Dispersal pattern of black-billed magpies (PICA Hudsonia) measured by molecular genetic (RAPD) anlaysis

Auk, The, Jan 2001 by Wang, Xiao-Hong, Trost, Charles H

ABSTRACT.-Black-billed Magpies (Pica hudsonia) are a relatively sedentary corvid, with greater dispersal of females than males. To genetically confirm that dispersal pattern, 29 reproductively active adults were captured over two years and were scored for primer-specific random amplified polymorphic DNA (RAPD) bands (53 polymorphic bands in 1996 and 104 in 1997). In both years, we captured more previously banded males than females, and individuals recaptured were more often the heaviest nestling in their brood from the previous years. Genetic distances between all possible adult pairs were calculated and degree of genetic similarity between pair-wise comparisons was assessed using the Mantel test. In the pair-wise comparison of genetic distances, data for both years showed that genetic distances between males were significantly less (P 0.05) than were random pairs of adults. Group-distance analysis using the computer package RAPD also showed higher genetic similarity among males than among females. We suggest that a plausible hypothesis to account for the relatively high genetic homogeneity of the breeding male population compared to the female population is that a long-lasting intraclutch dominance hierarchy exists among siblings. That hierarchy is sufficiently long-lasting to influence the breeding population, resulting in alpha males remaining in the natal area, whereas lower ranking birds, including females, are forced to disperse. Received 13 September 1999, accepted 8 August 2000.

FEMALE-BIASED natal dispersal occurs in the majority of bird species (Greenwood 1980, Greenwood and Harvey 1982), especially for most species of corvids (Woolfenden and Fitzpatrick 1984, Strickland 1991, Marzluff and Balda 1992), except for White-throated Magpie-- Jay (Calocitta formosa), which has an extremely male-biased dispersal pattern (Langen 1996a, b). In some cooperatively breeding corvid species with helpers at the nest, such as Florida Scrub-Jay (Aphelocoma coerulescens), females disperse substantially farther than males, but neither sex tends to disperse more than a few territories from the birthplace (Woolfenden and Fitzpatrick 1984). A similar pattern occurs in another cooperatively breeding corvid with helpers, Pinyon Jay (Gymnorhinus cyanocephalus), where there are more female wanderers (i.e. dispersers in the flock) in spring than males (Marzluff and Balda 1989, 1992). Most females breed in their first year, whereas some males stay and help their parents. Strickland (1991) showed in Gray Jays (Perisoreus canadensis), another cooperative breeder (Waite and Strickland 1997) that about two-thirds of the juvenile "stayers" were males.

Black-billed Magpies (Pica hudsonia) are a social corvid and are relatively sedentary, with an average dispersal distance of only 1.2 territories reported for populations in England (Birkhead 1991). North American Black-billed Magpies do not have helpers at the nests. Instead, philopatry in magpies seems to be associated with territorial acquisition (Trost 1999). North American Black-billed Magpies are not territorial year round; after fledging, territorial boundaries break down and magpies are seen in large social flocks. Dominance hierarchies are found within those flocks; males are dominant over females, and surprisingly, hatching-- year birds are often dominant over adults (Reese and Kadlec 1984, Komers and Komers 1992, Trost and Webb 1997). Hierarchies among young males are strongly linear, whereas female hierarchies are much less stable (Moholt 1989, Trost and Webb 1997) and may contain triangles (Komers and Komers 1992). Presumably, a dominant male would "squeeze into" the natal territory if its parents or their neighbors were to allow it, but usually such juvenile males are forced to wait for an opening caused by death of a parent or neighbor (Trost 1999). The more submissive siblings, including females, tend to disperse farther, and presumably will only establish a territory if they encounter a reliable food source (Reese and Kadlec 1984, Trost 1999). Such a pattern of sex-- biased dispersal leads to specific predictions with regard to the degree of genetic similarity between sexes. Breeding males, due to their tendency to establish territories in or near natal sites, should show higher levels of genetic similarity than breeding females in those same populations.

Dispersal of birds or mammals from natal sites typically is quantified by using direct methods (Payne 1991, Stenseth and Lidicker 1992) such as banding, or mark and recapture. A potential bias of direct methods is the low probability of finding marked individuals once they leave the population (Peacock 1997). According to Koenig et al. (1996), chance of detecting dispersal events in an area larger than the diameter of the study area is virtually zero using direct methods. To compensate for those limitations, recent dispersal studies have used genetic methods as well as direct banding or marking and recapturing (Dobson 1994, Horn et al. 1996, Peacock 1997).