The structure of song repertoires of the Tomsk population of the pied flycatcher (Ficedula hypoleuca) and their age-related variability

Abstract

Bird songs are one of the most difficult acoustic signals in wildlife, the main functions of which are to attract a sexual partner and protect the territory. The size of the song repertoire and the structure of the song are important indicators reflecting the "quality" of the male. They are related to its size, coloration, immune status, and attractiveness to females. At the same time, the variability of the repertoire with age remains an insufficiently studied aspect. The aim of this work was to study the main parameters of the repertoire, the structural elements of songs and their age-related changes in the “Tomsk” population of the pied flycatcher (Ficedula hypoleuca). We collected materials in the period from 2014 to 2020, in the area of natural habitat of the pied flycatcher, located 12 km from the city of Tomsk (56°20'56,1"N, 84°57'06"E). The study included individuals with precisely known ages. The recordings of singing of 26 males were analyzed, divided into three age groups: I - one-yearolds (n = 10); II - biennial (n = 11); III - males aged three years and older (n = 5). None of the birds in the study was harmed. Sequences of 75 songs were analyzed for all males. The number of figures and their types were counted in the song (see Fig. 1). At the first meeting, each type of figure was assigned an individual number and entered into the working directory. Song diversity was the ratio of the number of types of figures in a song to the total number of figures in it, expressed as a percentage. In the study group, 132 types of song figures were identified. The size of the individual repertoire ranged from 17 to 64 types of figures. The basis of the song repertoire can be considered 34 types of figures, the total share of which in the song sequences was 72.8%. They are noted in the repertoire of 88.5-42.3% of males. In our study, we relied on 12 types of figures, which are represented in each age group in 61.5-88.5% of males. With age, the pied flycatcher shows an increase in the size of the repertoire, the variety of songs (See Fig. 3), as well as a significant reduction in the number of figures in a song from the third year of life (See Table 1). The analysis of the general song sequence made it possible to distinguish the following stable structural components of the song: "bimotive", "monomotive", "trill", and "connector" starting element (See Fig. 5). The main element of the pied flycatcher's song is the bimotive - a construction consisting of two types of figures. The most common were bimotives 21→22, 24→62, 1→2, which account for 23.6% of the total song sequence. An analysis of the frequency of using these bimotives by birds of different ages showed that the figures in the bimotive 21→22 are similarly represented in all age groups (See Table 2). With age, the use of bimotive 24→62 significantly increases, while the use of bimotive 1→2 is steadily decreasing. The patterns of using bimotives with age are based on differences in the frequency of their repetition in a song. Thus, the male can repeat three popular bimotives (21→22; 24→62; 1→2) 5-7 times in a row (see Fig. 6). For bimotive 21→22, cases with a 2-fold execution of it (29.14%) dominated in young birds, whereas in adults these were cases with a 3-fold execution (38.82%). Bimotive 24→62, young individuals more often performed in double (44.4%) and single (41.97%) variants, while adults - in 3 (32.09%) and 4-fold (24.62%). For bimotive 1→2, no differences were observed between juveniles and adults. Monomotives, repetition of figures of the same type, are a common technique in the songs of the pied flycatcher. The most stable are 10 variants of monomotives, for which their repetition is the most frequent event observed with a probability of 0.37-0.59. Young birds use monomotives consisting of 1-7 figures, adults - up to 4 (see Fig. 7). Most often, there is a double execution of monomotives. The most complex song structure, which increases the variety of the song of males, is trills, consisting of 3-5 figures. The most common variant turned out to be the sequence 100→39→63→156 (n = 46). With age, the proportion of birds using such constructions increases (I - 36.6%; II - 60%; III - 100%). Between the abovedescribed constructions, the pied flycatcher often inserts single high-frequency figures - connectors. Adult singing uses more connector types than juveniles, which also contributes to increased song diversity with age. In the songs of the pied flycatcher, the function of the specialized starting element is performed to a greater extent by figure №77. The song began with her 8.3% of the time. At the beginning of the song, it appears 7.2 times more often than in its subsequent positions. With age, males reduce the use of specialized starting elements, starting the song immediately from its main variation part. No specialized finishing elements have been identified. Thus, the development of the song with age occurs in the direction of increasing the size of the repertoire and its diversity due to the development of complex structures by males and getting rid of redundant information.