Sexual selection is a major evolutionary force that may lead to changes
in morphology, behaviour, mating systems, life history evolution and
even speciation and extinction. In this thesis, I studied how abiotic
and biotic factors affect fitness, and the expression and evolution of
secondary sexual traits of pied flycatcher (Ficedula hypoleuca)
individuals with variable dichromatic plumage colouration. I studied
these questions using long-term data sets as well as manipulative and
interspecific approaches.



First, I studied whether the temperature-dependent breeding
success of pied flycatcher males with different dorsal plumage
colouration is due to the effects of the genotype of offspring or due to
post-hatching paternal effects. Additionally, I investigated how
increased or decreased parental effort affected future fitness of adult
pied flycatchers, and the future expression of male sexual
ornamentation. Furthermore, I examined the effects of environmental
conditions experienced during the wintering season (in sub-Saharan
Africa) on the expression of male secondary sexual traits, and the
potential interactive effects between wintering conditions and male
secondary sexual traits on future fitness of male and female pied
flycatchers. Finally, I studied the possible effects of social
environment on the development and signalling functions of male
ornamental traits.



I found that offspring raised by black pied flycatcher males
were lighter than offspring raised by brown males when it was cold, and
that this temperature-dependent breeding success of males with
different dorsal plumage colouration was due to post-hatching paternal
effects. Moreover, I found a plastic melanin response in dorsal plumage
colouration of young males experiencing brood enlargement so that males
became darker in the following breeding season. On the other hand,
females that experienced brood enlargement had a significantly lower
local survival in the next breeding season than those females in the
control or in the reduced treatment group.



I discovered that mean rainfall during the wintering season
and winter NAO (North Atlantic Oscillation) index predicted both a
plastic response and short-term microevolution on the wing patch size.
The wing patch size of individuals decreased in winters of low
precipitation and high NAO values, while in the same conditions
individuals with a small wing patch seemed to suffer lower local
survival than individuals with a large wing patch, and vice versa. I
also found that males breeding in our study area became darker after dry
and windy wintering conditions compared to moist and less windy
wintering conditions. Moreover, after dry and windy wintering conditions
females mated with large-patched males laid large clutches, whereas
females mated with small-patched males laid small clutches after such
wintering conditions. In addition, females paired with large-patched
males after dry and windy wintering conditions had a higher probability
of returning in the next breeding season than those that chose
small-patched males after such conditions.



I reveal that the forehead patch size of pied flycatcher
males affects the outcome of intraspecific male-male competition.
Moreover, in interspecific male-male competition between pied flycatcher
males and collared flycatcher (Ficedula albicollis) males (a closely
related sister species) it seems that the brown pied flycatcher males
mimic females of collared flycatchers. This most likely leads to reduced
aggression from more dominant collared flycatcher males.



The results of this thesis provide important novel information
on how changing environment affects phenotype expression and evolution
as well as phenotype-dependent selection in the pied flycatcher. It
seems to be that to a great extent the individual, population and year
variations of expressed male secondary sexual traits of pied flycatchers
are maintained by fluctuating environmental conditions experienced
during the breeding and wintering seasons. Thus, global climate change
can affect the population dynamics of long-distance migratory species in
many and various ways. Climate change can take very different
directions on the two different continents where breeding and wintering
take place. Such environmental changes may have strong influences on the
direction of sexual selection, on the expression of secondary sexual
traits and on geno-/phenotype-dependent fitness, and all these processes
shape populations.