Johan G. van Rhijn

Being honest about one's intentions: An evolutionary stable strategy for animal conflicts

Abstract In several vertebrate species it has been demonstrated that individuals recognize each other, and in many other species (even invertebrates) the conditions for it seem to be fulfilled. Individual recognition might therefore be a wide-spread phenomenon in the animal kingdom. This has consequences for the settlement of conflicts between animals: individuals may use information about the outcome of earlier conflicts even if the individuals do not differ in fighting ability. Conflicts will then be asymmetric, and bluff is very unlikely. Possible consequences of individual recognition for the settlement of conflicts are studied by means of simulation. Four strategies will be considered: (1) “Retaliator” (based on Maynard Smiths models and used as a control condition, (2) “Threat-right” (threatens towards a submissive and will follow with attack if the submissive does not retreat; retreats from a threatening or attacking dominant), (3) “Attack-right” [as (2), but without threatening], and (4) “Threatdominance” [as (2), but with a low probability of threatening and attacking a dominant]. If the knowledge about strength or dominance of the other individuals is perfect, the “Threat-right” strategy (thus a warning before a real attack) turns out to be most successful under a wide variety of conditions. If that knowledge is not perfect (during the learning phase), other strategies can yield better results. The effects of a number of possibilities to settle dominance will be considered (outcome first escalated conflict, last escalated conflict, etcetera). These possibilities will be related to the strategies of settling conflicts, to the expected number of conflicts within the pair of individuals, and to variations in the strength of an individual.

Mate guarding as a key factor in the evolution of parental care in birds

The evolution of male parental care in vertebrates with internal fertilization must have been preceded by a stage in which males profit by staying with the female after copulation. This paper discusses the results of a series of computer simulations to determine the pay-off to post-copulatory mate guarding under various conditions. Guarding is promoted by asynchrony in fertility of the females, high copulation frequencies of females, preference of females for males using the guarding strategy, and mate fidelity of guarded females. Moreover, it is demonstrated that, under several conditions, apparently those operating in a natural environment, the success of the guarding strategy is inversely related to its frequency in the population. This implies that both the guarding and the non-guarding strategy can be maintained in the same population. This phenomenon is put forward as a key factor determining the pathways in the evolution of parental care.

UNITS OF BEHAVIOUR IN THE BLACK-HEADED GULL, LARUS RIDIBUNDUS L.

Abstract The present paper makes a plea for the development of methods for clssifying behaviour in a systematic way, by which relevant sources of information are less likely to be neglected than in the conventional intuitive way of classifying behaviour. Film-analysis of the behaviour of the black-headed gull revealed that more details can be distinguished than was suggested by earlier descriptions. For a study on communicastion between individuals of this species, a new objectigve classification has been designed, in which special attention is paid to the position of the different parts of the body. It is based on a very large number of categories of behaviour. Most of these categories occur only a few times, and consequently the statistical treatment of the material is extremely difficult. The number of categories has therefore been reduced by joining categories which are similar with respect to form, motivation, and information content. This final classification is compared with earlier ones, and advantages are discussed, and examplified by two postures whose unique properties were revealed by the present procedure.

On the Mechanism of Mate Selection in Black-Headed Gulls

Processes leading to the establishment of social relationships are the main topic of the present study, which is based on observations of black-headed gulls kept in experimental groups in aviaries. Among these birds various kinds of social relationships were established, such as heterosexual monogamous pair-bonds, polygynous associations, and male-male relationships. Some of the factors which give rise to the formation of such relationships could be studied, because it was possible to watch all individuals during their whole life, and during all stages of their reproductive cycle. Quantitative data were collected on the influence of sex, age, reproductive state, physical condition, and various social factors on the incidence of social relationships. On the basis of these data it could be concluded that males and females play strongly differing roles. Unmated males advertize randomly. Unmated females direct their courtship to particular individuals, belonging to the category of advertizing unmated males, or of the active mated males. The final decision for pair-formation involves an acceptance by the approached male. Unsuccessful unmated males may adopt the female strategy, and thus establish homosexual social relationships. Mated males do not initiate courtship towards other females, but they may be courted by them. Mated females sometimes display social interest for other males. The main criteria for mate choice were advanced age, early nuptial moult, small body-size, and, in particular, social experience with one another. In contrast to our expectation, body weight and matching of nuptial moult and physical condition between partners were not important in our birds. The significance of the criteria age and moult may be directly related to fitness. The preference for small males had earlier (VAN RHIJN, 1985) been interpreted as a mechanism preventing polygyny, but additional data provide some inconsistencies with that idea. Finally, the significance of social experience with each other has been related with considerable investments of prospective mates before establishing a pair-bond, and with an increase of synchronization between mates as a result of long-lasting pair-bonds. Female-female pairing among some wild larids has been interpreted as a continuation of early bonds between nest companions.

