Bruno J. Ens

Green Light for Nocturnally Migrating Birds

The nighttime sky is increasingly illuminated by artificial light sources. Although this ecological light pollution is damaging ecosystems throughout the world, the topic has received relatively little attention. Many nocturnally migrating birds die or lose a large amount of their energy reserves during migration as a result of encountering artificial light sources. This happens, for instance, in the North Sea, where large numbers of nocturnally migrating birds are attracted to the many offshore platforms. Our aim is to develop bird-friendly artificial lighting that meets human demands for safety but does not attract and disorient birds. Our current working hypothesis is that artificial light interferes with the magnetic compass of the birds, one of several orientation mechanisms and especially important during overcast nights. Laboratory experiments have shown the magnetic compass to be wavelength dependent: migratory birds require light from the blue-green part of the spectrum for magnetic compass orientation, whereas red light (visible long-wavelength) disrupts magnetic orientation. We designed a field study to test if and how changing light color influenced migrating birds under field conditions. We found that nocturnally migrating birds were disoriented and attracted by red and white light (containing visible long-wavelength radiation), whereas they were clearly less disoriented by blue and green light (containing less or no visible long- wavelength radiation). This was especially the case on overcast nights. Our results clearly open perspective for the development of bird-friendly artificial lighting by manipulating wavelength characteristics. Preliminary results with an experimentally developed bird-friendly light source on an offshore platform are promising. What needs to be investigated is the impact of bird-friendly light on other organisms than birds.

Why Does the Typically Monogamous Oystercatcher (Haematopus Ostralegus) Engage in Extra-Pair Copulations?

(1) We attempted to identify the possible costs and benefits of Extra-Pair Copulations (EPCs) from field observations on a colour-marked population of Oystercatchers, a long lived monogamous species, where male and female share parental care and territory defence. (2) Despite our broad definition of EPCs, only 7.0% of successful copulations by male breeders and 5.1 % of successful copulations by female breeders were classified as EPCs. (3) Many pairs first copulated more than 2 months before egg-laying. The rate at which breeders engaged in Within-Pair Copulations (WPCs) was highest in the month preceding egg-laying: almost 1 copulation per hour during low tide. Males were increasingly likely to initiate WPCs close to the period of egg-laying. Once the clutch was complete copulation rates dropped to near zero. (4) Most EPCs were observed well before egg-laying. In the month preceding and including egg-laying only 3.3% of all successful copulations by females were EPCs. DNA-fingerprinting confirmed that of 65 chicks (from 26 clutches), only 1 was not fathered by the male partner, but by a neighbouring male, which was seen to copulate with the female before egg-laying. Thus, extra-pair paternity was extremely rare, comprising 1.5% of all chicks. Fingerprinting provided no evidence for intraspecific brood parasitism or quasi-parasitism. (5) Members of new pairs (a minority in the population) were observed to copulate with more mates than were members of old pairs, for the same number of copulations observed. (6) Two case studies suggested that EPCs by males and females of old pairs may be attempts to change mate. One female switched to a new mate after 2 years of EPCs with this bird, while the other female is expected to switch to a neighbouring male in 1992, after 3 years of EPCs with this neighbour. (7) The majority of EPCs by male breeders were in their own territory, while female breeders more often moved to the territory of the male, often the neighbour. This sex difference resembles the sex difference in breeding dispersal: female breeders are more likely to switch territory when switching mate (ENS et al., 1993). (8) Male breeders whose mate was absent sometimes evicted soliciting female intruders instantly. This suggest that EPCs were not necessarily beneficial, even when there was no apparent risk of a penalty by the mate. (9) IfEPCs are primarily attempts to change mate, we predict an increase in the probability of mate change when an individual has engaged in EPCs in the previous year. We surmise that we failed to establish this relationship, because our record of EPCs was incomplete and because attempts at mate change apparently often failed due to intra-sexual competition.

Experimental evidence for effects of human disturbance on foraging and parental care in oystercatchers

We carried out two experiments to quantify effects of human disturbance on foraging and parental care in European oystercatchers (Haematopus ostralegus). In experiment 1, pairs incubating a clutch were disturbed on their feeding territory on the mudflat. Disturbance significantly reduced the proportion of time that the clutch was incubated, but also the proportion of time that the pair spent on the mud flat. In experiment 2, foraging oystercatcher pairs with chicks were disturbed by two observers at different distances from the edge of the salt marsh where the chicks resided. Total food collected was independent of disturbance, but a smaller proportion of the food collected was allocated to the chicks with increasing disturbance level. Both experiments demonstrate that human disturbance of foraging in breeding oystercatchers reduced the amount of parental care, and thus presumably reproductive success.

