White-throated sparrows adjust behaviour in response to manipulations of barometric pressure and temperature

Brooding duration does not depend on cat predation risk but is related to weather and phenology in the wandering albatross (Diomedea exulans)

Parental investment increases offspring fitness at the expense of the parent's ability to invest in other offspring. In many animal species, parents guard their offspring after birth. The parental decision over the duration of this period is expected to be triggered by the associated fitness costs and benefits for both offspring and parents. Here, we evaluated the relevance of several intrinsic and environmental variables in determining brooding period duration in the wandering albatross (Diomedea exulans) and questioned whether brooding duration was related to chick subsequent survival and biometry prior to fledging. We used a semi-experimental design to increase the variance in cat abundance, a recent predator of albatross chicks, and predicted that an increased predation risk at the nest scale would trigger longer chick brooding and thus, protection. In addition, we questioned the influence of weather conditions, hatching date, and characteristics of chicks (sex and biometry) and parents (sex and age) on brooding duration. We report no effect of predation risk or parental characteristics on brooding duration. However, the probability for a parent to end brooding decreased with forthcoming unfavorable weather. Our data also revealed reduced brooding duration for late-hatched chicks and a positive association between brooding duration and chick structural size, and between the frequency of shifts between parents and chick structural size. Finally, brooding duration was not associated with chick survival or with chick biometry prior to fledging. We discuss these results in light of pre-existing hypotheses on fitness costs and benefits associated with brooding duration for chicks and parents.

L-band radar quantifies major disturbance of birds by fireworks in an urban area

Fireworks and other pyrotechnics are acknowledged as sources of disturbance to wildlife, with evidence that many species react adversely to their sight and sound at discharge. However, how firework releases impact wildlife within a city landscape is poorly understood. Here, we explore the effect of fireworks on urban birds using an L-band staring radar (90-degree sector out to a 5 km range) to capture bird activity derived from flight tracks (i.e. 3D visualisation of individual flying birds built from radar detections) within the city of Birmingham, UK. Comparing the tracks between baseline periods with no fireworks and periods where fireworks are commonly discharged using a null model indicated that birds flew at higher elevations during firework periods (standardised effect sizes of 17.11, 26.54 and 5.83, for Diwali, Bonfire Night, and New Year's Eve, respectively). Birds also flew in more significant numbers (standardised effect sizes of 23.41, 7.98 and 7.19 for Diwali, Bonfire Night, and New Year's Eve, respectively). Therefore, bird activity was elevated during firework events at a time of night when many would otherwise be roosting. Such disturbance may have implications for avian biology since large public firework events occur at colder times of the year in the UK when birds have elevated thermoregulatory costs.

The urge to breed early: Similar responses to environmental conditions in short- and long-distance migrants during spring migration

Birds migrating different distances experience different temporal, energetic, physiological, and physical constraints throughout migration, which is reflected in their migration strategy. Consequently, we predict different behavioral decisions to similar environmental cues between short- and long-distance migrants, which has been documented for autumn migration. Here, we focus on the question whether trade-off decisions regarding departure, routing, and landing when alternating between migratory endurance flights and stopovers also differ during spring migration. As early arrivals at the breeding grounds should be ultimately favored regardless of migration distance, selection may favor more similar behavioral decisions in spring than in autumn. We radio-tagged short- and long-distance migratory songbirds at stopover sites along the German North Sea coast during spring and automatically tracked their migratory behavior using a large-scale network of receiver stations. Once departed, birds could either cross the sea or detour along the coast. We corrected for spatially biased detection data, using a hierarchical multistate model to assess how birds respond to variation in environmental conditions in their day-to-day departure decisions and route selection. The day-to-day departure probability was higher in long-distance migrants independently of the routing decision. Irrespective of migration distance, all species more likely departed under light winds and rainless conditions, while the influence of air pressure change and relative humidity was species-specific. By accounting for detection probabilities, we estimated that about half of all individuals of each species crossed the sea but did not find differences between short- and long-distance migrants. Offshore flights were more likely when winds blew offshore and began earlier within the night compared with onshore flights. Our results suggest that selection more similarly affects birds of different migration distances in spring than in autumn. These findings put the focus toward how ultimate mechanisms may shape departure and routing decisions differently between migration seasons.