Black-Headed Gull or Black-Headed Girl? On the Advantage of Concealing Sex By Gulls and Other Colonial Birds

Mechanisms for sex-recognition in the Black-headed Gull, Larus ridibundus, were analysed in order to understand the occurrence of homo-sexual male pairs among captive members of this species. No discontinuities between males and females could be detected in either external characteristics or behaviour. Although most individuals select a mate of the opposite sex, there is no evidence that individuals which do not belong to the same pair are able to determine each others sex. The adaptiveness of concealing sex for colonial monogamous species is discussed. Its significance was tested in a group of captive gulls. Small female-like males appeared to be most successful in reproduction. The development of polygynous mating systems seems to be opposed by a preference of females for caring males.

Geographic variation in morphometrics, molt, and migration suggests ongoing subspeciation in Pacific golden-plovers (Pluvialis fulva)

ABSTRACT Breeding Pacific Golden-Plovers (Pluvialis fulva) cover 140 longitudinal degrees of Arctic tundra. Having examined 557 museum skins from across this huge distributional range, we conclude that Pacific Golden-Plovers breeding in Alaska are structurally larger than those breeding in Siberia, especially in wing length. Birds from Alaska also have more pointed wings and almost always postpone the initiation of primary molt until they reach their winter quarters, whereas many Siberian birds start primary molt in the breeding areas. These differences could have been favored by the longer transoceanic flights followed by the Alaskan populations to nonbreeding destinations in the Pacific Islands. We propose that the Alaskan and Siberian breeding birds be distinguished as distinct flyway populations to be used in conservation assessments by the international conservation community.

Diversity of nuptial plumages in male ruffs Philomachus pugnax

Nuptial ornament diversity was studied in 1814 individual male Ruffs that were caught in their spring staging areas in Friesland, The Netherlands. Ornaments (hereafter called plumages) comprised of a ruff, two head tufts and facial wattles. Individual feathers were found to be plain (white, black or one hue), or patterned with black and only one hue. Patterns of feathers varied modestly within males, and greatly between males. The colour of a males ruff plus head tufts consisted of black and/or white and/or only one other hue. Ruff, head tufts and facial wattles differed In coloration and pattern between individuals. Using seven criteria we counted 801 different plumage variants. Nevertheless, except for wattle colour, characteristics did not combine at random. Some combinations of characteristics, such as a white ruff with white head tufts, occurred much more often than expected by chance. Other combinations, such as a white ruff with black head tufts, a black ruff with white head tufts, and a regular ruff pattern with an Irregular pattern, were rare. Mostly there was conformity between ruff and head tufts: they were identical or had reversed primary and secondary colours. Nuptial plumage characteristics were only weakly associated with body size. Some associations between nuptial plumage and the timing of moult were remarkable: only white males and males with a plain ruff pattern tended to have completed moult by the time of their capture In April. We discuss our findings, including the extreme diversity of nuptial plumages among individuals, In the light of development, genetics and function.

In Tundra Plovers the Frequency of Inner Flight Feather Replacement Varies with Length of Long-Distance Flights

The repair jobs that birds have to do to maintain high quality plumage take energy and time, so should be under intense selection. Recently, we have shown that secondary moult in the Eurasian Golden Plover Pluvialis apricaria is incomplete, irregular and asymmetric between wings, and argued that this reflected their ‘relaxed’ migratory habits. On the basis of this hypothesis, we predict that relatives of this species that have to make long flights between breeding areas and winter quarters would invest more in the moult of secondaries. To test this we collected data on moult patterns in two populations of Pacific Golden Plovers Pluvialis fulva (Siberia and Alaska), on American Golden Plovers Pluvialis dominica (Alaska) and Grey Plovers Pluvialis squatarola. Data were from the breeding, staging (Mongolia and Indiana) and wintering (Uruguay) areas. We found patterns consistent with our prediction: Pacific Golden Plovers from Siberia, and Grey Plovers have a similar moulting pattern to Eurasian Golden Plovers. Although both go to remote winter quarters, they migrate mainly over land, probably making short or moderate flights, as they are able to stage frequently. In contrast, the Alaskan populations of Pacific Golden Plovers and American Golden Plovers tend to renew all their secondaries from their second wing moult onwards. In line with their moult patterns, these two populations are known to make very long non-stop flights between breeding and winter areas. We argue that irregular moult and the partial renewal of secondaries is a primitive character in all four species. The full replacement of secondaries in the Pacific Golden Plovers and American Golden Plovers from Alaska may be considered as an adaptation for very long demanding flights. Regular moult patterns of secondaries, as found in many other shorebirds, could be a further refinement of this adaptation.