Social System of Superb Fairy Wrens. The following table shows the percentage of male fairy-wrens in various age and social status categories.

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1 Social System of Superb Fairy Wrens Superb fairy-wrens are small (10g) insectivorous birds found in woodlands and edge habitat throughout eastern Australia. They live in cooperative social groups composed of a single breeding female, her social mate, and often one to four adult male helpers which defend a permanent territory. The social mate is behaviorally dominant over any other males (helpers) on his territory. Although females build the nest and incubate eggs without assistance, males contribute to next defense and feeding of young. All helpers assist with feeding and care of young. It is important to note that because superb fairy-wrens are long-lived and territorial, breeding vacancies are rare enough that females are generally unable to choose their social partner. There are three categories of males: paired dominant males without helpers, dominant males with one or more helpers, and helpers. Regardless of their social status, males become sexually active at age one when they develop adult breeding plumage and the ability to fertilize females. In addition to their complex social system, superb fairy-wrens are unique because they exhibit the highest known rates of extra-pair fertilization, a whopping 76%. This tutorial challenges you to identify some of the selective forces behind this social system. Male Status The following table shows the percentage of male fairy-wrens in various age and social status categories. TABLE 1. Percentage of male fairy-wrens in various age and social status categories, 1991 and 1992 seasons. Total n equals 135 male-seasons. Age (years) Dominant with helper Unassisted dominant Helper (0) (0) > What determines a male s social status? Why might this be the case? Why do helpers help? (Part I) The following table maps the effect of group size on reproductive success in a territory. TABLE 2. The effect of group size on reproductive success in a territory, 1991/1992 and 1992/1993 seasons Pairs Groups Group size effect Average number of fledglings p = Does the presence of helpers increase reproductive success in a territory? Average number of young surviving to 4 weeks Probability of fledgling no young p = p =.91

2 Page 2 Who are helpers helping? This table contains information about the relatedness of helpers to nestlings. TABLE 1. Percentage of help directed at relatives and non relatives, split by helper age. Relationship of helpers to nestlings 1 year old 2+ years First-order relatives ( r = 0.5 ) 35 % 23 % Second-order relatives ( r =.25) 46 % 23 % Unrelated young ( r <.25 ) 19 % 54 % Totals 100 % 100 % 1. Do you think that the levels of relatedness between helpers and nestlings is enough to be the sole reason that helpers stay and help instead of breeding on their own? Why might relatedness decrease as the helper gets older? 2. How might helpers deal with the problem of having some related young and some unrelated young in the nest they are helping? Remember that every time a helper feeds an unrelated young this is an evolutionary waste of energy. Feeding rate of helpers in relation to their mean relatedness to the young. 4. Do you see any correlation in the graph? Are helpers adjusting their feeding rates according to their relatedness to the young? Should they? Figure 1. The feeding rate of helpers in relation to their mean relatedness to the young they provisioned. Crosses represent nests with at least one young sired by the helper, open circles denote no paternity. Deletion of the arrowed point eliminates the weak negative correlation.

3 Page 3 Why do dominant males prefer or tolerate the presence of helpers? 1. Why do you think dominant males would choose to allow helpers on his territory? Remember that the presence of helpers does not increase the reproductive success in a territory. Paternity by the dominant male in his own nest is affected by the presence of helpers. 2. What clear correlation do you see? How do helpers affect the dominant male s paternity? From the above graphs, should dominant males accept helpers or not? Behavior of dominant males with and without helpers. Figure 1. Number of nest visits made by dominant males during a 30 min period in relation to the number of helpers. Figure 2. Probability that a dominant male will depart on an extra-group foray during a 30 min period in relation to the number of helpers, and the reproductive status of his mate.

4 Page 4 3. How is the behavior of dominant males affected by helpers? How could these behaviors be advantageous for the dominant male? 4. When the dominant male is not actively attending the nest, what do you think he is doing? How can he best spend the free time he gains from the presence of helpers? The following graphs give us insight into which males gain extra-pair fertilizations. 5. Which males are getting the most extra-pair fertilizations? Refer to the table about male status on page one. Fig. 3. (A) The effect of age on the probability of obtaining extra-group fertilization. (B) The effect of social status on the proportion of males obtaining an extra-group fertilization. Numbers indicate total males in each social category. 6. What proportion of the males sired extra-group offspring? Are the numerous extra group offspring being fathered by many different males or a select few? Here is the frequency with which males attain extra-pair fertilizations. Treat the shaded bars the same as white bars for now.

5 Page 5 Now for the shaded bars. One particular male in the study, identified by the researchers as BRY, was quite successful in his attempts to gain extra-pair fertilizations. This male fathered six young outside his group, and two generations of his descendents (a within-group son, an extra-group son, and an extra-group grandson) sired an additional 12 extra-group young. The shaded bars in the figure above identify males belonging to this illustrious BRY patriline. 7. In considering this information, what can you say about the tendency to sire extra-group young? Is it directly related to age and status, or are there more factors? Is this tendency inherited from the father or learned from the father? This table should answer an important question. In this study, the paternity of each offspring was determined (whether it was within-group or extra-group). This allowed for the calculation of a male s total reproductive success (inside and outside the group). Male survival was also calculated. Table 1. Average reproductive success and survival from one breeding season to the next (%) of dominant males living in four social categories: (0) pairs without helpers; (1) groups with a single helper; (2+) groups with two or more helpers; and (solitary) living without a female, or with a female which did not breed. Social category of the dominant male solitary significance Fledglings p < Independent young p < Male survival not significant 8. What benefits do males gain from the presence of helpers? In the case of reproductive success, where are these benefits taking place (inside our outside his territory)? Why do helpers help? (Part II)

6 Page 6 Why do young males usually stay at home and become helpers? Two hypotheses are the limited-mates hypothesis and the habitat-saturation hypothesis. In both cases, a necessary resource for successful breeding is limited to the point that males will not disperse due to a lack of this resource. In this study, the dominant male was removed from a pair (no helpers), thus presenting neighboring helper males with an opportunity to disperse to an available habitat and female. TABLE 1 Disposition of non-breeding males in superb fairy-wren groups Category Number Number of potential dispersers capable of dispersing given the opportunity 33 Number of potential dispersers above - that dispersed given the opportunity 31 - that did not disperse given the opportunity 2 1. What does this data suggest about the reason that males do not disperse and instead become helpers? This table presents data about dominant males responses to helper males who were absent from their territory. Aggression is characterized by extreme harassment by the dominant male, in the form of prolonged chases lasting up to 7 minutes. Table 1. Responses of dominant male superb fairy-wrens to the return of removed helper males in the first half-hour following the helper s return Stage of nesting N Aggression % Number of min. (out of 30) during which dominant male exhibited aggressive behavior Mean chase duration in seconds Non-breeding Fertilization period Incubation In addition to the reasons noted so far, why do helpers help? When does the dominant male respond most aggressively to a helper s departure? Why do you think he responds this way?