Bart Kranstauber

A dynamic Brownian bridge movement model to estimate utilization distributions for heterogeneous animal movement

1. The recently developed Brownian bridge movement model (BBMM) has advantages over traditional methods because it quantifies the utilization distribution of an animal based on its movement path rather than individual points and accounts for temporal autocorrelation and high data volumes. However, the BBMM assumes unrealistic homogeneous movement behaviour across all data. 2. Accurate quantification of the utilization distribution is important for identifying the way animals use the landscape. 3. We improve the BBMM by allowing for changes in behaviour, using likelihood statistics to determine change points along the animals movement path. 4. This novel extension, outperforms the current BBMM as indicated by simulations and examples of a territorial mammal and a migratory bird. The unique ability of our model to work with tracks that are not sampled regularly is especially important for GPS tags that have frequent failed fixes or dynamic sampling schedules. Moreover, our model extension provides a useful one-dimensional measure of behavioural change along animal tracks. 5. This new method provides a more accurate utilization distribution that better describes the space use of realistic, behaviourally heterogeneous tracks.

Individual‐based measurements of light intensity provide new insights into the effects of artificial light at night on daily rhythms of urban‐dwelling songbirds

The growing interest in the effects of light pollution on daily and seasonal cycles of animals has led to a boost of research in recent years. In birds, it has been hypothesized that artificial light at night can affect daily aspects of behaviour, but one caveat is the lack of knowledge about the light intensity that wild animals, such as birds, are exposed to during the night. Organisms have naturally evolved daily rhythms to adapt to the 24-h cycle of day and night, thus, it is important to investigate the potential shifts in daily cycles due to global anthropogenic processes such as urbanization. We captured adult male European blackbirds (Turdus merula) in one rural forest and two urban sites differing in the degree of anthropogenic disturbance. We tagged these birds with light loggers and simultaneously recorded changes in activity status (active/non-active) through an automated telemetry system. We first analysed the relationship between light at night, weather conditions and date with daily activity onset and end. We then compared activity, light at night exposure and noise levels between weekdays and weekends. Onset of daily activity was significantly advanced in both urban sites compared to the rural population, while end of daily activity did not vary either among sites. Birds exposed to higher amounts of light in the late night showed earlier onset of activity in the morning, but light at night did not influence end of daily activity. Light exposure at night and onset/end of daily activity timing was not different between weekdays and weekends, but all noise variables were. A strong seasonal effect was detected in both urban and rural populations, such as birds tended to be active earlier in the morning and later in the evening (relative to civil twilight) in the early breeding season than at later stages. Our results point at artificial light at night as a major driver of change in timing of daily activity. Future research should focus on the costs and benefits of altered daily rhythmicity in birds thriving in urban areas.

Group Hunting : A Reason for Sociality in Molossid Bats?

Many bat species live in groups, some of them in highly complex social systems, but the reasons for sociality in bats remain largely unresolved. Increased foraging efficiency through passive information transfer in species foraging for ephemeral insects has been postulated as a reason for group formation of male bats in the temperate zones. We hypothesized that benefits from group hunting might also entice tropical bats of both sexes to live in groups. Here we investigate whether Molossus molossus, a small insectivorous bat in Panama, hunts in groups. We use a phased antenna array setup to reduce error in telemetry bearings. Our results confirmed that simultaneously radiotracked individuals from the same colony foraged together significantly more than expected by chance. Our data are consistent with the hypothesis that many bats are social because of information transfer between foraging group members. We suggest this reason for sociality to be more widespread than currently assumed. Furthermore, benefits from group hunting may also have contributed to the evolution of group living in other animals specialized on ephemeral food sources.

Quantifying levels of animal activity using camera-trap data

Summary 1. Activity level (the proportion of time that animals spend active) is a behavioural and ecological metric that can provide an indicator of energetics, foraging effort and exposure to risk. However, activity level is poorly known for free-living animals because it is difficult to quantify activity in the field in a consistent, cost-effective and non-invasive way. 2. This article presents a new method to estimate activity level with time-of-detection data from camera traps (or more generally any remote sensors), fitting a flexible circular distribution to these data to describe the underlying activity schedule, and calculating overall proportion of time active from this. 3. Using simulations and a case study for a range of small- to medium-sized mammal species, we find that activity level can reliably be estimated using the new method. 4. The method depends on the key assumption that all individuals in the sampled population are active at the peak of the daily activity cycle. We provide theoretical and empirical evidence suggesting that this assumption is likely to be met for many species, but may be less likely met in large predators, or in high-latitude winters. Further research is needed to establish stronger evidence on the validity of this assumption in specific cases; however, the approach has the potential to provide an effective, non-invasive alternative to existing methods for quantifying population activity levels.

True navigation in migrating gulls requires intact olfactory nerves

During migratory journeys, birds may become displaced from their normal migratory route. Experimental evidence has shown that adult birds can correct for such displacements and return to their goal. However, the nature of the cues used by migratory birds to perform long distance navigation is still debated. In this experiment we subjected adult lesser black-backed gulls migrating from their Finnish/Russian breeding grounds (from >60°N) to Africa (to < 5°N) to sensory manipulation, to determine the sensory systems required for navigation. We translocated birds westward (1080 km) or eastward (885 km) to simulate natural navigational challenges. When translocated westwards and outside their migratory corridor birds with olfactory nerve section kept a clear directional preference (southerly) but were unable to compensate for the displacement, while intact birds and gulls with the ophthalmic branch of the trigeminal nerve sectioned oriented towards their population-specific migratory corridor. Thus, air-borne olfactory information seems to be important for migrating gulls to navigate successfully in some circumstances.