Atmospheric pressure predicts probability of departure for migratory songbirds

BACKGROUND

Weather can have both delayed and immediate impacts on animal populations, and species have evolved behavioral adaptions to respond to weather conditions. Weather has long been hypothesized to affect the timing and intensity of avian migration, and radar studies have demonstrated strong correlations between weather and broad-scale migration patterns. How weather affects individual decisions about the initiation of migratory flights, particularly at the beginning of migration, remains uncertain.

METHODS

Here, we combine automated radio telemetry data from four species of songbirds collected at five breeding and wintering sites in North America with hourly weather data from a global weather model. We use these data to determine how wind profit, atmospheric pressure, precipitation, and cloud cover affect probability of departure from breeding and wintering sites.

RESULTS

We found that the probability of departure was related to changes in atmospheric pressure, almost completely regardless of species, season, or location. Individuals were more likely to depart on nights when atmospheric pressure had been rising over the past 24 h, which is predictive of fair weather over the next several days. By contrast, wind profit, precipitation, and cloud cover were each only informative predictors of departure probability in a single species.

CONCLUSIONS

Our results suggest that individual birds actively use weather information to inform decision-making regarding the initiation of departure from the breeding and wintering grounds. We propose that birds likely choose which date to depart on migration in a hierarchical fashion with weather not influencing decision-making until after the departure window has already been narrowed down by other ultimate and proximate factors.

Departure, routing and landing decisions of long-distance migratory songbirds in relation to weather

Migrating birds flexibly adjust their individual migratory decisions, i.e. departing, routing and landing, based on intrinsic (e.g. energy stores) and extrinsic (e.g. landscape features and weather) factors modulating the endogenous stimuli. So far, these decisions have mostly been studied separately. Notably, we lack information on which factors landing decisions during active flight are based on. Therefore, we simultaneously recorded all three decisions in free-flying long-distance migratory songbirds in a coastal stopover area via regional-scale radio-telemetry and related them to the prevailing weather. Birds departed under favourable weather conditions resulting in specific nights with increased departure probability. Once departed, birds could either fly offshore or take a route along the coast, which was predicted by wind support. Radio-tracking revealed that departed individuals more likely interrupted their migratory endurance flight under overcast or headwind conditions. Studying departure, routing and landing decisions in concert, we highlight the importance of weather as a common driver across all migratory decisions. By radio-tracking individuals between stopovers, we provide evidence that avoidance of adverse weather conditions is an important function of stopover. Understanding how birds adjust migratory decisions and how they affect the timing of migration and survival is key to link migration performance to individual fitness.

On the use of body mass measures in severity assessment in laboratory passerine birds

Criteria for assessing the severity of scientific procedures in laboratory rodents include the loss of body mass. However, guidance is limited for passerine birds and application of criteria developed for mammals risks poor welfare decisions. Here, I ask whether, and how, body mass criteria could be incorporated into laboratory welfare assessment of passerines. Passerine birds strategically adjust their body mass to minimise combined mortality risk from starvation and predation. A systematic literature review found that strategic mass changes can be sizeable (sometimes > 10%) even over short timescales. Many aspects of a bird's current or past environment, including husbandry and experimental procedures, may alter perceived starvation or predation risks and thus drive strategic mass change via evolved mechanisms. Therefore, body mass criteria used for rodents may be too stringent for passerines, potentially leading to over-estimated severity. Strategic mass changes might obscure those stemming from experimental interventions yet could also offer insights into whether birds perceive an intervention or altered husbandry as a threat. Mass criteria for severity assessment should be species- and context-specific in order to balance needs for refinement and reduction. To guide the development of appropriate criteria, a future research priority is for greater data collection and sharing based on standardised routine monitoring of mass variation under a representative range of husbandry conditions and procedures.

