Bernd Leisler

A novel song parameter correlates with extra-pair paternity and reflects male longevity

Although elaborate bird song provides one of the prime examples of a trait that evolved under sexual selection, it is still unclear whether females judge the quality of males by attributes of their song and whether these song features honestly signal a mals genetic quality. We measured the ability of male dusky warblers Phylloscopus fuscatus to maintain a high sound amplitude during singing, which probably reflects an individuas physiological limitations. This new measure of singing performance was correlated with male longevity and with extra–pair paternity, indicating that females who copulated with better singers obtained ‘good gene’ for their offspring. Our findings are consistent with the idea that females assess male quality by subtle differences in their performance during the production of notes, rather than by the quantity or versatility of song. In addition, observations on territorial conflicts indicate that attractive males invest less in competition over territories because they can reproduce via extra–pair paternity.

Polygyny in the great reed warbler, Acrocephalus arundinaceus: a possible case of deception

SummaryPolygyny was studied for 1 year in a great reed warbler population breeding in southern Germany. Data on various parameters of male and territory characteristics were collected and subjected to multivariate statistical analysis in order to assess their relative importance in female choice. Differences in territory characteristics appeared to be more important (Tab. 1), but there was a strong correlation between male and territory characteristics (Fig. 2, 3). Although the evolutionary advantages of polygyny to males are obvious (Fig. 5), those to females are far from clear. Models based upon the polygyny threshold and sexy son hypotheses predict that females should gain evolutionary advantage in either the short or long-term. Our data did not confirm such predictions, and secondary females showed greatly reduced breeding success (Fig. 4). If females were able to assess the status of mated males, they should instead select unmated males. Great reed warblers are a migrant species with a short breeding cycle in which the male plays relatively little part. They defend large territories in reed beds where visibility is reduced. These factors may permit males to practice a form of deception, by moving and attracting a second female who has little chance to assess that he is already mated.

Ecomorphological predictors of natal dispersal distances in birds

1. Dispersal is one of the key ecological parameters but it is very difficult to quantify directly. As a consequence, empirical studies often ignore dispersal or use indirect measures. 2. Ringing data have previously been used to estimate the natal dispersal distances of 47 British passerine bird species. This provides an excellent opportunity to examine the potential of various indirect measures to predict natal dispersal distances in British birds. 3. We use a phylogenetic comparative framework and single- and multipredictor models including ecomorphological, behavioural or ecological traits to predict natal dispersal distance. 4. A multipredictor model that includes Kipps distance (a measure of wing tip length), bill depth and tail graduation explains 45% of the interspecific variation in natal dispersal distance. These morphological characters all relate to aerodynamics with stronger flyers dispersing further. 5. However, an index of migration is a strong (but less informative) correlate of dispersal distance and Kipps distance and bill depth are strong correlates of migration. Thus, we cannot disentangle whether these ecomorphological traits influence dispersal distance directly or whether the relationship between ecomorphology and dispersal is mediated through migratory behaviour. 6. Notwithstanding uncertainties regarding the causal links between dispersal distance and wing morphology, we suggest that two ecomorphological traits, Kipps distance and bill depth, may provide a useful surrogate.

Interspecific territorialism in reed warblers: a local effect revealed by playback experiments

