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Birds have been observed to have dietary preferences for unsaturated fatty acids during migration. Polyunsaturated fatty acids (PUFAs) may increase the exercise performance of migrant birds; however, PUFAs are also peroxidation prone and might therefore incur increased costs in terms of enhanced oxidative damage in migratory individuals. To shed light on this potential constraint, we analyzed plasma fatty acid (FA) composition and estimated the unsaturation index as a proxy for susceptibility to lipid peroxidation of migrants and residents of the partially migratory common blackbird (Turdus merula) at a stopover site during autumn migration. As predicted, migrant birds had higher relative and absolute levels of PUFAs compared to resident birds. This included the strictly dietary ω‐3 PUFA α‐linolenic acid, suggesting a dietary and/or storage preference for these FAs in migrants. Interestingly, the FA unsaturation index did not differ between migrants and residents. These findings suggest a mechanism where birds alter their levels of metabolic substrate without simultaneously increasing the susceptibility of the substrate to lipid peroxidation. In summary, our results are in line with the hypothesis that increased exercise performance during migration might be constrained by oxidative stress, which is manifested in changes in the composition of key FAs to retain the unsaturation index constant despite the increased levels of peroxidizable PUFAs., Migrating blackbirds, Turdus merula, have higher plasma levels of exercise‐enhancing polyunsaturated fatty acids compared to resident conspecifics at the same stopover site. High levels of such unsaturated fatty acids may incur costs in terms of increased lipid peroxidation, and hence oxidative stress. Yet, the fatty acid unsaturation index did not differ between migrants and residents, suggesting physiological mechanisms in migrants to mitigate this potential cost.

In many animals, catabolic and anabolic periods are temporally separated. Migratory birds alternate energy expenditure during flight with energy accumulation during stopover. The size of the energy stores at stopover affects the decision to resume migration and thus the temporal organization of migration. We now provide data suggesting that it is not only the size of the energy stores per se that may influence migration scheduling, but also the physiological consequences of flying. In two subspecies of the northern wheatear Oenanthe oenanthe, a long-distance migrant, estimated energy stores at a stopover during autumn migration were positively related with both constitutive innate and acquired immune function, and negatively related with oxidative damage to lipids. In other words, migrants’ physiological condition was associated with their energetic condition. Although time spent at stopover before sampling may have contributed to this relationship, our results suggest that migrants have to trade-off the depletion of energy stores during flight with incurring physiological costs. This will affect migrants’ decisions when to start and when to terminate a migratory flight. The physiological costs associated with the depletion of energy stores may also help explaining why migrants often arrive at and depart from stopover sites with larger energy stores than expected. We propose that studies on the role of energy stores as drivers of the temporal organization of (avian) migration need to consider physiological condition, such as immunological and oxidative states.

Most migratory songbirds travel between their breeding areas and wintering grounds through a series of nocturnal flights. The timing of their departures defines the potential flight duration and thus the distance covered during a migratory night. Yet, migratory songbirds show substantial variation in their nocturnal departure timing. With this study, we aim to assess whether the respective challenges of the migration route, namely its distance and nature, help to explain this variation. At a stopover site, we caught Northern Wheatears (Oenanthe oenanthe) of two subspecies that differ in distance and nature of their onward migration route in spring, but not in autumn. We determined the start of their nocturnal migratory restlessness during short-term captivity, and radiotracked their nocturnal departure timing after release in both migration seasons. Northern Wheatears started their nocturnal migratory restlessness earlier when facing a long remaining migration distance and an extended sea barrier in spring. Individual departure directions generally affected the nocturnal departure timing with early departures being directed towards the respective migratory destination. In spring, this pattern was predominantly found in birds carrying relatively large fuel stores, but was absent in lean birds. At the same time, birds facing a short remaining migration distance and no extended sea barrier strongly reacted to relatively large fuel stores by an early start of nocturnal migratory behaviour (migratory restlessness and departure timing), whereas this reaction was not found in birds facing a long remaining migration distance and sea barrier. These results suggest that the basic diel schedule of birds' migratory activity is adapted to the onward migration route. Further, they suggest that birds adjust their behavioural response, that is start of nocturnal migratory behaviour, to fuel stores in relation to their impending migratory challenges. This is a substantial step in understanding variation of nocturnal departure timing and its adjustments in migratory songbirds. Further, it emphasizes the importance of interpreting birds' nocturnal migratory behaviour in the respective ecological context.

