Complex patterns of collective escape in starling flocks under predation.

ABSTRACT: Collective behaviour of animals has been a main focus of recent research, yet few empirical studies deal with this issue in the context of predation, a major driver of social complexity in many animal species. When starling (Sturnus vulgaris) flocks are under attack by a raptor, such as a peregrine falcon (Falco peregrinus), they show a great diversity of patterns of collective escape. The corresponding structural complexity concerns rapid variation in density and shape of the flock over time. Here, we present a first step towards unravelling this complexity. We apply a time series analysis to video footage of 182 sequences of hunting by falcons on flocks of thousands of starlings close to two urban roosts during winter. We distinguish several types of collective escape by determining the position and movement of individuals relative to each other (which determines darkness and shape of the flock over time) as well as relative to the predator, namely 'flash expansion', 'blackening', 'wave event', 'vacuole', 'cordon' and 'split'. We show that the specific type of collective escape depends on the collective pattern that precedes it and on the level of threat posed by the raptor. A wave event was most likely to occur when the predator attacked at medium speed. Flash expansion occurred more frequently when the predator approached the flock at faster rather than slower speed and attacked from above rather than from the side or below. Flash expansion was often followed by split, but in many cases, the flock showed resilience by remaining intact. During a hunting sequence, the frequencies of different patterns of collective escape increased when the frequency of attack by the raptor was higher. Despite their complexity, we show that patterns of collective escape depend on the predatory threat, which resembles findings in fish. SIGNIFICANCE STATEMENT: Patterns of collective escape in flocks of starlings have always intrigued laymen and scientists. A detailed analysis of their complex dynamics has been lacking so far, and is the focus of our present study: we analysed video footage of hunting by falcons on flocks of thousands of starlings and show how patterns of collective escape (namely flash expansion, blackening, wave event, vacuole, cordon and split) depend on the preceding pattern and on details of attack. A higher frequency of attack during a hunting sequence resulted in a higher frequency of collective escape events. Flash expansion happened most often when the predator attacks at greater speed. A wave event was most likely when the raptor attacks at medium (rather than high or low) speed. These results provide a first quantitative approach to social complexity in collective avoidance of a predator.

Effect of social defeat in a territorial bird (Parus major) selected for different coping styles.

We addressed the questions (i) whether a social defeat triggers similar autonomic and behavioral responses in birds as is known from mammals and (ii) whether individuals that differ in coping style differ in their reaction to a social defeat. Adult captive male great tits (Parus major) from either of two different selection lines for coping style were used to test the effect of social defeat by an aggressive resident male conspecific on subsequent social and nonsocial behaviour, body temperature, breath rate and body mass. These parameters were measured 1 day before (baseline), immediately after and at Days 1 to 3 and 6 after the social interaction took place (Day 0). Social defeat decreased social exploration and increased body temperature substantially for at least 1 day in all birds. Breath rate and body mass were not affected. Birds belonging to the more aggressive and bolder line showed impairment in activity immediately after the social defeat. This is to our knowledge the first report showing that psychosocial stress in birds can have a similar impact as in rodents, but with a shorter recovery time. This might be due to species-specific differences in sensitivity to social stress, or to differences in the way social stress was induced.

Behavioural and nociceptive response in male and female spiny mice (Acomyscahirinus) upon exposure to snake odour.

Predator cues (both mammalian odour or avian vocalizations) are known to elicit fear-associated responses in rodents, including analgesia. In previous studies it was reported that spiny mice fail to show fear responses when presented with the calls of an owl. In order to test the hypothesis that this species (living in semiarid and rocky areas) may react to stimuli coming from reptilian predators, 40 sexually mature spiny mice (20 males, 20 females) were individually exposed to a small cylinder containing either fresh sawdust or snake odour. Behavioural changes (5 min before and 15 min after odour exposure) as well as the subsequent performance in a hot-plate test (50±0.5°C) were assessed. Results indicate that exposure to the odour of a sympatric terrestrial predator affected both behavioural and physiological responses of spiny mice. Upon exposure to snake odour both sexes showed significant changes in the patterns of inactivity, sniffing, grooming, sniffing the stimulus object (SO), withdraw reaction and in the frequency of somersaults. However, males increased the frequency of rearing, sniffing the SO, decreasing grooming more than females. No analgesic effect of odour exposure emerged; however, males showed significantly shorter latencies and higher frequencies of hindpaw licking compared to females.

