Konrad Halupka

Partial nest predation in an altricial bird selects for the accelerated development of young

In response to disturbance young of altricial open-nesting birds may leave the nest several days before the usual age for nest leaving. In Meadow Pipits Anthus pratensis, about 60% of broods attacked in the last quarter of the nestling period survived predation attempts. The probability that some nestlings would escape from nest predators increased with nestling age. The growth rate and number of nestlings did not predict whether or not the brood would be attacked by a predator. However, the probability that the attack would result in destruction of the entire brood decreased with increasing growth rate. Other sources of mortality (e.g. starvation, scramble competition) also affected slowly growing broods. It is concluded that the selection for accelerated nestling growth and earlier functional maturity acts via elimination of poorly nourished broods.

Male whitethroats, Sylvia communis, advertise their future contribution to parental care

Summary In altricial species participation of males in parental care enhances reproductive success of females. How does a female select a mate who will allocate time and energy to parental effort? In the whitethroat Sylvia communis, a socially monogamous bird, parental performance of males might be predicted on the basis of elaborated song flights displayed in courtship. The correlation analysis revealed that males which advertised intensively needed less time to attract a female and also their parental performance was better compared to males that produced cheaper signals. In the subsequent experiment, we handicapped males by increasing their body mass by 5% with a weight attached to tail feathers. Males in the treatment group reduced the proportion of showy song flights significantly more than controls and their mating success was significantly lower. We conclude that song flights in whitethroats honestly signal male quality and that the signalling system depends on a dynamic handicap trait which responds to a relatively small change in the male.

Coping with Shifting Nest Predation Refuges by European Reed Warblers Acrocephalus scirpaceus

Predation, the most important source of nest mortality in altricial birds, has been a subject of numerous studies during past decades. However, the temporal dynamics between changing predation pressures and parental responses remain poorly understood. We analysed characteristics of 524 nests of European reed warblers monitored during six consecutive breeding seasons in the same area, and found some support for the shifting nest predation refuge hypothesis. Nest site characteristics were correlated with nest fate, but a nest with the same nest-site attributes could be relatively safe in one season and vulnerable to predation in another. Thus nest predation refuges were ephemeral and there was no between-season consistency in nest predation patterns. Reed warblers that lost their first nests in a given season did not disperse farther for the subsequent reproductive attempt, compared to successful individuals, but they introduced more changes to their second nest sites. In subsequent nests, predation risk remained constant for birds that changed nest-site characteristics, but increased for those that did not. At the between-season temporal scale, individual birds did not perform better with age in terms of reducing nest predation risk. We conclude that the experience acquired in previous years may not be useful, given that nest predation refuges are not stable.

THE FREQUENCY AND TIMING OF COURTSHIP AND COPULATION IN BLACKBIRDS, TURDUS MERULA, REFLECT SPERM COMPETITION AND SEXUAL CONFLICT

In a long-term field study we estimated the number of copulations per pair in blackbirds at 24.2 in a single reproductive cycle. Thus it seems unlikely that the only function of copulation behaviour is just to guarantee fertilization. We considered three other hypotheses which might explain frequent pair copulation: (1) as an effect of sperm competition, (2) cooperation between pair members aiming at increasing joint production of offspring; or, (3) sexual conflict resulting in female manipulation of allocation of time and energy of the male. We found no positive correlation between the copulation frequency and reproductive performance, a result that essentially falsifies the cooperation hypothesis. Only two extra-pair copulations were recorded. However, attempts at forced extra-pair copulation were very common and a high pair copulation rate might have evolved in response to sperm competition. Alternately, relatively high copulation rate in the pre-fertile stage of the reproductive cycle and female control over timing and frequency of copulation (via solicitation of copulation and/or rejection of courtship by the male) also supports the sexual conflict scenario. We suggest that our findings support the idea that the female prolongs the period of sexual activity to induce her partner to guard her (which reduces sexual coercion from other males) and to monopolise his parental effort (the male while mate guarding cannot pursue extra-pair activity or search for better mating options).

Spreading information in a network of interacting neighbours.

Dispersed individuals can coordinate the onset of life history events, like reproduction or migration, on a large (population) spatial scale. However, the mechanism of this synchronisation has not yet been identified. In many species signals produced by one individual stimulate signalling activity of immediate neighbours. I propose that such local focuses of signalling could transform into waves propagating in space. This hypothesis predicts that signalling self-organizes into bursts, because neighbours tend to enter activity and refractory periods together. Temporal characteristics of such pulses should be more similar in locations proximate to one another than in distant ones. Finally, denser populations should produce relatively more complex wave patterns, since the number of propagating waves is proportional to the number of individuals. These predictions were tested in an analysis of time series of numbers of territorial songs in chaffinches, Fringilla coelebs, and the results supported the hypothesis. Time series of singing activity had memory of their past states: they were autoregressive processes with spectra in which low frequency oscillations predominated. The degree of similarity in two synchronously sampled time series, measured as a Euclidean distance between their spectra, decreased with the increasing physical distance of sampling spots and the number of signalling males. It is concluded that networks of interacting neighbours may integrate populations synchronising life cycles of dispersed individuals.