Migration is energetically demanding and physiologically challenging. Migrating birds, for example, need to boost their antioxidant defenses to defeat the pro‐oxidants produced during high energetic activity. The enhanced antioxidant defense possibly withdraws limited resources (e.g., energy or micronutrients) from other physiological functions, such as immune defense. Such a trade‐off might not occur outside the migration seasons or in resident individuals. Here, we investigate whether there is a negative relationship between innate immune function and antioxidant defense by sampling both migrating and resident blackbirds (Turdus merula) at the same location during the same period of the annual cycle. We show that in migrating blackbirds microbial killing capacity (BKA), an integrative measure of baseline innate immune function was negatively correlated with total nonenzymatic antioxidant capacity. In contrast, in resident conspecifics, sampled at the same time and location, these two physiological measures were not correlated. This suggests that migrating birds trade off innate immune function and antioxidant defense. Furthermore, and likely a consequence of this trade‐off, in migrant blackbirds BKA was positively correlated with oxidative damage to lipids. In resident blackbirds BKA and degree of lipid oxidation were uncorrelated. The mechanism and currencies of the supposed trade‐off are currently unknown, but energetic investments or micronutrients are likely candidates. Future experimental studies could provide more conclusive evidence for this trade‐off; yet, our results open up a new level of thinking about the physiological costs of migration.

For migrating birds, late arrival at the breeding grounds reduces reproductive success. Hence, while en route, late migrants are expected to accelerate migration. Most time can be saved by shortening stopovers, periods of rest and fuel replenishment (refuelling) in between flight bouts. In several field studies late migrants have indeed been observed to shorten stopovers. The length of stopovers is assumed to depend, at least partly, on refuelling rate (fuel deposition rate, FDR). Therefore, shorter stopovers later in the spring migration season may be explained by a seasonal increase in FDR. We tested this idea by measuring both migratory restlessness (an accurate proxy for departure likelihood from stopover) and FDR in the same set of temporarily caged wheatears, Oenanthe oenanthe, a long-distance songbird migrant, throughout spring migration. We found that the amount of migratory restlessness increased with the progress of the season, indicating higher departure likelihood in late migrants. Clearly, however, late migrants in our sample did not accelerate migration by increasing their FDR; daily FDR was relatively constant during the season in lean birds, and even decreased over the season in fat birds. There was little seasonal variation in daily food intake in both lean and fat birds. Because we eliminated and/or corrected for environmental factors influencing departure likelihood, these results strongly indicate that an endogenous time programme is involved in the regulation of departure from stopover in spring.

Corticosterone at baseline levels is thought to be mainly involved in the regulation of uptake, storage and release of energy, processes central to avian migration. Consequently, corticosterone levels are thought to be upregulated during migration, but the temporal pattern of its secretion during migration is not well defined. For example, although it appears that corticosterone levels decrease from flight to stopover, it is unknown if levels at stopover are still elevated and it is largely unclear how these levels compare to non-migratory life-history stages. Furthermore, what role corticosterone plays in crucial migratory processes, such as refueling and departure from stopover, is far from understood. We here determined baseline corticosterone levels in migrating and resident common blackbirds ( Turdus merula ), sampled simultaneously on Helgoland, a stopover site that also supports a sedentary breeding population. In autumn, migrants had higher corticosterone levels than residents, but in spring levels did not differ between the two groups. Corticosterone levels of migrants were very similar in spring and autumn, whereas in residents levels tended to be higher in spring than autumn. Higher levels in residents in spring than autumn most likely reflect the higher daily workload faced by birds during the pre-breeding than the post-breeding period. Our study thus indicates that, relative to the levels observed in residents in autumn, in spring baseline corticosterone levels were moderately elevated in both migrants and residents and that in autumn levels were moderately elevated in migrants only. Currently, corticosterone’s main function at stopover is thought to lie in the regulation of departure. Because most migrant blackbirds stay only one or two days on Helgoland, our results are in line with this idea and suggest that migrating blackbirds up-regulated their corticosterone level in anticipation of an oncoming flight bout.

