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Recent advances in spring arrival dates have been reported in many migratory species but the mechanism driving these advances is unknown. As population declines are most widely reported in species that are not advancing migration, there is an urgent need to identify the mechanisms facilitating and constraining these advances. Individual plasticity in timing of migration in response to changing climatic conditions is commonly proposed to drive these advances but plasticity in individual migratory timings is rarely observed. For a shorebird population that has significantly advanced migration in recent decades, we show that individual arrival dates are highly consistent between years, but that the arrival dates of new recruits to the population are significantly earlier now than in previous years. Several mechanisms could drive advances in recruit arrival, none of which require individual plasticity or rapid evolution of migration timings. In particular, advances in nest-laying dates could result in advanced recruit arrival, if benefits of early hatching facilitate early subsequent spring migration. This mechanism could also explain why arrival dates of short-distance migrants, which generally return to breeding sites earlier and have greater scope for advance laying, are advancing more rapidly than long-distance migrants.

In migratory birds, early arrival on breeding sites is typically associated with greater breeding success, but the mechanisms driving these benefits are rarely known. One mechanism through which greater breeding success among early arrivers can potentially be achieved is the increased time available for replacement clutches following nest loss. However, the contribution of replacement clutches to breeding success will depend on seasonal variation in nest survival rates, and the consequences for juvenile recruitment of hatching at different times in the season. In particular, lower recruitment rates of late‐hatched chicks could offset the benefits to early arrivers of being able to lay replacement clutches, which would reduce the likelihood of replacement clutch opportunities influencing selection on migratory timings. Using a simulation model of time‐constrained capacity for replacement clutches, paramaterized with empirically‐derived estimates from avian migratory systems, we show that greater reproductive success among early‐arriving individuals can arise solely through the greater time capacity for replacement clutches among early arrivers, even when later renesting attempts contribute fewer recruits to the population. However, these relationships vary depending on the seasonal pattern of nest survival. The benefits of early arrival are greatest when nest survival rates are constant or decline seasonally, and early arrival is least beneficial when nest success rates increase over the breeding season, although replacement clutches can mitigate this effect. The time benefits of early arrival facilitating replacement clutches following nest loss may therefore be an important but overlooked source of selection on migratory timings. Empirical measures of seasonal variation in nest survival, renesting, and juvenile recruitment rates are therefore needed in order to identify the costs and benefits associated with individual migration phenology, the selection pressures influencing migratory timings, and the implications for ongoing shifts in migration and breeding phenology. We use simulation models, paramaterized with empirically derived estimates from avian migratory systems, to show that greater reproductive success among early‐arriving individuals can arise solely through the greater time capacity for replacement clutches among early arrivers. The benefits of early arrival are greatest when nest survival rates are constant or decline seasonally, and early arrival is least beneficial when nest success rates increase over the breeding season, although replacement clutches can mitigate this effect. Current advances in timing of migration and breeding in many species means that the relative contribution of replacement clutches to productivity and recruitment may also be increasing and contributing to population changes.

Roads are among the most widespread anthropogenic structures, and their presence can impact biodiversity in surrounding landscapes through disturbance and collision risk, particularly when traffic volumes are high. However, the impact of roads with low traffic volumes in open landscapes is much less clear. In the open landscapes of lowland Iceland, road traffic is still relatively low but increasing, and the surrounding landscapes support internationally important populations of several breeding wader species. Here, we used transect counts perpendicular to low‐traffic (≤ 15 000 vehicles day−1) roads across the lowlands of southern Iceland to quantify variation in the densities of ground‐nesting birds with distance from roads, and to assess how far from the roads any such effects extended. The total abundance of birds increased significantly by 6% per 50 m interval from roads, and densities within 200 m of roads were ~ 20% lower than densities between 200 and 400 m from roads. Four species – whimbrel Numenius phaeopus, golden plover Pluvialis apricaria, dunlin Calidris alpina and meadow pipit Anthus pratensis – were found in significantly lower densities closer to roads, while four – black‐tailed godwit Limosa limosa, redshank Tringa totanus, snipe Gallinago gallinago and redwing Turdus iliacus – showed no change with distance from roads. Redwing was found in higher densities, and dunlin in lower densities, surrounding roads with higher traffic volumes. As approximately 20% of lowland Iceland is within 200 m of roads, the impact of roads on the overall abundance of ground‐nesting birds could be substantial. The results show that even relatively low‐traffic roads can have a significant impact on adjacent wildlife populations. Road construction, along with other anthropogenic structures, has been shown to have negative effects on bird abundance, and identifying areas for protection from such developments may be the most effective approach to reducing human impacts on the internationally important wildlife of lowland Iceland.