Community science reveals links between migration arrival timing advance, migration distance, and wing shape

Substantial global data show that many taxa are shifting their phenologies in response to climate change. For birds, migration arrival dates in breeding regions have been shifting earlier, and there is evidence that both evolutionary adaptation and behavioural flexibility influence these shifts. As more efficient flyers may be able to demonstrate more flexibility to respond to changing conditions during migratory flight, we hypothesize that differences among passerine species in flight efficiency, as reflected by morphology, may be associated with the magnitude of shifts in arrival date in response to climate warming. We applied a logistic model to eighteen years of eBird data to estimate mean arrival date for 44 common passerines migrating to northeast North America. We then used linear mixed-effects models to estimate changes in mean arrival date and compared these changes to morphological proxies for flight efficiency and migratory distance using phylogenetic generalized least squares models. On average, passerine species shifted their arrival dates 0.120 days earlier each year, with 27 of the 44 species shifting to significantly earlier arrival times, and two shifting to significantly later ones. Of the 15 species with non-significant shifts, 13 trended toward earlier arrivals. Longer migration distances and higher wing aspect ratios were associated with greater shifts towards earlier arrivals. Migration distance and aspect ratio were also significantly correlated to each other. This suggests that changes in arrival date are affected by factors pertaining to migratory flight over long distances namely, flight efficiency and migration distance. These traits may be able predict the magnitude of arrival date shift, and by extension identify species that are most at risk to climate change due to inflexible arrival timing.

Do Freshwater Turtles Use Rainfall to Increase Nest Success?

Rainfall following turtle nest construction has long been believed to increase nest survival by its effects on reducing the location cues used by nest predators. However, it is unclear if this is generally the case and if nesting turtles actively use this mechanism to increase their reproductive fitness by deliberately timing nesting to occur before or during rainfall. To address this question, we reviewed studies that examined freshwater turtle nesting behavior and nest predation rates in relation to rainfall. We supplemented our review with data on rainfall and nesting patterns from a 12-year study of two nesting populations of Ouachita Map Turtles (Graptemys ouachitensis). Our review revealed a diversity of responses in rainfall effects on predation and in the propensity for turtles to nest in association with rain. Our mixed findings could reflect a diversity of species- or population-specific responses, local adaptations, species composition of predator community, confounding abiotic factors (e.g., temperature decreases after rainfall) or methodology (e.g., most studies did not quantify rainfall amounts). Our case study on map turtles found very high yearly predation rates (75–100%), precluding our ability to rigorously analyze the association between nest predation and rainfall. However, close examination of the exact timing of both rainfall and predation revealed significantly lower predation rates when rain fell within 24 h after nesting, indicating that rainfall during or after nesting may reduce nest predation. Despite this effect, the best fitted model explaining the propensity to nest found that map turtles were more likely to nest after dry days than after days with rainfall, suggesting that rainfall was not a major factor driving turtles to nest in our populations. In both our review and in our map turtle populations there was little evidence that turtles can anticipate rainfall and nest prior to it occurring (e.g., in response to falling barometric pressure).

A partial migrant relies upon a range-wide cue set but uses population-specific weighting for migratory timing

BACKGROUND

Many birds species range over vast geographic regions and migrate seasonally between their breeding and overwintering sites. Deciding when to depart for migration is one of the most consequential life-history decisions an individual may make. However, it is still not fully understood which environmental cues are used to time the onset of migration and to what extent their relative importance differs across a range of migratory strategies. We focus on departure decisions of a songbird, the Eurasian blackbird Turdus merula, in which selected Russian and Polish populations are full migrants which travel relatively long-distances, whereas Finnish and German populations exhibit partial migration with shorter migration distances.

METHODS

We used telemetry data from the four populations (610 individuals) to determine which environmental cues individuals from each population use to initiate their autumn migration.

RESULTS

When departing, individuals in all populations selected nights with high atmospheric pressure and minimal cloud cover. Fully migratory populations departed earlier in autumn, at longer day length, at higher ambient temperatures, and during nights with higher relative atmospheric pressure and more supportive winds than partial migrants; however, they did not depart in higher synchrony. Thus, while all studied populations used the same environmental cues, they used population-specific and locally tuned thresholds to determine the day of departure.