In many migratory birds, males precede females during migration and arrival at the breeding sites. Three proximate mechanisms are proposed to explain this phenomenon of protandry: males 1) winter closer to breeding sites, 2) start spring migration earlier, and/or 3) migrate faster than females. So far, the relative contribution of these mechanisms to protandry is unknown. The present study investigated the importance of each of the 3 proximate mechanisms of protandry for a songbird migrant wintering in Africa, the northern wheatear (Oenanthe oenanthe). Two subspecies co-occur in Europe on migration, of which the leucorhoa northern wheatears breeding from Iceland to Canada have to cross the North Atlantic, whereas the nominate form breeding in Europe does not face any significant sea barrier. We show that the leucorhoa subspecies had a significantly higher degree of protandry at stopover sites across Europe than the oenanthe subspecies (−6 vs. −2 days). Leucorhoa northern wheatear’s higher degree of protandry was associated with a larger age effect, in which old males preceded young males, and greater sex-specific differences in wing shape and refueling yielding higher migration speeds in males than females. In oenanthe northern wheatears, light-level geolocators revealed that males did not winter closer to the breeding sites or migrate faster than females, but initiated spring migration earlier. Our results demonstrate that the significance of the mechanisms causing protandry can differ between related taxa and highlight the importance of the advancement in male arrival date with age as a potential factor shaping the degree of protandry.

A widespread phenomenon in migrant birds is that they travel faster in spring than in autumn. During migration birds spend most time at stopover sites and, correspondingly, the faster spring migration is mainly explained by shorter stopovers in spring than autumn. Because a main purpose of stopovers is to replenish the fuel used in flight, a higher rate of fuel deposition (FDR) in spring is thought to explain the shorter stopovers and hence shorter total duration of migration in spring. Critical migratory processes, including the onset and extent of pre-migratory fueling, are endogenously regulated. It is therefore not unlikely that refueling at stopover sites is, at least partly, also under endogenous control. We here tested whether there is an endogenous seasonal difference in food intake and FDR, which could contribute to shorter stopovers and hence faster migration in spring. We measured daily food intake and daily FDR in two subspecies of the northern wheatear Oenanthe oenanthe, temporarily confined at stopover under identical constant indoor conditions in spring and autumn. The two wheatear subspecies differed markedly in absolute food intake and FDR. Within subspecies, however, food intake and FDR did not differ between spring and autumn, indicating that faster spring migration in northern wheatears is not explained by an endogenously controlled seasonal difference in birds’ motivation to refuel. To further substantiate this claim, similar measurements should be taken at other locations along northern wheatears’ migration routes. Comparable experiments in other species could test the generality of our results.

Numerous studies in both the field and laboratory illustrate factors involved in the endogenous, intrinsic and environmental control of avian migration, but we are lacking an integrated individual-based approach, connecting field and laboratory studies in a single species. The Northern Wheatear Oenanthe oenanthe appeared very feasible for an integrated individual-based one-species approach in the study of the control of bird migration as it can be studied individually in the field as well as kept indoor under controlled ‘common-garden’ conditions. We here present a brief overview of the results collected in our wheatear studies on the intrinsic disposition and extrinsic factors that control migratory behavior in an obligate migratory species.

A variety of methods have been used to study the relationship between wind conditions and departure decisions of migrant birds at stopover sites. These methods are either costly or suffer from inaccuracy in determining whether or not an individual has resumed migration. Here we present a novel and simple approach to studying the relationship between wind conditions and departure likelihood. Northern Wheatears Oenanthe oenanthe caught during stopover were temporarily caged to measure their nocturnal migratory restlessness, which is an accurate proxy for their individual departure likelihood. We then related the degree of nocturnal restlessness to wind conditions prevailing at the time of capture. Confirming the general pattern from previous studies of departure, the intensity of nocturnal migratory restlessness, and hence departure likelihood, increased with increasing wind support towards the migratory goal. This suggests that approximating the propensity to depart by measuring nocturnal migratory restlessness is a reliable way to study the effect that wind conditions experienced during stopover has on the departure decision of migrants. Our study also shows that nocturnal migrants possess the ability to use information gathered during the day for their departure decisions at night. Because measuring migratory restlessness is straightforward and inexpensive, our approach is ideally suited to test hypotheses regarding spatio-temporal variation in wind selectivity in migrating birds.

