Peter B. Stacey

Cooperative Breeding in Birds: long-term studies of ecology and behavior

Animals have evolved to behave in ways that maximize their reproductive success. This makes behavior that reduces their reproduction, but helps someone elses, di cult to understand. Possible solutions to this dilemma include helping relatives, improving the chance of inheriting a good reproductive position in the future, and maximizing access to resources. Examples of this phenomenon are particularly widespread within populations of birds engaged in cooperative breeding, where individuals heavily invest in rearing o spring that are not their own. Biological parents and one or more foster parents cooperate to raise a nests o spring. This helpful behavior results in a foster parent foregoing its opportunity to have progeny of its own during that breeding season. Despite this signi cant cost, there are many major bene ts to being a helper. This section will examine these bene ts and the ecological variables that determine whether or not an individual will participate in cooperative breeding. Author: Omar Metwalli

Territory Quality and Dispersal Options in the Acorn Woodpecker, and a Challenge to the Habitat-Saturation Model of Cooperative Breeding

The central feature of cooperative breeding systems is the presence of one or more nonbreeding birds in a social group that devote time and energy to help raise the offspring of other group members. The most widely accepted model for the evolution of this behavior is that it arises when there is some ecological constraint to independent breeding by young birds that otherwise would disperse from their natal groups. Among territorial species, this constraint is usually hypothesized to be saturation of suitable habitat by sedentary established groups. We use data collected over a 10-yr period to test this model for a population of acorn woodpeckers (Melanerpes formicivorus) in central New Mexico. Acorn woodpeckers store large quantities of mast, and territories vary greatly in the amount of storage facilities present. Storage facilities significantly affect the survival and reproductive success of groups occupying those territories. Most helpers occur in high-quality territories; low-quality territories are frequently unoccupied and thus available for colonization. We estimate lifetime direct and indirect offspring production for yearlings that help in territories of differing quality and for varying numbers of years. Birds that forgo individual reproduction for one or more years in high-quality territories but eventually breed there can have greater lifetime fitness than those that disperse at 1 yr of age and breed immediately in low-quality territories. Had we combined data from all territories and all groups, these findings would have been obscured; rather, the resulting calculations would have erroneously indicated that, for yearlings, dispersing and breeding is always the preferred option. Our analyses indicate that habitat saturation or some other constraint on the opportunity to breed successfully is not required to explain the existence of cooperative breeding in this population of woodpeckers. Instead, we suggest that competition for space may be the outcome of factors that promote philopatry, such as access to a critical resource and/or mutualistic benefits of living in a group and eventually breeding in the natal territory. If this scenario is correct, habitat limitation is a result of other factors favoring cooperative breeding rather than the basis for this kind of social system.

Habitat saturation and communal breeding in the acorn woodpecker

Abstract The acorn woodpecker (Melanerpes formicivorus) typically lives in social groups that breed communally: all group members help to feed the young of a single nest. The behaviour of acorn woodpeckers was compared in two locations (New Mexico and California) to test the hypothesis that saturation of the habitat by established groups may lead to more than a single male-female pair living in groups during reproduction. Several lines of evidence indicated that the habitat was less saturated in New Mexico. This difference was reflected in a lower tendency for juveniles and adults to remain in groups, higher reproductive rates and, most important, smaller group sizes. These results suggest that habitat saturation plays an important role in communal breeding in this species.

Kinship, promiscuity, and communal breeding in the acorn woodpecker

Summary1.Acorn woodpeckers typically live in permanent social groups in which most adults help to incubate and feed the young of a single nest. The selective basis of communal breeding in this species is examined.2.At the study area in New Mexico, most juveniles did not remain on their natal territories during subsequent breeding seasons. There was also a high rate of turnover among adults within each group. The groups were not composed entirely of extended families, and unrelated adults fed young. Kin selection alone does not appear to be adequate to explain all instances of communal breeding.3.Immigrants that joined groups before courtship in the spring participated in all phases of reproduction in their new groups. However, immigrants that joined groups after eggs were laid did not incubate or feed the young. This, combined with the high rate of turnover within groups, indicates that hypotheses based upon individual benefits through increased production of young and/or reciprocity are also not sufficient for this species.4.An examination of the reproductive behavior within groups suggests that the mating system may be promiscuous. It was not possible to identify which individuals were the parents of the young, and all group members normally participated in every phase of reproduction. A promiscuous mating system means that each group member could make a genetic contribution to the young and that all would help to raise the young to protect their genetic investment.5.Larger groups successfully fledged more young than did smaller groups, and promiscuity may be an individual strategy to insure that all individuals remain within the group and participate in reproduction.

Social plasticity in the acorn woodpecker.

Acorn woodpeckers (Melanerpes formicivorus) in southeastern Arizona exhibited two different types of social organization: one of highly cooperative and resident groups and another of birds that migrated and formed only temporary male-female pairs during reproduction. The occurrence of both patterns in the same population indicates a high degree of social flexibility in this species.