CONCLUSIONS

Our data support the idea that migratory timing is controlled by general, species-wide mechanisms, but fine-tuned thresholds in response to local conditions.

Magnetic Compass Orientation in a Palaearctic-Indian Night Migrant, the Red-Headed Bunting

Simple Summary

The earth’s magnetic field, celestial cues, and retention of geographical cues en route provide birds with compass knowledge during migration. The magnetic compass works on the direction of the magnetic field, specifically, the course of the field lines. We tested Red-headed Buntings in orientation cages in the evening during spring migration. Simulated overcast testing resulted in a northerly mean direction, while in clear skies, birds oriented in an NNW (north–northwest) direction. Buntings were exposed to 120° anticlockwise shifted magnetic fields under simulated overcast skies and responded by shifting their orientation accordingly. The results showed that this Palaearctic night migrant possesses a magnetic compass, as well as the fact that magnetic cues act as primary directional messengers. When birds were exposed to different environmental conditions at 22 °C and 38 °C temperatures under simulated overcast conditions, they showed a delay in Zugunruhe (migratory restlessness) at 22 °C, while an advance migratory restlessness was observed under 38 °C conditions. Hot and cold weather clearly influenced the timing of migrations in Red-headed Buntings, but not the direction.

Abstract

Red-headed Buntings (Emberiza bruniceps) perform long-distance migrations within their southerly overwintering grounds and breeding areas in the northern hemisphere. Long-distance migration demands essential orientation mechanisms. The earth’s magnetic field, celestial cues, and memorization of geographical cues en route provide birds with compass knowledge during migration. Birds were tested during spring migration for orientation under natural clear skies, simulated overcast skies at natural day length and temperature, simulated overcast at 22 °C and 38 °C temperatures, and in the deflected (−120°) magnetic field. Under clear skies, the Red-headed Buntings were oriented NNW (north–northwest); simulated overcast testing resulted in a northerly mean direction at local temperatures as well as at 22 °C and 38 °C. The Buntings reacted strongly in favor of the rotated magnetic field under the simulated overcast sky, demonstrating the use of a magnetic compass for migrating in a specific direction.

Environmental cue affects the hearing-related behaviors of Drosophila melanogaster by targeting the redox pathways

Environmental cues like noise, pressure, and circadian rhythm can affect the hearing ability of human beings. Nevertheless, the complex physiology of the human being does not allow us to understand how these factors can affect hearing and hearing-related behaviors. Conversely, these effects can be easily checked using the hearing organ of Drosophila melanogaster, the Johnston organ. In the current study, the Drosophila was exposed to challenging environments like noise, low pressure, and altered circadian rhythm. The hearing organ of larvae, as well as adults, was analyzed for hearing-related defects. In the third instar larva, the cell deaths were detected in the antenna imaginal disc, the precursor of Johnston's organ. Elevated levels of reactive oxygen species and antioxidant enzymes were also detected in the adult antennae of environmentally challenged flies. The ultrastructure of the antennae suggests the presence of abundant mitochondria in the scolopidia of control. Fewer amounts of mitochondria are found in the environmentally challenged adult antennae. In adults, various hearing-related behaviors were analyzed as a readout of functionality of the hearing organ. Analysis of climbing, aggressive, and courtship behaviors suggests abnormal behavior in environmentally challenged flies than the control. The current study suggests that the environmental cues can alter hearing-related behaviors in Drosophila. The methods used in this study can be used to monitor the environmental pollution or to study the effect of alteration of noise, pressure, and circadian rhythm on hearing-related behaviors taking Drosophila melanogaster as a model organism. Graphical abstract.