Migrating birds spend most of their time at stopover sites where they replenish the fuel used during flight, termed refueling. The overall time of migration thus largely depends on the duration of stopovers, and factors shaping stopover duration therefore are of interest. A handful of field studies have shown that the likelihood of departure from stopover sites increases with poor feeding conditions. However, food availability and stopover duration are generally difficult to quantify accurately in the field. Results of fasting-refueling experiments on captive birds using migratory restlessness (Zugunruhe) as a proxy for departure likelihood are mixed. Although Zugunruhe usually decreased with refueling, fasting often failed to increase Zugunruhe. In addition, some experiments lacked randomization. In a fasting-refueling experiment on Northern Wheatears (Oenanthe oenanthe), using birds as their own control in a randomized fashion, we found that fasting increased Zugunruhe, whereas refueling decreased Zugunruhe. These results show that the motivation to migrate, quantified by Zugunruhe, is affected by changes in food availability. Furthermore, Zugunruhe during refueling did not depend on fuel reserves left after fasting, but tended to decrease with the amount of fuel lost during fasting. We discuss why extent of fuel loss may be a better predictor of stopover duration than fuel reserves.

Migratory birds rely on fueling prior to migratory flights. Fueling in migrants is controlled by intrinsic as well as extrinsic factors. From captive studies we have started understanding the internal mechanisms controlling bird migration. Field studies have demonstrated the effects of external factors, such as food availability, weather, competitors, parasites or diseases, on the stopover behavior of migrants. However, an integrated approach is still missing to study coherently how the innate migration program interacts with the varying environmental cues and to estimate the contribution of the innate migration program and the environment to realized migration. The northern wheatear Oenanthe oenanthe offers a unique opportunity for integrated studies. It breeds across almost the whole Holarctic with just a “gap” between eastern Canada and Alaska. All breeding populations overwinter in sub-Saharan Africa which makes the northern wheatear one of the most long-distant migratory songbirds with extraordinary long non-stop flights across oceans. It is a nocturnal migrant which travels without parental or social aid/guidance. Thus, young birds rely entirely on endogenous mechanisms of timing, route selection and fueling on their first outbound migration. By establishing indoor housing under controlled conditions the endogenous control mechanisms of northern wheatear migration could be revealed. At the same time, environmental factors controlling fueling could be investigated in the field. On migration wheatears occur in a variety of habitats with sparse vegetation where their stopover behavior could be quantitatively studied in the light of “optimal migration” theory by the use of remote balances, radio-tagging and even experimentally manipulated food availability. The present paper summarizes our approach to understand the control of migration in northern wheatears by combining field and laboratory studies at various spatial and temporal scales, and linking various sub-disciplines.

Corticosterone, at baseline and moderately elevated levels, is thought to regulate energy mobilization during the predictable life-history cycle. In birds, corticosterone is known to be moderately elevated during migration, and some experiments on captive, but migratory active birds have shown that exogenous corticosterone can positively affect food intake and fat deposition, i.e. fueling. We present observations which indicate that in wild birds endogenous corticosterone does not promote refueling during migratory stopovers. We took a comparative approach and studied two subspecies of Northern wheatears (Oenanthe oenanthe) during their simultaneous spring stopovers on Helgoland, a small island some 50km off the German coast. In spring O. oenanthe have to travel relatively short distances from Helgoland to their next stopover or breeding sites, whereas Oenanthe leucorhoa face a lengthy overseas journey. Consequently, for their next flight bout leucorhoa wheatears deposit more fuel, more rapidly than oenanthe wheatears. Corticosterone levels, however, were lower in leucorhoa than oenanthe wheatears, contradicting the idea that corticosterone promotes migratory refueling. This finding was solidified by the observation that actual fuel deposition rate was negatively correlated with corticosterone level. We also observed a positive correlation between corticosterone level and fuel stores. Together these findings suggest that, rather than promoting migratory refueling, corticosterone may function as a readiness cue, with levels increasing towards departure from the stopover site.