Intake rates and the functional response in shorebirds (Charadriiformes) eating macro-invertebrates

As field determinations take much effort, it would be useful to be able to predict easily the coefficients describing the functional response of free‐living predators, the function relating food intake rate to the abundance of food organisms in the environment. As a means easily to parameterise an individual‐based model of shorebird Charadriiformes populations, we attempted this for shorebirds eating macro‐invertebrates. Intake rate is measured as the ash‐free dry mass (AFDM) per second of active foraging; i.e. excluding time spent on digestive pauses and other activities, such as preening. The present and previous studies show that the general shape of the functional response in shorebirds eating approximately the same size of prey across the full range of prey density is a decelerating rise to a plateau, thus approximating the Holling type II (‘disc equation’) formulation. But field studies confirmed that the asymptote was not set by handling time, as assumed by the disc equation, because only about half the foraging time was spent in successfully or unsuccessfully attacking and handling prey, the rest being devoted to searching.

From Sensor Data to Animal Behaviour: An Oystercatcher Example

Animal-borne sensors enable researchers to remotely track animals, their physiological state and body movements. Accelerometers, for example, have been used in several studies to measure body movement, posture, and energy expenditure, although predominantly in marine animals. In many studies, behaviour is often inferred from expert interpretation of sensor data and not validated with direct observations of the animal. The aim of this study was to derive models that could be used to classify oystercatcher (Haematopus ostralegus) behaviour based on sensor data. We measured the location, speed, and tri-axial acceleration of three oystercatchers using a flexible GPS tracking system and conducted simultaneous visual observations of the behaviour of these birds in their natural environment. We then used these data to develop three supervised classification trees of behaviour and finally applied one of the models to calculate time-activity budgets. The model based on accelerometer data developed to classify three behaviours (fly, terrestrial locomotion, and no movement) was much more accurate (cross-validation error = 0.14) than the model based on GPS-speed alone (cross-validation error = 0.35). The most parsimonious acceleration model designed to classify eight behaviours could distinguish five: fly, forage, body care, stand, and sit (cross-validation error = 0.28); other behaviours that were observed, such as aggression or handling of prey, could not be distinguished. Model limitations and potential improvements are discussed. The workflow design presented in this study can facilitate model development, be adapted to a wide range of species, and together with the appropriate measurements, can foster the study of behaviour and habitat use of free living animals throughout their annual routine.

Flocking and feeding in the fiddler crab (UCA tangeri): prey availability as risk-taking behaviour

For a full understanding of prey availability, it is necessary to study risk-taking behaviour of the prey. Fiddler crabs are ideally suited for such a study, as they have to leave their safe burrow to feed on the surface of the intertidal flats during low tide, thereby exposing themselves to avian predators. A study in an intertidal area along the coast of Mauritania showed that small crabs always stayed in the vicinity of their burrow, but large crabs wandered in large flocks (also referred to as droves) to feed on sea-grass beds downshore. Transplanting downshore feeding substrate to the burrowing zone of the small crabs proved that they too preferred to feed on it. Since small crabs can be preyed upon by more species of birds, this suggests that the decision not to leave the burrowing zone might be related to the risk of being fed upon by birds. We calculated predation risk from measurements on the density and feeding activity of the crabs, as well as the feeding density, the intake rate and the size selection of the avian predators. Per hour on the surface, crabs in a flock were more at risk than crabs feeding near their burrow. Thus, though flocking crabs may have benefited from ‘swamping the predator’ by emerging in maximum numbers during some tides only, this did not reduce their risk of predation below that of non-flocking crabs. Furthermore we found that irrespective of activity, large crabs suffered a higher mortality per tide from avian predators than small crabs. This suggests that large crabs could not sufficiently reduce their foraging time to compensate for the increased risk while foraging in a flock, even though they probably experienced better feeding conditions than small crabs staying near their burrow. The greater energy demands of large crabs were reflected in a greater surface area grazed. Thus, with increasing size a fiddler crab has to feed further away from its burrow and so may derive less protection from staying near to it. It seems that growing big does not reduce the risk of predation for fiddler crabs, as it does in many other species with indeterminate growth. As in such species, the most probable advantage of growing big is increased mating success. Ultimately, therefore, prey availability must be understood from the life-history decisions of the prey species.