Stopover refuelling, movement and departure decisions in the white-throated sparrow: The influence of intrinsic and extrinsic factors during spring migration

Differential migration timing between sex or age classes is an example of how migratory movement strategies can differ among subgroups within a population. However, in songbirds, evidence for intrinsic differences in en route migratory behaviour is often mixed, suggesting that the local environmental context may play a role in accentuating or diminishing patterns. We evaluated how multiple intrinsic and extrinsic variables influenced refuelling rates, local movement behaviour and departure decisions in the white-throated sparrow Zonotrichia albicollis during spring migration. This species exhibits a unique genetically based plumage dimorphism, providing a unique class of individual in which to evaluate patterns and processes of differential migration, in addition to sex, age and migration distance. At a migratory stopover site, plasma metabolite analysis was used to quantify individual variation in stopover refuelling rate. In after second year adults, automated and manual radio telemetry was used to quantify daily activity timing, daily movement distances, stopover duration and departure time. Arrival timing to the stopover site was determined using capture data. Non-breeding and previous breeding/natal latitude were determined using analysis of hydrogen isotopes in claws and feathers. Males arrived at the stopover site 11 days on average before females, but no difference in migration timing was observed between plumage morph or age classes. After second year, adults with more southern previous breeding latitudes arrived at stopover earlier, whereas second year birds making their first return migration arrived at stopover in an inverse relationship to non-breeding latitude. Stopover refuelling rate did not differ between ages, sexes or plumage morphs, and daily departure probability of adults was higher under warmer temperatures and favourable tailwinds. White-striped morphs moved greater distances during stopover, initiated daily activity earlier in the morning and departed for migration earlier in the evening than tan-striped morphs. Our results show that while individual phenotype can influence some aspects of local stopover-scale movement behaviour, evidence for differential stopover behaviour was weak. Differential migration timing is unlikely to result from intrinsic differences in en route refuelling rate and departure decisions, especially because the latter is strongly influenced by meteorological conditions.

Tropical cyclones alter short-term activity patterns of a coastal seabird

BACKGROUND

Mobile organisms in marine environments are expected to modify their behavior in response to external stressors. Among environmental drivers of animal movement are long-term climatic indices influencing organism distribution and short-term meteorological events anticipated to alter acute movement behavior. However, few studies exist documenting the response of vagile species to meteorological anomalies in coastal and marine systems.

METHODS

Here we examined the movements of Eastern brown pelicans (Pelecanus occidentalis carolinensis) in the South Atlantic Bight in response to the passage of three separate hurricane events in 2 years. Pelicans (n = 32) were tracked with GPS satellite transmitters from four colonies in coastal South Carolina, USA, for the entirety of at least one storm event. An Expectation Maximization binary Clustering algorithm was used to discretize pelican behavioral states, which were pooled into 'active' versus 'inactive' states. Multinomial logistic regression was used to assess behavioral state probabilities in relation to changes in barometric pressure and wind velocity.

RESULTS

Individual pelicans were more likely to remain inactive during tropical cyclone passage compared to baseline conditions generally, although responses varied by hurricane. When inactive, pelicans tended to seek shelter using local geomorphological features along the coastline such as barrier islands and estuarine systems.

CONCLUSIONS

Our telemetry data showed that large subtropical seabirds such as pelicans may mitigate risk associated with spatially-extensive meteorological events by decreasing daily movements. Sheltering may be related to changes in barometric pressure and wind velocity, and represents a strategy common to several other classes of marine vertebrate predators for increasing survival probabilities.

The role of humidity and metabolic status on lean mass catabolism in migratory Swainson's thrushes (Catharus ustulatus)

Migratory birds use protein as a fuel source during flight, but the mechanisms and benefits of protein catabolism during migration are poorly understood. The tissue-specific turnover rate hypothesis proposes that lean mass loss depends solely on the constitutive rate of protein degradation for a given tissue, and is therefore independent of metabolic rate or environmental stimuli. However, it has been demonstrated that environmental stressors such as humidity affect the rate of lean mass catabolism during flight, a finding that seemingly contradicts the tissue-specific turnover rate hypothesis. In order to resolve this, we placed migratory Swainson's thrushes in either high (HEWL) or low (LEWL) evaporative water loss conditions at rest and while undergoing simulated migratory flight at 8 m s^-1 in a wind tunnel to test the impact of both environmental stressors and metabolic rate on the rate of protein breakdown. The total quantity and rate of lean mass loss was not different between flight and rest birds, but was affected by humidity condition, with HEWL losing significantly more lean mass. These results show that the rate of protein breakdown in migratory birds is independent of metabolic rate, but it can be augmented in response to environmental stressors.