In birds, accumulating energy is far slower than spending energy during flight. During migration, birds spend, therefore, most of the time at stopover refueling energy used during the previous flight. This elucidates why current energy stores and actual rate of accumulating energy are likely crucial factors influencing bird's decision when to resume migration in addition to other intrinsic (sex, age) and extrinsic (predation, weather) factors modulating the decision within the innate migration program. After first summarizing how energy stores and stopover durations are generally determined, we critically review that high-energy stores and low rates of accumulating energy were significantly related to high departure probabilities in several bird groups. There are, however, also many studies showing no effect at all. Recent radio-tracking studies highlighted that migrants leave a site either to resume migration or to search for a better stopover location, so-called "landscape movements". Erroneously treating such movements as departures increases the likelihood of type II errors which might mistakenly suggest no effect of either trait on departure. Furthermore, we propose that energy loss during the previous migratory flight in relation to bird's current energy stores and migration strategy significantly affects its urge to refuel and hence its departure decision.

Bird migration entails replenishing fuel stores at stopover sites. There, individuals make daily decisions whether to resume migration, and must also decide their time of departure. Variation in departure timing affects the total time required to complete a migratory journey, which in turn affects fitness through arrival time at the breeding and wintering grounds. It is well established that stopover departure decisions are based on cues from innate rhythms, intrinsic factors and extrinsic factors. Yet, virtually nothing is known about the physiological mechanism(s) linking these cues to departure decisions. Here, we show for a nocturnal migratory songbird, the northern wheatear ( Oenanthe oenanthe ), that baseline corticosterone levels of birds at stopover increased both over the migratory season and with wind assistance towards the migratory destination. Corticosterone in turn predicted departure probability; individuals with high baseline corticosterone levels were more likely to resume migration on a given night. Corticosterone further predicted the departure time within the night, with high baseline levels being associated with early departures. These novel findings indicate that corticosterone may be mediating between departure cues and the timing of departure from a stopover site, which is a major step towards understanding the hormonal control of animal migration.

Animals need a well-functioning immune system to protect themselves against pathogens. The immune system, however, is costly and resource trade-offs with other demands exist. For migratory animals several (not mutually exclusive) hypotheses exist. First, migrants reduce immune function to be able to allocate resources to migration. Second, migrants boost immune function to cope with more and/or novel pathogens encountered during migration. Third, migrants reallocate resources within the immune system. We tested these hypotheses by comparing baseline immune function in resident and migratory common blackbirds ( Turdus merula ), both caught during the autumn migration season on the island of Helgoland, Germany. Indices of baseline innate immune function (microbial killing capacity and haptoglobin-like activity) were lower in migrants than in residents. There was no difference between the groups in total immunoglobulins, a measure of baseline acquired immune function. Our study on a short-distance avian migrant supports the hypothesis that innate immune function is compromised during migration.

Knowing why certain individuals refrain from reproduction is fundamental to our understanding of the evolution of cooperative breeding. Although many studies have focused on the role of life history and ecological factors, the proximate mechanisms underlying this behaviour have received considerably less attention. We determined the natural variation in plasma testosterone (T) level and cloacal protuberance (CP) size - an indication of sperm storage - in successfully reproducing primary male and in reproductively inactive subordinate male Seychelles warblers (Acrocephalus sechellensis). Primary males had significantly higher T levels during female's fertile period and significantly larger CPs throughout the breeding season than subordinate males. To test if the lower T levels of subordinates were caused by their social status per se, we experimentally promoted subordinates to primary males, by permanent removal of primary males from their territories. The experimentally promoted males increased T production and CP sizes significantly, suggesting former suppression by primary males. However, T levels of the promoted males only marginally increased. As this could not be explained by their younger age, we conclude that recovery from social suppression may take a substantial time period.

In many species, territory ownership is a prerequisite for reproduction; consequently, factors that affect success in territory acquisition can have a large impact on fitness. When competing for territories, some individuals may have an advantage if, for example, they are phenotypically superior or more familiar with the site than others. The relative importance of the many factors involved in territory acquisition is, at present, unclear. We studied patterns of natural territory acquisition in a closed and saturated population of Seychelles warblers. Furthermore, by removing breeders, we experimentally investigated the relative importance, to territory acquisition, of a range of factors and assessed whether this differed between the sexes. In both sexes, the main route to natural territory acquisition was to disperse from the natal territory to immediately claim a vacant dominant position. Males were older than females when acquiring a territory for the first time. In the removal experiment, for both sexes, the proximity of an individual's natal territory to a vacant dominant position was positively related to the individual's chance of claiming the vacancy. Older males were more likely to gain an experimental vacant dominant position than were young males, whereas age did not affect territory acquisition in females. In the Seychelles warbler, the degree of intrasexual competition for territory ownership may be stronger for males than for females because territory ownership is a prerequisite for male reproduction, whereas females can reproduce on their natal territory. In such competition, young males subsequently lose out to older ones. Copyright 2009, Oxford University Press.