Shared paternity in the acorn woodpecker (Melanerpes formicivorus)

SummaryWe investigated multiple paternity in acorn woodpeckers (Melanerpes formicivorus) from central New Mexico by starch-gel electrophoresis. Seven of thirty-one presumptive genetic loci were polymorphic. Multiple paternity was revealed in one of three multi-male groups sampled on the basis of offspring and adult electrophoretic phendtypes, and was suggested in the two other groups. This study provides the first documentation of multiple paternity for any avian cooperative breeder.

Body color pattern and the aggressive behavior of male pumpkinseed sunfish (Lepomis gibbosus) during the reproductive season.

Male pumpkinseed sunfish (Lepomis gibbosus) readily attacked plywood models placed into their nests. Models containing red features (iris, opercular flap) received more attacks and aggressive displays than models lacking these features or than models which had these features painted black. This indicates that in the pattern of the male pumpkinseed, the red portion of the opercular flap and the red iris are social releasers for aggressive behavior. These features fade in subordinate pumpkinseeds in the laboratory and also in female pumpkinseeds before they enter the males nest during reproduction. This pattern change would therefore function to decrease aggressive behavior directed at these individuals. A conspicuous feature of the female pumpkinseed is the presence of lateral bars. When bars were added to models, reductions in aggressive behavior were consistently observed. Hence, bars appear to inhibit male aggression. Bluegill sunfish (L. macrochirus) nest sympatrically with pumpkinseeds and interspecific nest defense was commonly seen. A conspicuous feature of male bluegills is a dark spot in the area of the dorsal fin rays. When such a spot is added to models, increases in aggressive behavior were observed in male pumpkinseeds. Hence, this feature may provide a basis for interspecific recognition and reproductive isolation. Finally, pumpkinseeds responded more vigorously to models than did bluegills. This may imply that the former are more attuned to morphological features than the latter. Bluegills, on the other hand, may be more attuned to the behavior of nest intruders. This hypothesis agrees with differences in the nesting ecology of these species.

Changes in the darkness of four body features of bluegill sunfish (Lepomis macrochirus Rafinesque) during aggressive encounters.

The darkness of four body features (general body surface, GBS; lateral stripe pattern, LSP; opercular flap spot, OFS; and dorsal fin-ray spot, DFRS) was observed during the formation of 16 pairwise status relationships among bluegill sunfish (Lepomis macrochirus Rafinesque). In eight cases (Method A) an intruder fish was placed into a 110 liter aquarium occupied for 7 previous days by a resident fish. In eight additional cases (Method B) two fish lived on opposite sides of a 110 liter aquarium divided by an opaque screen for 7 days prior to removal of the screen. Dominance was established by one of the fish in all but two cases (both were Method B) and aggressive encounters proceeded through five clearly identifiable stages: (1) a stage of inactivity just after pair formation, (2) a stage in which the fish oriented and approached each other tentatively and without further interaction, (3) a stage in which threat displays are emitted, often by both fish, but no actual body contact occurs, (4) a stage of vigorous attacks, and (5) a stage where one fish is clearly emitting all attacks and where the other fish is fleeing and/or hiding. Darkness of each feature in each fish was evaluated with a statistically reliable three-point scale at each stage during the contest. GBS increased in darkness equally for dominant and submissive fish. LSP, OFS and DFRS became darker in dominant fish and lighter in submissives, suggesting that these three features may have status-related signal value. Method of pair formation did not interact with these effects.

Cooperative Breeding in Birds: Contents

Animals have evolved to behave in ways that maximize their reproductive success. This makes behavior that reduces their reproduction, but helps someone elses, di cult to understand. Possible solutions to this dilemma include helping relatives, improving the chance of inheriting a good reproductive position in the future, and maximizing access to resources. Examples of this phenomenon are particularly widespread within populations of birds engaged in cooperative breeding, where individuals heavily invest in rearing o spring that are not their own. Biological parents and one or more foster parents cooperate to raise a nests o spring. This helpful behavior results in a foster parent foregoing its opportunity to have progeny of its own during that breeding season. Despite this signi cant cost, there are many major bene ts to being a helper. This section will examine these bene ts and the ecological variables that determine whether or not an individual will participate in cooperative breeding. Author: Omar Metwalli

Cooperative Breeding in Birds: Contributors

Animals have evolved to behave in ways that maximize their reproductive success. This makes behavior that reduces their reproduction, but helps someone elses, di cult to understand. Possible solutions to this dilemma include helping relatives, improving the chance of inheriting a good reproductive position in the future, and maximizing access to resources. Examples of this phenomenon are particularly widespread within populations of birds engaged in cooperative breeding, where individuals heavily invest in rearing o spring that are not their own. Biological parents and one or more foster parents cooperate to raise a nests o spring. This helpful behavior results in a foster parent foregoing its opportunity to have progeny of its own during that breeding season. Despite this signi cant cost, there are many major bene ts to being a helper. This section will examine these bene ts and the ecological variables that determine whether or not an individual will participate in cooperative breeding. Author: Omar Metwalli