Understanding how individual tradeoffs and carry‐over effects along the annual cycle influence fitness is fundamental to unravel population dynamics, but such data is particularly challenging to collect in long‐distance migrants. Here, with a full annual cycle perspective of Icelandic whimbrels Numenius phaeopus islandicus, we investigate tradeoffs across the entire distribution, assessing migration costs and wintering energetic balance experienced throughout the wintering range (from temperate to tropical regions), and link these to breeding parameters for two wintering regions. We found that Icelandic whimbrels traded off higher costs of migration with more favourable wintering conditions, in terms of energetic balance. By migrating further, whimbrels experience lower thermoregulatory costs and higher net energetic intake rates, resulting in a more positive energetic balance during winter. However, these differences did not appear to carry‐over into the breeding season in terms of measurable effects on laying date (and, consequently, fledging success) or egg volume, suggesting that individual fitness is unlikely to be significantly influenced by previous wintering conditions, although effects on other traits may potentially occur. Nevertheless, Icelandic whimbrels seem to favour wintering locations where the conditions are more advantageous, as the abundance of individuals at the wintering sites reflects the variation in site quality.

Shifts in phenology are widely reported across taxa and, among migratory birds, advancing timing of breeding has occurred predominantly in short‐distance migrants. Long‐distance migrants might be less able to advance breeding if they arrive later and breed soon after arrival, but opportunities to quantify trends in phenology across species that experience similar breeding conditions but vary in migration distances are rare. Between 2007 and 2022, we recorded arrival and laying dates across lowland Iceland for nine wader species that vary in migration distances. Waders wintering closer to Iceland arrived 6 weeks earlier than those wintering further away, yet laying dates differed by only 1–2 weeks. Over this survey period, short‐distance migrants advanced laying despite little or no advance in arrival, while long‐distance species advanced both arrival and laying dates. The longer arrival‐laying interval in species travelling shorter distances appears to allow earlier laying in warm springs, a flexibility less available to later‐arriving species. Due to the benefits of breeding early in migratory systems, the opportunity of early nesting in warming springs could be contributing to divergent population trajectories of short‐ and long‐distance migrants. Quantifying the phenology of nest and fledging success of species migrating over different distances will help to identify the costs of travelling further and arriving later during this period of rapid environmental change. Short‐distance migrants arrive earlier and are advancing laying at a faster rate than long‐distance species, particularly in warmer springs. Early arrival does not always result in earlier laying, but short‐distance migrants can exploit favourable conditions and lay before many long‐distance migrants arrive. Migration‐timing constraints may therefore limit access to early breeding opportunities and contribute disproportionately to population declines in long‐distance migrants.

Several expressions of sexual segregation have been described in animals, especially in those exhibiting conspicuous dimorphism. Outside the breeding season, segregation has been mostly attributed to size or age-mediated dominance or to trophic niche divergence. Regardless of the recognized implications for population dynamics, the ecological causes and consequences of sexual segregation are still poorly understood. We investigate the foraging habits of a shorebird showing reversed sexual dimorphism, the black-tailed godwit Limosa limosa, during the winter season, and found extensive segregation between sexes in spatial distribution, microhabitat use and dietary composition. Males and females exhibited high site-fidelity but differed in their distributions at estuary-scale. Male godwits (shorter-billed) foraged more frequently in exposed mudflats than in patches with higher water levels, and consumed more bivalves and gastropods and fewer polychaetes than females. Females tended to be more frequently involved and to win more aggressive interactions than males. However, the number of aggressions recorded was low, suggesting that sexual dominance plays a lesser role in segregation, although its importance cannot be ruled out. Dimorphism in the feeding apparatus has been used to explain sex differences in foraging ecology and behaviour of many avian species, but few studies confirmed that morphologic characteristics drive individual differences within each sex. We found a relationship between resource use and bill size when pooling data from males and females. However, this relationship did not hold for either sex separately, suggesting that differences in foraging habits of godwits are primarily a function of sex, rather than bill size. Hence, the exact mechanisms through which this segregation operates are still unknown. The recorded differences in spatial distribution and resource use might expose male and female to distinct threats, thus affecting population dynamics through differential mortality. Therefore, population models and effective conservation strategies should increasingly take sex-specific requirements into consideration.

Distinct preference of species for habitats is most often driven by long term differences in demographic rates between habitats. Estimating variation in those rates is key for developing successful conservation strategies. Stochastic events can interact with underlying variation in habitat quality in regulating demography but the opportunities to explore such interactions are rare. Whimbrels in Iceland show a strong preference for sparsely vegetated riverplains. Such habitats in Iceland face various threats, e.g., climate change, river regulation and spread of alien plant species. In this study we compared demographic parameters of breeding Whimbrels between riverplains and other habitats before, during and after volcanic eruption events to estimate the importance of the habitats for the species and the effect of ash deposit on breeding success. We found that an estimated minimum of 23% of the Icelandic population of Whimbrels and c. 10% of the world population of the species breed in riverplain habitats in Iceland. Whimbrels bred consistently at much higher densities in riverplain habitats than in other habitats and riverplains also had higher densities of pairs with fledglings although the proportion of successful breeders was similar between habitats. Predation by livestock may have had a considerable negative effect on breeding success on our study sites. Breeding was negatively affected by the volcanic activity, probably through the effects of ash on the invertebrate food supply, with breeding success being gradually worse closer to the eruption. Breeding success was equally affected by volcanism across habitats which differed in underlying habitat quality. This study gives an example of how populations can be regulated by factors which operate at different spatial scales, such as local variation in habitat quality and stochastic events which impact larger areas.