The presence of parents in the natal territory may play an important, but often overlooked, role in natal dispersal and the consequent acquisition of a territory. Living with parents in a territory may confer a fitness advantage to subordinates through, for example, the nepotistic behavior of the parents or indirect benefits gained by helping to raise nondescendent kin. When a parent is replaced by a stepparent, such advantages are reduced or disappear and, as a result, subordinates may disperse. Subordinates that disperse after parent replacement may be constrained in their timing of dispersal, which could have negative fitness consequences. In the cooperatively breeding Seychelles warbler, we show that when a parent was naturally replaced or experimentally removed and subsequently replaced by a stepparent from outside the territory, subordinates were more likely to disperse than when both parents remained in the natal territory. Furthermore, subordinates dispersing from territories in which one or both parents had been replaced were less likely to acquire a breeder position than subordinates dispersing when both parents were still on the natal territory. Our findings suggest that the presence of parents in the natal territory may promote delayed dispersal and facilitate the eventual acquisition of a breeder position outside the natal territory. Our results support the idea that the prolonged parental care, which long-lived species are able to provide, may have selected for family living. Copyright 2007, Oxford University Press.

1. Variation in survival, a major determinant of fitness, may be caused by individual or environmental characteristics. Furthermore, interactions between individuals may influence survival through the negative feedback effects of density dependence. Compared to species in temperate regions, we have little knowledge about population processes and variation in fitness in tropical bird species.2. To investigate whether variation in survival could be explained by population size or climatic variables we used capture-recapture models in conjunction with a long-term data set from an island population of the territorial, cooperatively breeding Seychelles warbler (Acrocephalus sechellensis). The lack of migration out of the study population means that our results are not confounded by dispersal.3. Annual survival was high, both for adults (84%) and juveniles (61%), and did not differ between the sexes. Although there was significant variation in survival between years, this variation could not be explained by overall population size or weather variables.4. For territorial species, resource competition will work mainly on a local scale. The size of a territory and number of individuals living in it will therefore be a more appropriate measure of density than overall population density. Consequently, both an index of territory quality per individual (food availability) and local density, measured as group size, were included as individual covariates in our analyses.5. Local density had a negative effect on survival; birds living in larger groups had lower survival probabilities than those living in small groups. Food availability did not affect survival.6. Our study shows that, in a territorial species, although density-dependent effects might not be detectable at the population level they can be detected at the individual territory level - the scale at which individuals compete. These results will help to provide a better understanding of the small-scale processes involved in the dynamics of a population in general, but in particular in tropical species living in relatively stable environments.

Th e latitudinal increase in extra-pair paternity (EPP) rates in birds suggests broad selective benefi ts to low EPP rates in the tropics. However, we have few EPP data from tropical birds, particularly from species with close relatives at high latitudes. Here, we report EPP rates in two resident equatorial populations of rufous-collared sparrow Zonotrichia capensis, a genus well-represented at high latitudes. We found 64% and 60% of broods contained extra-pair off spring, and 42% and 52% of all young were extra-pair. EPP rates were similar in these populations, despite clear diff erences in elevation, temperature, rainfall, and breeding season length. Th ese fi ndings provide evidence that EPP rates in tropical birds can be as high as those observed in temperate birds, and suggest that the selective pressures acting on EPP rates vary markedly across tropical birds.

In captivity, migratory birds show increased activity during the time that they would normally migrate. The phenology and intensity of such ‘migratory restlessness’ has been shown to mirror species- and population-specific migration patterns observed in the wild and has consequently been used as a proxy for the motivation to migrate. Many studies doing so, however, were aiming to explain among-individual variation in migratory behaviour or traits, and not species- or population-specific traits. These studies thus assumed that, also at the level of the individual, migratory restlessness is an accurate proxy for the motivation to migrate. We tested this assumption for the first time and found that it holds; individuals showing very little migratory restlessness remained at stopover for longer than one night, whereas most individuals showing more restlessness departed sooner. This finding validates the use of migratory restlessness as a proxy for the motivation to migrate, thereby justifying the conclusions made in a large body of research on avian migration.

