies in wild populations.

Population regulation of territorial species: both site dependence and interference mechanisms matter

Spatial patterns of site occupancy are commonly driven by habitat heterogeneity and are thought to shape population dynamics through a site-dependent regulatory mechanism. When examining this, however, most studies have only focused on a single vital rate (reproduction), and little is known about how space effectively contributes to the regulation of population dynamics. We investigated the underlying mechanisms driving density-dependent processes in vital rates in a Mauritius kestrel population where almost every individual was monitored. Different mechanisms acted on different vital rates, with breeding success regulated by site dependence (differential use of space) and juvenile survival by interference (density-dependent competition for resources). Although territorial species are frequently assumed to be regulated through site dependence, we show that interference was the key regulatory mechanism in this population. Our integrated approach demonstrates that the presence of spatial processes regarding one trait does not mean that they necessarily play an important role in regulating population growth, and demonstrates the complexity of the regulatory process.

Contrast in Edge Vegetation Structure Modifies the Predation Risk of Natural Ground Nests in an Agricultural Landscape

Nest predation risk generally increases nearer forest-field edges in agricultural landscapes. However, few studies test whether differences in edge contrast (i.e. hard versus soft edges based on vegetation structure and height) affect edge-related predation patterns and if such patterns are related to changes in nest conspicuousness between incubation and nestling feeding. Using data on 923 nesting attempts we analyse factors influencing nest predation risk at different edge types in an agricultural landscape of a ground-cavity breeding bird species, the Northern Wheatear (Oenanthe oenanthe). As for many other bird species, nest predation is a major determinant of reproductive success in this migratory passerine. Nest predation risk was higher closer to woodland and crop field edges, but only when these were hard edges in terms of ground vegetation structure (clear contrast between tall vs short ground vegetation). No such edge effect was observed at soft edges where adjacent habitats had tall ground vegetation (crop, ungrazed grassland). This edge effect on nest predation risk was evident during the incubation stage but not the nestling feeding stage. Since wheatear nests are depredated by ground-living animals our results demonstrate: (i) that edge effects depend on edge contrast, (ii) that edge-related nest predation patterns vary across the breeding period probably resulting from changes in parental activity at the nest between the incubation and nestling feeding stage. Edge effects should be put in the context of the nest predator community as illustrated by the elevated nest predation risk at hard but not soft habitat edges when an edge is defined in terms of ground vegetation. These results thus can potentially explain previously observed variations in edge-related nest predation risk.