Earlier playback experiments on the reed warbler Acrocephalus scirpaceus established a clear case of interspecific territorialism with a sympatric species in West Germany (Catchpole 1978), where male reed warblers responded aggressively to playback of recorded song from a marsh warbler A . palustris . Marsh warblers are locally abundant and regularly breed in adjacent territories where interspecific interactions are common. This is not the case in England and no response was obtained from a population of reed warblers tested with the same experiment . These and other results suggest that interspecific territorialism in Acrocephalus warblers is adaptive and can largely be explained by competition between closely related species with similar ecology . In particular, a species like the reed warbler which arrives back from migration first . and later finds its habitat invaded by a competitor, attempts to exclude the rival species . The relationship between interspecific territorialism and competition has been pointed out by early reviewers (Simmons 1951 ; Orians & Willson 1964) . Later Cody (1969, 1974) suggested a hypothesis of physical or vocal convergence as a special mechanism in the evolution of interspecific territorialism . However, Murray (1971, 1976, 1981) favours the view that interspecific aggression could occur through mistaken identity as misdirected intraspecific aggression . If a general failure to recognize closely related species is responsible for interspecitic aggression, then presumably different populations would show a similar response to playback of closely related species song . By demonstrating a clear differential response related to the presence of a sympatric competitor, the earlier experiment (Catchpole 1978) provided some evidence against the mistaken identity hypothesis . It was concluded that individual males learn to recognize and respond to the songs of congenerics who are regularly encountered in competitive situations. One weakness in the argument is that the experiments were carried out in different countries, and there might he some inherent differences between British and continental populations of reed warblers which could explain the differential response obtained . Reed (1982) has since reported on a differential response between two unrelated competitors . the chaffinch Fringilla coelebs and the great tit Parus mayor, on the mainland and islands off Scotland . In this case there can be no question of confusion in morphology or song structure, and Reed explains his results by the different ecology and competitive relationships on the mainland and islands . We now report another case concerning the reed Short Communications 299

Does song reflect age and viability? A comparison between two populations of the great reed warbler Acrocephalus arundinaceus

Song complexity is often regarded as a sexually selected trait that reflects the overall quality of a male. In many passerine species, old males possess larger song repertoires than younger males. This may be either because individual males improve their performance as they get older (longitudinal increase) or because poor singers have reduced viability and, hence, are underrepresented in old age classes (cross-sectional increase). We studied the age dependence of repertoire size and other song traits in a German and a Swedish great reed warbler (Acrocephalus arundinaceus) population. We found marked differences between longitudinal and cross-sectional approaches, as well as between the two study populations. In the German population, we found that syllable switching, a measure of immediate versatility and strophe length, increased with age in a cross-sectional analysis. This was not because birds improved with age (longitudinally) but because syllable switching was positively correlated with male longevity. However, in Sweden, syllable switching seemed to be unrelated to age and longevity. In the Swedish population, individual males increased their repertoire size as they got older (longitudinal increase), but this did not happen in the German population. Hence, two populations, even when belonging to the same subspecies, may differ in whether or not they show delayed song maturation.

Ecological constraints on the evolution of avian brains

Birds have brains that are comparable in size to those of mammals. However, variation in relative avian brain size is greater in birds. Thus, birds are ideal subjects for comparative studies on the ecological and behavioral influences on the evolution of the brain and its components. Previous studies of ecological or behavioral correlates in relative brain size were mainly based on gross comparisons between higher taxa or focussed on the relationships between the sizes of specific brain structures and the complexity of different tasks. Here we examine variation in dimensions of the braincase, relative overall brain size and size of its components, in reference to one general ecological and behavioral task: migration. We used data from three lineages of closely related species (14 Acrocephalines, 17 Sylvia and 49 parulid warblers). Within each group, species vary in their migratory tendencies. We found that species migrating long distances have lower skulls and smaller forebrains than resident species. We discuss four hypotheses that could explain smaller forebrain sizes, and suggest relevant taxa to use in comparative analyses to examine each of these hypotheses:Brain size is energetically constrained. Contrasts can be made not only between migrants and residents, but also between birds in habitats with high and low levels of available food.Brain size is developmentally constrained; birds with short growing periods should have smaller forebrains. Comparisons need to be made between birds living in habitats with long and short breeding seasons.Bill adaptations for foraging constrain braincase dimensions. Further analyses would need to be done on groups with high variation in bill dimensions and foraging modes.Birds with small brains have to migrate to compensate for low behavioral flexibility. Contrasts between members of families containing tropical residents and migrants need to be made.We also raise the question of whether only those parts of the brain are reduced that are most dispensable and whether brain size reduction limits foraging skills and social competence.