Forecasting the response to global warming in a heat-sensitive species

Avoiding hyperthermia entails considerable metabolic costs for endotherms. Such costs increase in warm conditions, when endotherms may trade food intake for cooler areas to avoid heat stress and maximize their energy balance. The need to reduce heat stress may involve the adoption of tactics affecting space use and foraging behaviour, which are important to understand and predict the effects of climate change and inform conservation. We used resource selection models to examine the behavioural response to heat stress in the Alpine ibex (Capra ibex), a cold-adapted endotherm particularly prone to overheating. Ibex avoided heat stress by selecting the space based on the maximum daily temperature rather than moving hourly to ‘surf the heat wave’, which minimised movement costs but prevented optimal foraging. By integrating these findings with new climate forecasts, we predict that rising temperatures will force mountain ungulates to move upward and overcrowd thermal refugia with reduced carrying capacity. Our approach helps in identifying priority areas for the conservation of mountain species.

A Nearctic-Neotropical Migratory Songbird's Nesting Phenology and Clutch Size are Predictors of Accumulated Cyclone Energy

The breeding season phenology of Nearctic-Neotropical migratory songbirds is constrained by subsequent seasons resulting in single-brooded behavior (one successful clutch per year) in some species. Early cessation of the nesting season prior to an active hurricane season will allow for behavioral plasticity during a physiologically challenging migration. Hurricane activity shows a high degree of inter-annual variability. I show that a single-brooded Nearctic-breeding species' (Catharus fuscescens) nesting phenology and clutch size are significant predictors of Accumulated Cyclone Energy. The most skilled predictive model includes both mean clutch initiation date and mean clutch size (R² = 0.84). Spearman rank correlation coefficients for both predictors with subsequent major hurricanes (1998-2016) are -0.55 and 0.52, respectively. Therefore, May and June clutch initiation and clutch size showed stronger correlations with subsequent hurricanes than early season (prior to August) meteorological predictions widely publicized by CSU, NOAA, and TSR (≤0.45, 2003-2014). Rainfall anomalies in the southern Amazon basin associated with ENSO cycles are a possible proximate cue affecting phenology and clutch size. This discovery potentially has far-reaching ornithological, meteorological, and social implications and shows that tropical storms significantly constrain breeding season behavior providing renewed evidence that hurricane activity is a primary factor regulating Nearctic-Neotropical migratory songbird populations.

Understanding variation in migratory movements: A mechanistic approach

Spatial and temporal fluctuations in resource availability have led to the evolution of varied migration patterns. In order to appropriately time movements in relation to resources, environmental cues are used to provide proximate information for timing and the endocrine system serves to integrate these external cues and behavioral and physiological responses. Yet, the regulatory mechanisms underlying migratory timing have rarely been compared across a broad range of migratory patterns. First, we offer an updated nomenclature of migration using a mechanistic perspective to clarify terminology describing migratory types in relation to ecology, behavior and endocrinology. We divide migratory patterns into three types: obligate, nomadic, and fugitive. Obligate migration is characterized by regular and directed annual movements between locations, most commonly for breeding and overwintering, where resources are predictable and sufficient. Nomadic migrations occur less predictably than do obligate migrations as animals make use of potentially rich but ephemeral resources that occur unpredictably in space or time. Fugitive migrations move animals away from an area in response to severe disruption of environmental conditions and occur as part of an emergency life history stage. We also consider partially migratory populations, which include a mix of sedentary and migratory individuals; the movement patterns of partial migrants are expected to fall into one of the three types above. For these various forms of migration, we review our understanding of the environmental cues and endocrine mechanisms that underlie the expression of a migratory state. Several common hormonal mechanisms exist across the varied migratory forms, but there are also important areas where further investigations are needed in order to gain broad insight into the origin of movements and the diversity of migratory patterns. We propose that taking a comparative approach across the migratory types that considers endocrine mechanisms will advance a new understanding of migration biology.