The decision made by migrating birds to stop refueling and to depart from stopover depends on cues from innate rhythms, intrinsic factors such as fuel reserves, and extrinsic factors such as weather conditions. The physiological mechanism behind this decision, however, is largely unexplored. The transition from refueling to flight involves an increase in both locomotion and energetic demands. Because, at baseline levels, corticosterone stimulates locomotion and is involved in the mobilization of energy, this hormone could encourage departure of migrants. We collected field data on baseline corticosterone, migratory restlessness, and actual departure in northern wheatears at stopover. Additionally, in refueling long-term captive conspecifics, we measured migratory restlessness while simultaneously collecting droppings to determine glucocorticoid metabolite (GCM) levels. We found that migratory restlessness at stopover was positively correlated with corticosterone level. Similarly, in refueling long-term captive birds, migratory restlessness was positively correlated with concurrently measured GCM levels in droppings. To our best knowledge, our study is the first to simultaneously measure a hormonal signal and migratory restlessness. In conclusion, our results are consistent with the hypothesis that, by increasing locomotor activity, baseline corticosterone is involved in the regulation of departure of migrants at stopover. Future studies could reveal how corticosterone is up-regulated in migrants that are ready to depart.

Mating systems can vary among species and populations and thus influence evolutionary trajectories, ecological traits, and population demography. The siring of offspring by an extrapair male, or extrapair paternity (EPP), is a widespread and varied phenomenon in all vertebrate classes. However, we do not understand all of the factors associated with variation in EPP rates. The breeding synchrony hypothesis suggests that EPP rates should increase with latitude and elevation, whereas the paternal care hypothesis predicts that EPP rates should decrease with elevation. To address these hypotheses, we investigated how population EPP rates vary over elevation and latitude in emberizid sparrows. In comparative analyses including the effects of phylogeny, the relationship between EPP rates and elevation depended on latitude. EPP rates were greater in higher-latitude populations. But within higher-latitude populations, EPP rates decreased with increasing elevation. These findings provide support for both the breeding synchrony and paternal care hypotheses, suggesting that in lower-latitude, higher-elevation populations, the need for male parental care does not outweigh the benefits of seeking extrapair fertilizations in populations with relatively synchronous breeding. In contrast, at higher-latitude, higher-elevation sites, the need for male parental care is greater and might drive lower rates of extrapair mating despite highly synchronous breeding.

In most monogamous bird species, circulating testosterone concentration in males is elevated around the social female's fertile period. Variation in elevated testosterone concentrations among males may have a considerable impact on fitness. For example, testosterone implants enhance behaviours important for social and extra-pair mate choice. However, little is known about the relationship between natural male testosterone concentration and sexual selection. To investigate this relationship we measured testosterone concentration and sexual signals (ventral plumage colour and tail length), and determined within and extra-pair fertilization success in male North American barn swallows (Hirundo rustica erythrogaster). Dark rusty coloured males had higher testosterone concentrations than drab males. Extra-pair paternity was common (42% and 31% of young in 2009 and 2010, respectively), but neither within- nor extra-pair fertilization success was related to male testosterone concentration. Dark rusty males were less often cuckolded, but did not have higher extra-pair or total fertilization success than drab males. Tail length did not affect within- or extra-pair fertilization success. Our findings suggest that, in North American barn swallows, male testosterone concentration does not play a significant direct role in female mate choice and sexual selection. Possibly plumage colour co-varies with a male behavioural trait, such as aggressiveness, that reduces the chance of cuckoldry. This could also explain why dark males have higher testosterone concentrations than drab males. Published version

We observed a change in the sex-specific rate of delayed natal dispersal in a stable population of Seychelles warblers over a period of 20 years. At first, females were more likely to delay dispersal in their first year of life than were males, whereas later there was no sex bias in the rate of delayed natal dispersal. Similarly, the female-bias in helping-at-the-nest and the male-bias in floating have also weakened over time. These changes may have resulted from the decrease in variation in territory quality observed in the population over the study period. Our findings strengthen the view that natal dispersal is a highly plastic response to local ecological and social circumstances, and clearly show that rates of sex-biased dispersal cannot be considered a species or population constant. Our study also highlights the importance of collecting long-term datasets to understand complex behaviour such as natal dispersal.