Morphological Consequences of Migration in Passerines

Migration, like any other ecological factor, shapes the morphology of birds. Most ecomorphological studies of migration have mainly considered external features (e.g. Cox 1968; Gaston 1974; Greenberg 1981). Winkler and Leisler (1992) partly overcame this deficit and related skeletal and myological measurements to migration in several passerine and non-passerine groups. Their main findings were: aspect ratio was higher in migrants in all groups studied; migration apparently set upper boundaries on the development of the hind limbs, especially in small birds. Similarly, Calmaestra and Moreno (1998a, 2000) analyzed forelimb muscles and skeletal elements that may distinguish migrant and resident passerine species by pairwise comparisons in 10, and 15, passerine genera. After correcting for phylogeny, they found, not surprisingly, that migrants possess larger sternal dimensions and longer coracoids, thus offering greater surface for attachment of the main flight muscles. Hindlimb elements were not related to migration but rather to habitat use.

The evolution of polygamy in european reed warblers of the genus acrocephalus: A comparative approach

A comparative survey of mating systems in six European Acrocephalus species revealed that two of the species can be classified as polygamous and four monogamous. Several correlations were then found between mating system and various aspects of behaviour, ecology and morphology. Males of polygamous species invested little or nothing in parental care, whereas males of monogamous species shared it equally with females. Males of polygamous species defended large territories in high productivity marshland ecotones rich in arthropod food. Monogamous males defended smaller territories in less productive marshland. Females of polygamous species fed their young on larger prey collected within territory near the nest. In monogamous species, smaller prey was collected from longer flights often outside territory. Polygamous species also had larger bills as an adaptation to take larger prey. Males of polygamous species had short, simple songs used in territorial defense, whereas males of monogamous species had long, c...

EVOLUTION OF BREEDING SYSTEMS IN ACROCEPHALINE WARBLERS

Abstract Comparative analyses constitute an important complement to studies of adaptive behavior. Previous studies of avian mating systems considered the role of paternal care and habitat type on the evolution of polygyny. We extended those studies and included in our analyses the role of habitat quality, as characterized by food supply. Species in the monophyletic lineage of acrocephaline warblers (Acrocephalus, Chloropeta, Hippolais) are widely distributed, inhabit a variety of different habitats, and show a variety of breeding systems. We present a phylogenetic analysis of parental care and mating system characteristics in relation to ecological traits in 17 species. On the basis of a molecular phylogeny, we reconstructed patterns of changes from social monogamy to polygyny, and in paternal brood care. Specifically, we analyze the coevolution of brood care participation of males and social system, and how it relates to habitat quality. Furthermore, we assessed the phylogenetic inertia of mating systems. We found support for the hypothesis that change to highly productive habitats was associated with a greater emancipation of males from brood care, and with polygyny and promiscuity. Poor habitats, on the other hand, were associated with monogamy and the occurrence of helpers. In contrast to some morphological characters, mating systems appear to be phylogenetically labile.

MALE REPRODUCTIVE SUCCESS IS CORRELATED WITH BLOOD PARASITE LEVELS AND BODY CONDITION IN THE PROMISCUOUS AQUATIC WARBLER (ACROCEPHALUS PALUDICOLA)

Abstract The Aquatic Warbler (Acrocephalus paludicola) has a unique mating system. Males are free of any parental duties and can fertilize many females during a single breeding season. Females also usually copulate with more than one male, and nestlings in a single nest may be sired by as many as five males. Paternal success of 33 Aquatic Warbler males in a Polish population was determined by microsatellite polymerase-chain-reaction analysis. Males infected by blood parasites (trypanosomes) sired fewer offspring, weighed less, and probably arrived later to breeding grounds than uninfected males. Number of nestlings sired by a male correlated positively with its fat deposits and wing length. These findings indicate that male body condition directly influences paternal success. Number of sired nestlings per individual male ranged from one to eight. Reproductive success among males was uneven, with six males (18%) fathering 44% of nestlings in the study area. Males with high reproductive success arrived at the breeding grounds earlier in spring than males with low reproductive success.