The effects of parental age difference on the offspring sex and fitness of European blackbirds
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Background. Many studies of birds have indicated that offspring sex ratios can vary
with environmental and parental traits. On the basis of long-term research, we first
evaluated the possible influence of parental age difference and brood characteristics on
offspring sex and fitness in multi-brooded Blackbirds Turdus merula.
Methodology. The study was conducted in the city-centre Stefan Żeromski Park in
Szczecin, NW Poland, where the local population of Blackbirds has been studied since
1996. Data on the offspring sex and fitness were collected in five years, 2005–2007 and
2016–2017. During the breeding season we inspected the study area to locate the pairs’
territories and to track their nests and clutches.
Results. We found that the overall sex ratio did not differ statistically from 50:50,
but that younger females bonded with older mates did tend to produce more sons,
probably because of the greater fitness of male descendants. Accordingly, the sons’
breeding success increased with the father’s age, but this relationship was close to nonlinear, which may indicate that the transgenerational effect of paternal senescence could
negatively affect progeny fitness despite the high-quality of older fathers. Older females
mated with younger males produced more daughters, which could have been due to
the lesser attractiveness of the males and the mothers’ poorer condition caused by
accelerated senescence. We found that neither offspring hatching sequence nor hatching
date or clutch sequence were significant for sex determination.
Conclusions. We consider that in our Blackbird population, parental age could make
a more significant contribution to shaping offspring sex and reproductive success
with environmental and parental traits. On the basis of long-term research, we first
evaluated the possible influence of parental age difference and brood characteristics on
offspring sex and fitness in multi-brooded Blackbirds Turdus merula.
Methodology. The study was conducted in the city-centre Stefan Żeromski Park in
Szczecin, NW Poland, where the local population of Blackbirds has been studied since
1996. Data on the offspring sex and fitness were collected in five years, 2005–2007 and
2016–2017. During the breeding season we inspected the study area to locate the pairs’
territories and to track their nests and clutches.
Results. We found that the overall sex ratio did not differ statistically from 50:50,
but that younger females bonded with older mates did tend to produce more sons,
probably because of the greater fitness of male descendants. Accordingly, the sons’
breeding success increased with the father’s age, but this relationship was close to nonlinear, which may indicate that the transgenerational effect of paternal senescence could
negatively affect progeny fitness despite the high-quality of older fathers. Older females
mated with younger males produced more daughters, which could have been due to
the lesser attractiveness of the males and the mothers’ poorer condition caused by
accelerated senescence. We found that neither offspring hatching sequence nor hatching
date or clutch sequence were significant for sex determination.
Conclusions. We consider that in our Blackbird population, parental age could make
a more significant contribution to shaping offspring sex and reproductive success