Fast time-resolved Fourier-transform spectroscopy for the study of transient chemical reactions.

We describe the most recent implementation of the data acquisition system which we have developed for fast time-resolved (FTR) Fourier transform spectroscopy (FTS) and report spectra that were obtained by using this instrument. This FTRPTS data system operates in conjunction with any continuous-scan Michelson interferometer, giving it the capability to record many time-delayed spectra of a transient event, with a minimum time resolution of 1 µs. The sensitivity and the spectral resolution of the complete system are the same as those that would be obtained if the interferometer were used in conventional steady-state spectroscopy. To illustrate the performance of the FTRPTS system, we recorded emission spectra from the products of transient chemical reactions of H atoms with CF(3)Cl, CF(2)Cl(2), CFCL(3), and NO(2). These are laser-initiated reactions involving atoms with energies that correspond to a temperature of approximately 27,000 K and lifetimes of a few microseconds, but the FTRPTS system records the time evolution of their products with high signal-to-noise ratio.

Fecal corticosteroids in a territorial bird selected for different personalities: daily rhythm and the response to social stress.

In this study we tested the hypothesis that in a passerine bird (great tit, Parus major) individuals differing for coping strategies differ in the magnitude of the adrenocortical response to social stress as well. Furthermore, we aimed at characterizing daily rhythms in corticosteroid release before and after social stress. We used 16 males from either of two lines bidirectionally selected for different coping strategies (fast and slow explorers). Social stress was induced by confrontation with an aggressive resident male. Corticosteroid metabolites were analyzed in feces collected at 90-min intervals from 900 to 1630 h on a baseline day, on the day of the social conflict, and on the following day. In both days and in both lines levels varied with time of day in a robust rhythm with a peak in the first sample of the morning and a trough at the end of the light phase. This rhythm correlates with activity (perch hopping). An overall increase in levels relative to baseline day was observed between 30 and 140 min after the challenge. Birds of the less aggressive and more cautious line (slow explorers) showed a trend for a higher response compared to birds of the more aggressive and bolder line (fast explorers), which showed almost no response. On the day after the challenge the birds of the slow line exhibited significantly reduced corticosteroid secretion, probably due to an increased negative feedback. The results provide evidence for a physiological basis of different coping strategies in birds, emerging in response to social stress and with a pattern similar to that in other vertebrates.

Parental recycling of nestling faeces in the common swift.

Several hypotheses have been proposed to explain why parent birds eat their nestlings' faecal sacs. Among them, the parental nutrition hypothesis suggests that faeces may provide alternative food to parent birds, while the economic disposal hypothesis proposes that eating faeces represents an economical alternative to carrying them away when birds are engaged in parental activities. In both cases the elimination of faeces could prevent contamination of the nest (nest sanitation). In the present study, we tested these hypotheses on the common swift, Apus apus. The behaviour of parents at the nest was videotaped and analysed in relation to sex, brood size and nestling age. The parents regularly swallowed faecal sacs during the first 3 weeks after the nestlings hatched, but only occasionally thereafter until they fledged. Both sexes ingested sacs at a similar rate in the first week after hatching, and ingestion rate was directly related to the number of feeding visits. Females ate significantly more sacs than males overall. The parents consumed faeces after actively searching into and around the nest cup, also during brooding spells. However, brooding decreased with increasing brood size and nestling age, while faeces consumption increased. Our results are consistent with the hypothesis that parent birds ingest their nestling faeces to recycle water and nutrients, making the 'best of a bad job' during periods of high energetic requirements. In addition, ingestion of faeces may be an alternative strategy to delay hunger and to facilitate the allocation of food to the offspring. As the nestlings grew, parents ate fewer faecal sacs. The increased begging behaviour of the young appeared to be an important factor in determining the decline of faeces consumption, as it hindered parents approaching the nest. Copyright 1998 The Association for the Study of Animal Behaviour.