Variation in Lapwing Breeding Densities

The goal of this project is to study two questions related to colony formation in birds. The first question is why do many lapwings (Vanellus vanellus) breed in nesting aggregations while others breed solitarily? The second is whether male display flight and mobbing performance advertises a male`s ability to deter predators, as previously proposed, or whether it is a general indicator of male quality. One clear benefit of high density breeding has thus far been identified: lapwings mob avian egg predators such as crows much more effectively when they breed in clusters and thereby significantly reduce nest predation. The existence of this benefit creates the enigma of why many lapwings breed away from clusters when suitable nesting habitat often exists near conspecifics. To answer this question we will examine potential costs of high density breeding. One unexamined potential cost is that nesting clusters attract large mammalian predators that are undeterred by communal defense, a hypothesis we will test experimentally. Extra-pair paternity may be a cost of breeding in clusters to some males and a benefit to others, a question which we will study by performing the first DNA fingerprinting study of the species. We will combine paternity data with behavioral observations to determine whether certain males increase their reproductive success from high density breeding at the expense of their neighbors. We will also determine whether females pursue extra-pair copulations with high quality males. Male quality is linked to colony formation by the `hidden lek` hypothesis which predicts that low quality males aggregate around more attractive males to obtain mates. This will be tested by examining whether singly-paired males obtain mates sooner by nesting near more attractive polygynous males or by nesting solitarily. In tandem, we will experimentally examine another proposed indicator of male quality, namely the number of nesting scrapes constructed by males in their breeding territories.

Our project has focused on lapwings (Vanellus vanellus) a shorebird that many Europeans are familiar with because of their wide geographic range and conspicuous call flights on spring days. Many lapwings breed in socially monogamous pairs in which the male and female share parental care such as incubating and keeping predators away from eggs and fledglings. Our Swedish population on Oland, Island in the Baltic Sea, also comprised many polygynous breeders where one male would reproduce with two or three (and in one case four) females in his territory. This provided us with an excellent opportunity to study polygyny, which has not been explained despite many studies over the last 40 years. The conventional theory for explaining variation in harem size is that polygynous males are higher in quality in than monogamous or unpaired males. This might lead to polygynous males being able to attract females better than monogamous males. Our study surprisingly contradicts this assumption. We found that polygynous and monogamous males were the same in numerous traits, including body size and condition, leg coloration (which is an indicator of condition), date of arrival at the breeding areas and annual survival. Lapwings are conspicuous for the long crests on top of their heads which is much longer in males than females and appears to be an ornament to attract females. However, there was also no difference in crest length between polygynous and monogamous males. We also examined an important criterion of male mating success, namely whether the two types of males differed in the number of offspring in their nests that another male had sired. Ever since the invention of genetic analyses such as DNA fingerprinting, behavioral biologists have discovered that extra-pair (i.e. "adulterous") matings are common in birds. We performed the first genetic analyses of lapwings by obtaining blood samples from approximately 100 families over three years. We found exceptionally high levels of extra-pair paternity for a shorebird, with 19% of broods containing at least one chick that did not belong to the resident male. However, there was also no difference in the frequency with which monogamous and ploygynous males lost paternity, again arguing against a difference in quality. So the question had remained, what maintains variation in harem size? We then examined body sizes of both sexes and discovered that monogamous males were paired to females approximately their own size while the first female of polygynous males were significantly smaller than their mates. Because first females attempt to aggressively drive away their mates` potential second mates, our finding suggests that smaller females are unable to prevent their male from aquiring a second female, whereas larger females can. Thus we propose that variation in lapwing harem size is mediated by sexual conflict.