Reyer Heinz-Ulrich

Age-related reproductive success and the function of delayed plumage maturation in male black redstarts Phoenicurus ochruros

Project Number: CH-4397
Project Type: Research_Project
Project Duration: 02/01/1994 - 11/30/1997 project completed
Funding Source: other ,
Project Leader: Prof. em. Heinz-Ulrich Reyer
pensioniert 2012

related to this project.
for which the project has a relevance.


Research Areas:
Biodiversity

Disciplines:
general biology
zoology

Keywords:
Black Redstart, Phoeniurus ochruros, Reproductive success, delayed plumage maturation, yearlings

Abstract:
Males of many species of passerine birds do not display mature plumage colour for months or years after reaching sexual maturity. Such delayed acquisition of adult plumage by sexually mature birds is termed delayed plumage maturation (DPM). I investigated two competing hypotheses for the adaptive value of DPM in the Black Redstart Phoenicus ochruros. Black Redstarts are small, insectivorous and migratory passerine birds. Approximately 90% of the yearling males breed in a dull, female-like plumage. Full adult plumage is displayed only by males in their second or later breeding season.
The study was carried out between March 1994 and May 1997 on a population of individually marked birds breeding in the two small villages, Jeizinen and Engersch (46° 20’ N/7° 43’ E), in the Central Alps of Switzerland. General field procedures included daily censuses to record the temporal pattern of the bird’ presence on the breeding ground, the mating system, all nesting attempts and reproductive success. True parentage was revealed by DNA-fingerprinting.
In the first chapter I examine if yearling males in non-mature plumage are reproductively constrained. I tested the hypothesis that such a constraint may be the underlying cause for the evolution and maintenance of DPM. Annual reproductive success of yearling males was only half the value of older males. A multifactorial examination of the variation in reproductive success revealed that old males did not invest more in reproduction. However, they outcompeted yearling males in their access to (multiple) females and adult females. Mating with adult females turned out to be beneficial, because adult females had higher reproductive output than females breeding for their first time.
Possible processes that promote the observed age-related reproductive success in males are evaluated in chapter 2. Late summer territoriality, a widely neglected trait in passerine birds, is proposed to play a key role. Experienced breeders, females and males, remained after the end of the breeding season for almost two months on the breeding ground. Males present in late summer displayed territorial and mate guarding behaviour similar as in spring. Yearling males with only one breeding season of experience and unmated males usually abandoned their former breeding territory and occupied a new late summer territory where they displayed territorial behaviour. Postbreeding dispersal enhanced the frequency with which males had access to experience females. This suggests, that experience males actively select territories and mates already postbreeding in anticipation of the next breeding season. Juvenile males – the yearlings of the next breeding season – did not occupy territories in late summer and hence did not preestablish pair bonds with experienced females. In spring, territory positions occupied in late summer were reclaimed and pair-bonds among experienced breeders frequently re-established. High winter survival and site fidelity favoured the retention of pairs. Consequently, yearling males in spring had to fill gaps and mated more frequently with yearling, inexperienced females than expected by chance. It is concluded that age-related reproductive success in males emerged because the start of territory selection and pair formation was temporarily segregated in the two age-classes by over six months.
In Chapter 3 I assess the female mimicry hypothesis as one possible functional explanation for DPM. This hypothesis states that dull coloured yearling males may deceive rival males about their real sex and thereby gain easier access to territory positions or sneaky copulations. Five predictions derived from the female mimicry hypothesis were all rejected: First, during territory establishment dull coloured yearlings did not intrude into territories of adult males more frequently than other adults did. Second, the first sites selected by yearling males tended to be in accordance with the prediction that they should be located close to territories of adult males. However, the same was true for adult males. Third, yearling males did not suppress conspicuous male behaviour such as singing or perching on sentinel position during any time of the season. Fourth, the frequency of extra-pair fertilization revealed by multilocus DNA-fingerprinting on 215 chicks was only 3% and there was no trend for yearling males to obtain extra-pair fertilization more frequently than adult males. Fifth, sneaky nest inspections that are probably part of a reconnaissance tactic to enhance future reproductive perspectives were not conducted specifically by yearling males. Overall, there was no indication that the non-mature plumage of yearling males served to deceptively acquire any relevant breeding resources.
I presented model males to territorial males to examine predictions of the “status signal hypothesis”, and report the results in chapter 4. According to the status signalling hypothesis, non-mature plumage is an honest signal of lower fighting ability that allows to settle contests without costly fights. It was tested a) if yearling male territory owners attack artificial intruders less vigorously than adult male conspecifics and b) if dull coloured intruders elicit less aggressive responses than bright coloured ones. The aggressive response showed a condition dependent pattern. Before pairing, yearling territory owners reacted as aggressively as adults. After pairing, however, adult males were more aggressive than yearlings No conclusive results were obtained regarding the effect of intruder coloration because territory owners reacted even to blue-coloured control males. These results are most consistent with the view that DPM in the Black Redstart serves as a status signal that honestly indicates fighting motivation. However, the status signalling system was not operating during the time males fought over access to females. These findings are consistent with previously reported predictions from game theory stressing that status signalling is not operational when the value of the contested resource is high.
In conclusion this study showed that the preselection of territories in late summer by experienced breeders reproductively handicaps yearling male Black Redstarts substantially in terms of their access to (multiple) females and adult females. These age-specific asymmetries presumably favoured the evolution of a special plumage in yearling males. The pattern of aggressive motivation is most consistent with the view that the special yearling plumage serves as an honest signal of reduced fighting motivation during the post mating phase when the value of the contested resources is low.

Leading questions:
  • Do have yearlings of Black Redstarts lower annual reproductive success than older males?

  • What is the significance of late summer territoriality for age-related asymmetries in male Black Redstarts?

  • Is the female mimicry a possible functional explanation for delayed plumage maturation?

  • Is there an effect of age and mating status on the aggressive reaction of territorial Black Redstarts towards differently coloured intruders?


    Publications:
    Weggler, M. 1997. Age-related reproductive success and the function of delayed plumage maturation in male Black Redstarts Phoenicurus ochruros. Ph.D. Thesis, University of Zurich, Zurich, Switzerland.


    Last update: 12/29/16
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