Song repertoire is not affected by stress in an adult male songbird, the Whitethroat Sylvia communis

Summary It is believed that bird song has evolved as a reliable signal of quality of displaying individuals. Recent research has focused on costs of development of complex song. In the present paper we test if the acquired repertoire size is costly to maintain. We compared changes in song structure in male Whitethroats (Sylvia communis) after 48 h exposure to a stressor (5% body mass weight attached to the tail feathers) vs. changes observed within the same time interval in the control group. The strophe length was marginally significantly shorter in the handicapped males comparing to controls. However, the repertoire size (i.e., a measure of diversity of different song elements) remained intact in both groups. We concluded that the song repertoire in Whitethroats is a static secondary sexual trait. A review of literature has revealed no convincing examples of decreasing repertoire size in adult male songbirds. Further research is needed to improve our understanding of evolutionary and proximate mechanisms maintaining the stability of song repertoires.

Alarm calls and chick reaction: comments on Kleindorfer et al. (1996)

Correspondence: K. Halupka, Department of Avian Ecology, University of Wroclaw, Sienkiewicza 21, 50335 Wroclaw, Poland (email: halupka@biol.uni.wroc.pl). In a study of relationships between alarm calling by parents and the reactions of nestlings in the moustached warbler, Acrocephalus melanopogon, Kleindorfer et al. (1996) concluded that the antipredator response of chicks is the proximate cue for adult alarm calls. In this note we review sampling errors in the study and the reasoning behind the chick reaction hypothesis. The most serious problems in Kleindorfer et al.’s paper are pseudoreplication and lack of control for potential confounding factors. In 1991–1994, during 5 months of the breeding season, they made 191 1-h observations of 33 nests. The data (frequency of alarm calling) were treated as independent and pooled without any control for the influence of the nest (parent birds, nest site) advancement of the breeding season and year (all these factors could affect alarm-calling frequency: Montgomerie & Weatherhead 1988). Kleindorfer et al. used 23 nests to perform 250 experiments (observations of the parental and offspring reactions to three species of predators). Thus, on average, there were 10.9 tests per nest and each predator was exposed more than three times at the same nest. However, the data were treated as if they were independent. The following passage (Kleindorfer et al. 1996, page 1201) indicates another problem with sampling. ‘The cases ‘‘without alarm calls’’ include approaches (. . .) to three nests where alarm calls began after 1 min at the nest (these three nests are repeated entries in the ‘‘with call’’ category, because later calls elicited chick jumping).’ Thus, the data set was ‘enlarged’ by using invalid definitions of displays: the same parent birds in the same test could be both active and silent and the same offspring could both jump out and remain in the nest (data prepared in this way were

Group foraging by wintering rooks: combined effects of safety and energy requirements

We report the results of feeding experiments with free-ranging gregarious corvids, rooks (Corvus frugilegus), at varying ambient temperatures and risk of interruption. Feeding behaviour depended on the photoperiod and the interaction between ambient temperature and safety of the feeder. Birds did not trade off safety for increasing food intake; instead their behaviour became more risk-averse during short photoperiods or low ambient temperatures, when energy requirements were presumably the highest. We propose that rooks are not severely limited by food supply and, when environmental conditions deteriorate, can quickly increase body reserves. The added fat load, however, handicaps their ability to escape from a danger, thus increasing risk aversion. Only the rooks which initiated foraging were sensitive to safety of the feeder and similar reactions were not observed among birds which joined later. We suggest that rooks, while foraging gregariously, rely on the ‘safety in numbers’ effect, which enhances feeding success. Some other observations also supported this hypothesis. Thus, food intake by small groups of foragers was higher on ‘safe’ than on ‘risky’ feeders, but this pattern was reversed with increasing group size. This suggests that individuals in a large group can reduce scanning for predators and focus their attention on collecting food. An increased feeding rate allows the time exposed to danger to be shortened. Foraging behaviour of birds showed greater synchronisation at risky places than at safe ones. This might be a reaction to the increased costs of feeding, which is predicted by theoretical models of group foraging by selfish individuals.

The effect of climate change on the duration of avian breeding seasons: a meta-analysis

Many bird species are advancing the timing of their egg-laying in response to a warming climate. Little is known, however, of whether this advancement affects the respective length of the breeding seasons. A meta-analysis of 65 long-term studies of 54 species from the Northern Hemisphere has revealed that within the last 45 years an average population has lengthened the season by 1.4 days per decade, which was independent from changes in mean laying dates. Multi-brooded birds have prolonged their seasons by 4 days per decade, while single-brooded have shortened by 2 days. Changes in season lengths covaried with local climate changes: warming was correlated with prolonged seasons in multi-brooded species, but not in single-brooders. This might be a result of higher ecological flexibility of multi-brooded birds, whereas single brooders may have problems with synchronizing their reproduction with the peak of food resources. Sedentary species and short-distance migrants prolonged their breeding seasons more than long-distance migrants, which probably cannot track conditions at their breeding grounds. We conclude that as long as climate warming continues without major changes in ecological conditions, multi-brooded or sedentary species will probably increase their reproductive output, while the opposite effect may occur in single-brooded or migratory birds.