Cooperative breeding – in which some adults forgo independent breeding and remain as subordinates within a group helping to raise the offspring of others – occurs in between 3% and 10% of vertebrates. The structure of such systems varies greatly, from pairs with helpers-at-the-nest to communal breeders, andmayinclude young helpers or post-reproductive ‘grandparents’. That some individuals spend part, or all, of their lives helping others to reproduce contradicts the concept of ‘selfish’ natural selection and provides an intriguing evolutionary paradox. When and why such apparently altruistic behaviour occurs has, therefore, been the focus of much study. Although constraints and the benefits of group living, may persuade individuals to remain as subordinates, indirect ‘kin’ benefits or direct benefits (such as becoming a breeding helper) appear to favour the subsequent evolution of helping. Cooperative behaviour can have wide-ranging consequences, not only on the biology of the cooperative species, but also on those species it interacts with.

In this study, we test whether patterns of territory inheritance, social mate choice and female-biased natal dispersal act as inbreeding avoidance mechanisms in the cooperatively breeding Seychelles warbler. Our results show that Seychelles warblers do not reduce the likelihood of inbreeding by avoiding related individuals as mates. The occurrence of natural and experimentally induced territory inheritance did not depend on whether the remaining breeder was a parent of the potential inheritor or an unrelated breeder. Furthermore, dispersing individuals were no less related to their eventual mates than expected given the pool of candidates they could mate with. The female bias in natal dispersal distance observed in the Seychelles warbler does not facilitate inbreeding avoidance because, contrary to our prediction, there was no sex difference in the clustering of related opposite sex breeders around the natal territories of dispersers. As a result, the chance of females mating with relatives was not reduced by their greater dispersal distance compared with that of males.

Early arrival at the breeding site positively affects the breeding success of migratory birds. During migration, birds spend most of their time at stopovers. Therefore, determining which factors shape stopover duration is essential to our understanding of avian migration. Because the main purpose of stopover is to accumulate fat as fuel for the next flight bout, fuel reserves at arrival and the accumulation of fuel are both expected to affect stopover departure decisions. Here, we determined whether fuel reserves and fuel accumulation predict a bird's motivation to depart, as quantified by nocturnal migratory restlessness (Zugunruhe), using northern wheatears ( Oenanthe oenanthe ) that were captured and temporarily contained at spring stopover. We found that fuel reserves at capture were positively correlated with Zugunruhe, and negatively correlated with fuel accumulation. This indicates that fat birds were motivated to depart, whereas lean birds were set on staying and accumulating fuel. Moreover, the change in fuel reserves was positively correlated with the concurrent change in Zugunruhe, providing the first empirical evidence for a direct link between fuel accumulation and Zugunruhe during stopover. Our study indicates that, together with innate rhythms and weather, the size and accumulation of fuel reserves shape stopover duration, and hence overall migration time.

The distances that individuals disperse, from their natal site to the site of first breeding and between breeding sites, have important consequences for the dynamics and genetic structure of a population. Nearly all previous studies on dispersal have the problem that, because the study area encompassed only a part of the population, emigration may have been confounded with mortality. As a result long-distance dispersers may have been overlooked and dispersal data biased towards short distances. By studying a virtually closed population of Seychelles warblers Acrocephalus sechellensis we obtained almost unbiased results on several aspects of dispersal. As in the majority of other avian species, natal dispersal distance was female biased in the Seychelles warbler. Female offspring also forayed further from the natal territory in search of breeding vacancies than male offspring. The sex bias in natal dispersal distance did, however, depend on local breeding density. In males, dispersal distance decreased as the number of territories bordering the natal territory increased, while in females, dispersal distance did not vary with local density. Dispersal by breeders was rare and, unlike in most species, distances did not differ between the sexes. We argue that our results favour the idea that the sex bias in natal dispersal distance in the Seychelles warbler is due to inbreeding avoidance and not resource competition or intrasexual competition for mates. The distances between natal sites and the sites of first breeding (natal dispersal distance), and between breeding sites (breeding dispersal distance), have important conse