Parental investment of the two sexes has been proposed as a key promoter of sexual
selection in most species of the animal kingdom 1], 2]. In species where both parents are involved in bringing up the young, paternal care
is a crucial variable of female choice since females directly benefit from high levels
of male investment 3], 4]. Biparental care is especially widespread in birds (~81Â % of all species, 5]) with most of the species being socially monogamous 6]. In birds, females have been found to adjust their mating strategies and their own
investment according to the expected male participation in offspring rearing 7], 8]. Females may assess paternal qualities on the basis of male secondary traits (‘good-parent’
model 9], but see also 10] and studies cited therein for conflicting results). Mechanisms that have been proposed
to explain female assessment of male parenting abilities are similar to ‘good-genes’
or ‘handicap’ models of female choice 11]. These models assume that male extravagant traits come with costs and only males
of high phenotypic and/or genetic quality manage to sustain these traits 12], 13]. Traits that have been shown to be related to paternal qualities include morphological
characteristics such as plumage coloration 14], 15] or courtship related behaviors 16]–19].
In passerines, male song is such a courtship behavior 20] and there is evidence that song serves as predictor for paternal effort. For example,
high song rates were found to be associated with higher feeding rates in a few species,
and it was suggested that high song rates possibly indicate a male’s foraging efficiency,
his ability to defend a high quality territory or individual quality in general 21]–23]. Elaborate song flights which are associated with social costs for males advertise
future paternal performance in whitethroats Sylvia communis24]. Large repertoires indicate higher feeding rates in male sedge warblers Acrocephalus schoenobaenus25] whereas similar studies in other species failed to find a correlation between measures
of song complexity and male care 22], 26]–28]. Assuming that song complexity reflects a superior male status 29]–31] which has been mostly linked to indirect fitness benefits for females, it might at
the same time be an indicator of more direct benefits of female choice (i.e. a male’s
ability to provide paternal care). In summary, female songbirds may choose prospective
good fathers by paying attention to male song prior to pair formation.
The common nightingale Luscinia megarhynchos is a socially monogamous passerine and from few previous observations it is known
that males provide paternal care during breeding. They, for example, feed the female
during incubation, provide food to nestlings and readily defend the nest against potential
predators 32], 33]; CB, personal observation). Thus, reproductive success seems to be highly dependent
on male contribution, making it most probable that in nightingales male parenting
abilities are a crucial factor in mate choice. Furthermore, nightingales belong to
the most versatile singers of the temperate zone 20]. Males have very large song repertoires of up to 250 different song types (approx.
180 different song types per male, e.g. 34], 35]) with large repertoire birds being older 36], being in better condition and arriving earlier at the breeding grounds 35]. Within repertoires, specific features of certain songs seem to carry information
that might be important during different behavioral contexts, which constitutes the
basis for the formation of song categories (e.g. buzz songs: 37]; trill songs: 38]; whistle songs: 39], 40]; see also Fig. 1). Furthermore, the nightingale is an excellent model to study the functional aspect
of complex ‘syntax-like’ rules of song delivery. Song sequencing in nightingales is
neither fully fixed (e.g. singing in an A-Z manner), nor random, but instead follows
sequential rules that are partly controlled by early learning and memory retrieval
processes, e.g. 41]–43]. However, these sequential rules are not fully deterministic, but offer potential
for behavioral plasticity both short- and long-term. For example, males change their
repertoire composition and song sequencing with age, which is most probably attributed
to an adjustment to the breeding population 44]–46], or they change the sequencing of songs in response to playbacks 47]. Also, the usage of specific song types (i.e. song categories) is related to different
behavioral contexts or different times during the day and the breeding cycle, e.g.
39], 48]–52] which additionally affects the sequencing of songs. Very recently it has been shown
that the sequential ordering of song types is related to male attributes. For example,
older males sing their songs more ordered (i.e. they sing song sequences repeatedly
in the same order) and membership to a population seems to be encoded in song sequencing
(i.e. members of the same population share song sequences) 46], 47]. These findings imply that the sequential ordering of nightingale song may carry
information which is relevant for females during mate choice.
Fig. 1. Example songs of nightingale song categories. The acoustic patterns that constitute
categories are underlined in grey. (a): whistle song, characterized by a homotype series of repeated elements with a narrow
frequency band and little or no frequency modulation. (b): trill song, characterized by a rapid broad-band trill consisting of repeated elements
with little frequency modulation but covering a large frequency range. (c): buzz song, characterized by a long, non-repeated buzz element produced by a very
fast repetition of sound units in a narrow and rather low frequency range
If a male’s song is also related to his parenting qualities, and if females use song
to choose ‘good fathers’ has not been investigated in nightingales so far. To date,
there are neither studies systematically investigating paternal care in the nightingale,
nor studies looking at the relationship between song and parenting qualities. Here,
we investigated the potential function of male song as an indicator for future male
parental care. To do so, we analyzed paternal care by continuous recording of male
provisioning behavior at the nest across several days. Also, we analyzed male nocturnal
song with special focus on repertoire size, repertoire composition, and the sequencing
of songs as possible candidates for the advertising function of male song. We hypothesized
that males who are more committed to paternal care are ‘better’ singers.