Electronic Supplementary material
1. Detailed Methods
The accuracy of the data obtained was constrained by several features of the biology
of the study species. These generate a number of difficulties and require assumptions
that need careful explanation.
Age is not always known precisely. Early in the study the precise age of many birds is
unknown, as they were already adult when first encountered. While by 1991 it was
possible to classify these as four years or older (eg Fig 1c), it was not possible to
include them in analyses treating age as a continuous variable (Fig 1a). This problem
had largely disappeared by 1997, as many old birds died in 1996.
Female recruitment cannot be measured. All females undertake at least one dispersal
movement before acquiring a breeding vacancy (Cockburn et al. 2003). While some
birds disperse within the study area, many move out of the area and sometimes travel
several kilometres to gain a vacancy. Hence, we can measure the number of sons
recruited during each year of selection, but not recruitment of daughters, as we cannot
reliably distinguish between female death and dispersal. Hence, in this case we use
recruitment of sons as the fecundity measure in the calculations of fitness. This
limitation will be occur commonly in field studies, and is exacerbated because the
trait under sexual selection cannot be scored in females. Coulson et al. (Coulson et al.
2006) report a comparable example in their calculation of selection in Soay sheep.
The assumption that males are an adequate measure of fecundity would be
particularly problematic if there were strong facultative adjustment of the offspring
sex ratio in response to the same environmental features that are influencing sexual
selection gradients. That difficulty does not arise in M. cyaneus because there is no
facultative adjustment of the sex ratio in this species (Cockburn & Double 2007).
Male reproductive success if imperfectly known. Our population is not closed.
Because males can achieve extra-group success some territories from their own
(Double & Cockburn 2000), some males recruited into the study area are sired by
males from outside the study area, and some of the males in the study area
presumably also sire males elsewhere. We have tried to minimise this problem by
excluding marginal territories that were not largely surrounded by neighbours, but this
is not a complete solution, and hence our data must be viewed as a partial estimate of
male success. Our assumption that we can proceed despite this difficulty is supported
by the strong patterns we have uncovered, and the observation that males on or
towards the edge of the study area attained some of the highest levels of reproductive
success in this study.
The precise date that some males acquire nuptial plumage is unknown. It is easy to
determine precisely the moult date of a male that undergoes a conventional moult
from eclipse plumage to nuptial plumage and vice versa. However, the moult date of
some males is more difficult to record. These fall into five classes:
1. Some males use the post-breeding moult to renew their nuptial plumage, and hence
never exhibit eclipse plumage. The completion of this process is difficult to
determine, so we have assumed a value of 8 weeks, earlier than any male that gained
at least some eclipse feathers (Fig 1a).
2. Similarly, some males (almost always males in their first year) initiate the moult
but do not complete it, retaining a variable amount of eclipse plumage. We cannot
always tell when the moult has stopped, so we have arbitrarily given these males a
value of 48 weeks, after the last date for conventional acquisition of males in their
first year (Fig 1a).
3. A much smaller number of males never moult. There is some evidence that this
failure is pathological rather than part of a continuum (Mulder & Cockburn 1993), but
we have assigned these males a value of 52 weeks.
4. Some males die during the process of moult, particularly during the Pre-breeding
phase when mortality is generally high (Fig. 2c). We have assigned them a moult
completion date estimated from males that survived but were in a similar stage of
moult when the original bird died.
5. Most difficult, some birds died without initiating moult. Failure to include these
birds would distort the calculation of pt(i), as by definition these birds biased towards
the lowest possible level of fitness in the season in which they die, and the calculation
is sensitive to the number of males included in the sample (Coulson et al. 2006). We
therefore assigned these birds a moult date and class in the following way.
a. for the birds that were three years or older, we assigned moult date based on
the observation that older birds typically retain approximately the same moult
rank within their cohort from year-to-year. We therefore calculated their
percentile position within their cohort, and assigned a moult date accordingly.
b. this approach was not appropriate for one and two year old males, as we had
no prior information on one year old males, and moult is not correlated over the
first two years of life. However, these cohorts show much less variance than the
other age classes (95% of individuals generally complete moult within a three
week period in any year), so we randomly assigned them a value based on the
distribution for their cohort in that season. This is obviously not entirely
satisfactory, but we experimented with alternative methods of assignment and
the exclusion of these birds, and obtained qualitatively similar results in each
case. In addition, the measurement we ultimately used to look at variation in
reproductive monopolisation is completely insensitive to these assumptions.
References
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no facultative manipulation of offspring sex ratio despite plausible benefits.
Behavioral Ecology and Sociobiology, in review.
Cockburn, A., Osmond, H. L., Mulder, R. A., Green, D. J. & Double, M. C. 2003
Divorce, dispersal and incest avoidance in the cooperatively breeding superb
fairy-wren Malurus cyaneus. Journal of Animal Ecology 72, 189-202.
Coulson, T., Benton, T. G., Lundberg, P., Dall, S. R. X., Kendall, B. E. & Gaillard, J.M. 2006 Estimating individual contributions to population growth:
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Double, M. & Cockburn, A. 2000 Pre-dawn infidelity: females control extra-pair
mating in superb fairy-wrens. Proceedings of the Royal Society of London
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Mulder, R. A. & Cockburn, A. 1993 Sperm competition and the reproductive anatomy
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