Low metabolism in a tropical bat from lowland Panama measured using heart rate telemetry: an unexpected life in the slow lane

SUMMARY Animals must optimize their daily energy budgets, particularly if energy expenditures are as high as they are in flying animals. However, energy budgets of free-ranging tropical animals are poorly known. Newly miniaturized heart rate transmitters enabled this to be addressed this in the small, energetically limited, neotropical bat Molossus molossus. High-resolution 48 h energy budgets showed that this species significantly lowers its metabolism on a daily basis, even though ambient temperatures remain high. Mean roosting heart rate was 144 beats min–1, much lower than expected for a 10 g bat. Low roosting heart rates combined with short nightly foraging times (37 min night–1) resulted in an estimated energy consumption of 4.08 kJ day–1, less than one-quarter of the predicted field metabolic rate. Our results indicate that future research may reveal this as a more common pattern than currently assumed in tropical animals, which may have implications in the context of the effect of even small temperature changes on tropical species.

Costs of sleeping in: circadian rhythms influence cuckoldry risk in a songbird

Summary Circadian (i.e. daily) regulation of behaviours is thought to provide fitness benefits to organisms by enabling them to anticipate diel changes in the environment, such as sunrise. A common behaviour among socially monogamous songbirds that usually takes place in the early mornings is extra-pair mating, that is copulating with partners outside of the social pair bond. Thus, variation in when individuals begin their daily activity may influence their reproductive success; early risers may be better able to gain copulations and guard their partners, thus minimizing their risk of being cuckolded compared with late risers. Sexual selection may thus play an important role in shaping circadian behaviours, but this assumption has yet to be tested in free-living animals. Here, we experimentally weakened endogenous circadian rhythmicity, and thus, anticipation of dawn in male great tits (Parus major) in the wild through the subcutaneous administration of implants filled with melatonin shortly before egg-laying began in this population. Melatonin is a hormone released during the dark phase at night and is one important cue animals use to entrain their circadian clock. Experimental individuals delayed the onset of daily activity compared with controls and were more likely to be cuckolded compared with control males. Manipulation did not alter other behavioural traits observed; no difference between treatments was observed in activity levels during the day or in the end time of daily activity. These results strongly support the assumption that selection, particularly sexual selection, shapes the circadian phenotypes of wild vertebrates which enable anticipation of important and predictive diel changes in an individuals biotic and abiotic environment.

Bivariate Gaussian bridges: directional factorization of diffusion in Brownian bridge models

BackgroundIn recent years high resolution animal tracking data has become the standard in movement ecology. The Brownian Bridge Movement Model (BBMM) is a widely adopted approach to describe animal space use from such high resolution tracks. One of the underlying assumptions of the BBMM is isotropic diffusive motion between consecutive locations, i.e. invariant with respect to the direction.Here we propose to relax this often unrealistic assumption by separating the Brownian motion variance into two directional components, one parallel and one orthogonal to the direction of the motion.ResultsOur new model, the Bivariate Gaussian bridge (BGB), tracks movement heterogeneity across time. Using the BGB and identifying directed and non-directed movement within a trajectory resulted in more accurate utilisation distributions compared to dynamic Brownian bridges, especially for trajectories with a non-isotropic diffusion, such as directed movement or Lévy like movements. We evaluated our model with simulated trajectories and observed tracks, demonstrating that the improvement of our model scales with the directional correlation of a correlated random walk.ConclusionWe find that many of the animal trajectories do not adhere to the assumptions of the BBMM. The proposed model improves accuracy when describing the space use both in simulated correlated random walks as well as observed animal tracks. Our novel approach is implemented and available within the “move” package for R.

Bats Swarm Where They Hibernate : Compositional Similarity between Autumn Swarming and Winter Hibernation Assemblages at Five Underground Sites

During autumn in the temperate zone of both the new and old world, bats of many species assemble at underground sites in a behaviour known as swarming. Autumn swarming behaviour is thought to primarily serve as a promiscuous mating system, but may also be related to the localization and assessment of hibernacula. Bats subsequently make use of the same underground sites during winter hibernation, however it is currently unknown if the assemblages that make use of a site are comparable across swarming and hibernation seasons. Our purpose was to characterize the bat assemblages found at five underground sites during both the swarming and the hibernation season and compare the assemblages found during the two seasons both across sites and within species. We found that the relative abundance of individual species per site, as well as the relative proportion of a species that makes use of each site, were both significantly correlated between the swarming and hibernation seasons. These results suggest that swarming may indeed play a role in the localization of suitable hibernation sites. Additionally, these findings have important conservation implications, as this correlation can be used to improve monitoring of underground sites and predict the importance of certain sites for rare and cryptic bat species.

Similarity in spatial utilization distributions measured by the earth mover's distance

Estimating the similarity in space use (spatio-temporal home range overlap) of animals is important for many questions regarding behavioural ecology, wildlife management and conservation. The current methods that calculate joint space use generally do not account for proximity in space use, as all of them rely on the differences between the exact spatial overlay of utilization distributions, while spatial distances between distributions should be considered to truly quantify similarity. We implemented the earth movers distance (EMD), a spatially explicit method, that quantifies similarity between utilization distributions by calculating the effort it takes to shape one utilization distribution landscape into another, hence EMD. The EMD is a method commonly used in image retrieval applications, and we propose its use to calculate similarity in space use in the framework of movement ecology. We show that the EMD is a consistent and useful as well as versatile measure of overlap and provide an easy to use implementation in the r package move.