Paweł Ręk

No male identity information loss during call propagation through dense vegetation: the case of the corncrake.

Individually specific acoustic signals in birds are used in territorial defence. These signals enable a reduction of energy expenditure due to individual recognition between rivals and the associated threat levels. Mechanisms and acoustic cues used for individual recognition seem to be versatile among birds. However, most studies so far have been conducted on oscine species. Few studies have focused on exactly how the potential for individual recognition changes with distance between the signaller and receiver. We studied a nocturnally active rail species, the corncrake, which utters a seemingly simple disyllabic call. The inner call structure, however, is quite complex and expressed as intervals between maximal amplitude peaks, called pulse-to-pulse durations (PPD). The inner call is characterized by very low within-individual variation and high between-individuals difference. These variations and differences enable recognition of individuals. We conducted our propagation experiments in a natural corncrake habitat. We found that PPD was not affected by transmission. Correct individual identification was possible regardless of the distance and position of the microphone which was above the ground. The results for sounds from the extreme distance propagated through the vegetation compared to those transmitted above the vegetation were even better. These results support the idea that PPD structure has evolved under selection favouring individual recognition in a species signalling at night, in a dense environment and close to the ground.

Functionally similar acoustic signals in the corncrake (Crex crex) transmit information about different states of the sender during aggressive interactions.

We combined playback experiments with hormonal manipulations to study the information content of acoustic signals during aggressive interactions between male corncrakes. During territorial conflicts, fights are uncommon, but the intensity of signaling usually increases. Such signals can be temporally and contextually associated with many aggressive behaviors and most likely function as threats or as indicators of the senders quality or motivation. However, such correlational data are unsatisfactory for the proper interpretation of the function and information content of signals. Experimental tests are required to determine whether signals and aggressive behaviors are controlled by common or independent mechanisms. In our experiment, we assigned subjects to four groups: testosterone-implanted birds, flutamide-implanted birds, birds with empty implants, and non-captured control birds. Males produced two types of calls (quiet soft calls and loud broadcast calls), both of which are known to be reliable predictors of aggressive escalation. When testosterone action was blocked with flutamide, males significantly limited the amount of time spent close to the playback speaker and stopped responding to playback with soft calls. Broadcast calling was unaffected by the blockage of testosterone. Conversely, increased levels of testosterone neither affected calling nor the time spent near the speaker, indicating a permissive, rather than a graded effect of androgens. We concluded that, despite the seemingly similar function, both signals may transmit information about different states of the sender; soft calls seem to imply a threat of force, while broadcast calls appear to be more similar to an announcement, which is only indirectly associated with a males aggressive behavior.

Are Changes in Predatory Species Composition and Breeding Performance Responsible for the Decline of Coots Fulica atra in Milicz Ponds Reserve (SW Poland)

Abstract. Both native and non-native predators should strongly affect resident fauna. Nevertheless, because of a lack of defensive mechanisms in potential prey, the influence of non-native predators should have longer-lasting and more deleterious repercussions. The breeding ecology of the Coot was studied in the Milicz Ponds reserve and compared with data from 20 years earlier. In the meantime, non-native, mammalian predators (American Mink Mustela vison, Raccoon Dog Nyctereutes procyonoides and Raccoon Procyon lotor) turned up in this area, while the numbers of Hooded Crow Corvus cornix, the main predator of Coot nests, decreased. Compared with 1980–1982, the number of Coots in 2002–2003 dropped by more than half and mean clutch size decreased. Breeding success and the number of hatchlings per pair remained unchanged; in the 1980s, however, Coots renested more frequently, there was greater nesting synchrony and breeding seasons were demonstrably shorter. Moreover, although predation still remained the main cause of losses, its numbers were decreasing. Coots nesting on islands were the most successful, which was due to the weaker predation on the part of Crows. In contrast, the mammalian (non-native) predators did not appear to play a significant role in the breeding success in 2002–2003. Nevertheless, taking into account the breeding parameters of the Coot population and the nature of the relationship between mammalian/bird predators and Coots, it does seem that the low density of Coots in 2002–2003 was a reaction to the pressure of mammalian predators.

Does mating experience of male house crickets affect their behavior to subsequent females and female choice

Male mating experience was shown to play an important role in settling conflicts between males; however, little is known about whether and how prior access to females influences male behavior during intersexual interactions and female choice itself. Here, I experimentally test this relationship in the house cricket (Acheta domesticus) by combining one-on-one interaction between the male and female with direct comparison of males by the female, but precluding aggression between males. I found that solitary males were more active during subsequent courtship displays than paired males, suggesting the detrimental effect of mating on courtship performance. At the same time, females spent significantly more time close to solitary males or playbacks of males natural courtship songs, and responded positively to the condition of males, ignoring body size of males. In contrast, females responded similarly to computer-modified playbacks of courtship songs of solitary and paired males with standardized rate of phrases and amplitudes; however, when females were additionally allowed to contact with anesthetized males they spent more time close to bigger males, irrespective of the acoustic parameters of courtship songs. These results show that although females were able to differentiate between many behavioral and morphological characteristics of males, including voluntary and intrinsic ones, they preferred traits conditional upon the costliness of males displays. In addition, mating experience appeared to be a crucial factor in the choice of a particular costly mating strategy by males.

Acoustic location of conspecifics in a nocturnal bird: the corncrake Crex crex

Although the use of sounds in spatial orientation is widespread among animals, only a few groups advanced such specific adaptations as echolocation. In contrast, practically all animals and night-active species in particular, must occasionally orient themselves relative to invisible but audible objects such as a hidden rival or predator. In this study, I would like to determine the impact of locating which involves the use of acoustic parameters of sender’s vocalisations by receivers and changes of positions and triangulation of sender’s vocalisations by receivers in estimating the distance to the sender during night-time territorial interactions of the corncrake (Crex crex). Males were subjected to two kinds of stimuli: approaching one, imitating the change of the distance of the calling intruder toward the focal male while keeping the direction constant, or stationary stimuli, involving acoustic stimulation with no motion. Although males subjected to approaching stimulation moved longer distances, in both stimuli groups, males moved predominantly toward or out of the playback speaker, and only occasionally made sideway movements. However, the results gave no evidence of corncrakes moving specifically in order to locate the source of the sound; they suggest that males moved toward or away from the already located sound. The fact that males moved longer distances in response to approaching than stationary stimuli indicates that they were able to perceive the change of the distance to the playback speaker based only on structural parameters or amplitude of the calls played.