For many migratory bird species, the latitudinal range of the winter distribution spans thousands of kilometres, thus encompassing considerable variation in individual migration distances. Pressure to winter near breeding areas is thought to be a strong driver of the evolution of migration patterns, as individuals undertaking a shorter migration are generally considered to benefit from earlier arrival on the breeding grounds. However, the influence of migration distance on timing of arrival is difficult to quantify because of the large scales over which individuals must be tracked. Using a unique dataset of individually-marked Icelandic black-tailed godwits Limosa limosa islándica tracked throughout the migratory range by a network of hundreds of volunteer observers, we quantify the consequences of migrating different distances for the use of stop-over sites and timing of arrival in Iceland. Modelling of potential flight distances and tracking of individuals from across the winter range shows that individuals wintering further from the breeding grounds must undertake a stop-over during spring migration. However, despite travelling twice the distance and undertaking a stop-over, individuals wintering furthest from the breeding grounds are able to overtake their conspecifics on spring migration and arrive earlier in Iceland.Wintering further from the breeding grounds can therefore be advantageous in migratory species, even when this requires the use of stop-over sites which lengthen the migratory journey. As early arrival on breeding sites confers advantages for breeding success, the capacity of longer distance migrants to overtake conspecifics is likely to influence the fitness consequences of individual migration strategies. Variation in the quality of wintering and stopover sites throughout the range can therefore outweigh the benefits of wintering close to the breeding grounds, and may be a primary driver of the evolution of specific migration routes and patterns.

In migratory species, sexual size dimorphism can mean differing energetic requirements for males and females. Differences in the costs of migration and in the environmental conditions occurring throughout the range may therefore result in sex‐biases in distribution and resource use at different spatial scales. In order to identify the scale at which sexual segregation operates, and thus the scale at which environmental changes may have sex‐biased impacts, we use range‐wide tracking of individually color‐ringed Icelandic black‐tailed godwits (Limosa limosa islandica) to quantify sexual segregation at scales ranging from the occupation of sites throughout the non‐breeding range to within‐site differences in distribution and resource use. Throughout the range of this migratory shorebird, there is no evidence of large‐scale sex differences in distribution during the non‐breeding season. However, the sexes differ in their selection of prey types and sizes, which results in small‐scale sexual segregation within estuaries. The scale of sexual segregation therefore depends on the scale of variation in resource distribution, which, in this system, is primarily within estuaries. Sexual segregation in within‐site distribution and resource use means that local‐scale anthropogenic impacts on estuarine benthic prey communities may disproportionately affect the sexes in these migratory shorebirds. Migratory species are extremely mobile and often range over large geographic areas, potentially creating distinct distribution patterns for each sex, which may be linked to resource use, given sex‐specific energetic requirements. Icelandic black‐tailed godwits mostly segregate spatially at the local scale as a consequence of sex differences in prey selection. Sex differences in resource exploitation have implications for the capacity of males and females to meet their energetic requirements, and for sex differences in the response to changing environmental conditions. Photo by Graham Catley.

Intensified agricultural practices have driven biodiversity loss throughout the world, and although many actions aimed at halting and reversing these declines have been developed, their effectiveness depends greatly on the willingness of stakeholders to take part in conservation management. Knowledge of the willingness and capacity of landowners to engage with conservation can therefore be key to designing successful management strategies in agricultural land. In Iceland, agriculture is currently at a relatively low intensity but is very likely to expand in the near future. At the same time, Iceland supports internationally important breeding populations of many ground-nesting birds that could be seriously impacted by further expansion of agricultural activities. To understand the views of Icelandic farmers toward bird conservation, given the current potential for agricultural expansion, 62 farms across Iceland were visited and farmers were interviewed, using a structured questionnaire survey in which respondents indicated of a series of future actions. Most farmers intend to increase the area of cultivated land in the near future, and despite considering having rich birdlife on their land to be very important, most also report they are unlikely to specifically consider bird conservation in their management, even if financial compensation were available. However, as no agri-environment schemes are currently in place in Iceland, this concept is highly unfamiliar to Icelandic farmers. Nearly all respondents were unwilling, and thought it would be impossible, to delay harvest, but many were willing to consider sparing important patches of land and/or maintaining existing pools within fields (a key habitat feature for breeding waders). Farmers’ views on the importance of having rich birdlife on their land and their willingness to participate in bird conservation provide a potential platform for the codesign of conservation management with landowners before further substantial changes in the extent of agriculture take place in this subarctic landscape.