LOCAL DOMINANCE AND TERRITORIAL SETTLEMENT OF NONBREEDING OYSTERCATCHERS

1. We studied the mechanisms of territory acquisition in an oystercatcher (Haematopus ostralegus) breeding population on Schiermonnikoog (1983-1997), competing for low quality (leapfrog) and high quality (resident) territories. 2. Numbers of nonbreeders o

Kleptoparasitism as a problem of prey choice: a study on mudflat-feeding curlews, Numenius arquata

Curlews feeding on intertidal flats attacked mainly conspecifics for prey, whereas they themselves also fell victim to kleptoparasitic attacks from gulls. There is no convincing evidence that robbers obtained other benefits, such as access to a good feeding site, besides the stolen prey itself. The data are consistent with the view that kleptoparasitism should be treated as a problem of prey choice, where attacks are initiated so as to maximize the net rate of energy gain. Attacks were directed mainly towards conspecifics handling large prey animals. There was no evidence of a simple producer-scrounger dichotomy. Instead, a dominance hierarchy seems more likely, where each individual could initiate attacks, but did so predominantly towards subdominant individuals, according to immediate profitability.

Fitness consequences of divorce in the oystercatcher, Haematopus ostralegus

We investigated the fitness consequences of divorce in oystercatchers. We made a distinction between two types of divorce: in desertions the disruption of the pair bond is initiated by one of the pair members, and in usurpations by a conspecific individual. Survival and reproduction prospects for oystercatchers are largely determined by their social status (nonbreeder or breeding bird at a site of a specific quality). Changes in social status in relation to divorce showed that birds taking the initiative to leave their mate increased in fitness, relative to birds that were forced to leave their partner. Status of individuals that remained in their territory after divorce was unaffected if their mate was expelled, but declined if their mate deserted. Survival after divorce was significantly lower for birds that were expelled than for those deserting. Divorce rate, and especially desertion rate, was higher among occupants of low- than high-quality territories. In general, divorce rate increased following elevated mortality. In high-quality territories usurpations increased with increasing breeder mortality, but at low-quality territories this relation was absent. Desertion rates were similarly related to mortality in both territory types. Divorce participants thus differed strongly in their fitness prospects, depending on the type of divorce, the role played in the divorce and the quality of the territory where divorce took place. Studies that do not observe the birds during divorce cannot determine the type of divorce and the role played by the individuals, and this may lead to misleading conclusions on the costs and benefits of divorce.  2003 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour.

FLUCTUATING SELECTION AND THE MAINTENANCE OF INDIVIDUAL AND SEX-SPECIFIC DIET SPECIALIZATION IN FREE-LIVING OYSTERCATCHERS

Fluctuating and disruptive selection are important mechanisms for maintaining intrapopulation trait variation. Nonetheless, few field studies quantify selection pressures over long periods and identify what causes them to fluctuate. Diet specialists in oystercatchers differ in short‐term payoffs (intake), but their long‐term payoffs are hypothesized to be condition dependent. We test whether phenotypic selection on diet specialization fluctuates between years due to the frequency of specialists, competitor density, prey abundance, and environmental conditions. Short‐term payoffs proved to be poor predictors of long‐term fitness payoffs of specialization. Sex‐differences in diet specialization were maintained by opposing directional fecundity and viability selection between the sexes. Contrasting other studies, selection on individual diet specialization was neither negative frequency‐ or density‐dependent nor dependent on prey abundance. Notwithstanding, viability selection fluctuated strongly (stabilizing↔disruptive) over the 26‐year study period: slightly favoring generalists in most years, but strongly disfavoring generalists in rare harsh winters, suggesting generalists cannot cope with extreme conditions. Although selection fluctuated, mean selection on specialists was weak, which can explain how individual specialization can persist over long periods. Because rare events can dramatically affect long‐term selective landscapes, more care should be taken to match the timescale of evolutionary studies to the temporal variability of critical environmental conditions.