Experimental temperature manipulations alter songbird autumnal nocturnal migratory restlessness

Migrating birds may respond to a variety of environmental cues in order to time migration. During the migration season nocturnally migrating songbirds may migrate or stop-over at their current location, and when migrating they may vary the rate or distance of migration on any given night. It has long been known that a variety of weather-related factors including wind speed and direction, and temperature, are correlated with migration in free-living birds, however these variables are often correlated with each other. In this study we experimentally manipulated temperature to determine if it would directly modulate nocturnal migratory restlessness in songbirds. We experimentally manipulated temperature between 4, 14, and 24°C and monitored nocturnal migratory restlessness during autumn in white-throated sparrows (Zonotrichia albicollis). White-throated sparrows are relatively shortdistance migrants with a prolonged autumnal migration, and we thus predicted they might be sensitive to weatherrelated cues when deciding whether to migrate or stopover. At warm temperatures (24°C) none of the birds exhibited migratory restlessness. The probability of exhibiting migratory restlessness, and the intensity of this restlessness (number of infra-red beam breaks) increased at cooler (14°C, 4°C) temperatures. These data support the hypothesis that one of the many factors that birds use when making behavioural decisions during migration is temperature, and that birds can respond to temperature directly independently of other weather-related cues.

Towards a conceptual framework for explaining variation in nocturnal departure time of songbird migrants

Most songbird migrants travel between their breeding areas and wintering grounds by a series of nocturnal flights. The exact nocturnal departure time for these flights varies considerably between individuals even of the same species. Although the basic circannual and circadian rhythms of songbirds, their adaptation to migration, and the factors influencing the birds' day-to-day departure decision are reasonably well studied, we do not understand how birds time their departures within the night. These decisions are crucial, because the nocturnal departure time defines the potential flight duration of the migratory night. The distances covered during the nocturnal migratory flights in the course of migration in turn directly affect the overall speed of migration. To understand the factors influencing the arrival of the birds in the breeding/wintering areas, we need to investigate the mechanisms that control nocturnal departure time. Here, we provide the first conceptual framework for explaining the variation commonly observed in this migratory trait. The basic schedule of nocturnal departure is likely regulated by both the circannual and circadian rhythms of the innate migration program. We postulate that the endogenously controlled schedule of nocturnal departures is modified by intrinsic and extrinsic factors. So far there is only correlative evidence that birds with a high fuel load or a considerable increase in fuel load and significant wind (flow) assistance towards their migratory goal depart early within the night. In contrast, birds migrating with little fuel and under unfavorable wind conditions show high variation in their nocturnal departure time. The latter may contain an unknown proportion of nocturnal movements not directly related to migratory flights. Excluding such movements is crucial to clearly identify the main drivers of the variation in nocturnal departure time. In general we assume that the observed variation in the nocturnal departure time is explained by individually different reactions norms of the innate migration program to both intrinsic and extrinsic factors.

When Siberia came to the Netherlands: the response of continental black-tailed godwits to a rare spring weather event

1. Extreme weather events have the potential to alter both short- and long-term population dynamics as well as community- and ecosystem-level function. Such events are rare and stochastic, making it difficult to fully document how organisms respond to them and predict the repercussions of similar events in the future. 2. To improve our understanding of the mechanisms by which short-term events can incur long-term consequences, we documented the behavioural responses and fitness consequences for a long-distance migratory bird, the continental black-tailed godwit Limosa limosa limosa, resulting from a spring snowstorm and three-week period of record low temperatures. 3. The event caused measurable responses at three spatial scales - continental, regional and local - including migratory delays (+19 days), reverse migrations (>90 km), elevated metabolic costs (+8·8% maintenance metabolic rate) and increased foraging rates (+37%). 4. There were few long-term fitness consequences, however, and subsequent breeding seasons instead witnessed high levels of reproductive success and little evidence of carry-over effects. 5. This suggests that populations with continued access to food, behavioural flexibility and time to dissipate the costs of the event can likely withstand the consequences of an extreme weather event. For populations constrained in one of these respects, though, extreme events may entail extreme ecological consequences.