Male Traits and Female Choice in Java Sparrows: Preference for Large Body Size

Ornithol Sci 10: 73–80 (2011) SHORT COMMUNICATION Male traits and female choice in Java Sparrows: preference for large body size Ai HASEGAWA1, #, Masayo SOMA2 and Toshikazu HASEGAWA1 1 Department of Cognitive and Behavioral Science, Graduate School of Arts and Sciences, University of Tokyo, 3–8–1 Komaba, Meguro-ku, Tokyo 153–8902, Japan 2 Department of Biology, Faculty of Science, Hokkaido University, Kita 10 Nishi 8, Kita-ku, Sapporo, Hokkaido 060–0810, Japan Abstract Estrildine finches are important model species in experimental studies on ORNITHOLOGICAL female mating preferences, but research has focused on only two species from this SCIENCE family, namely, the Zebra Finch Taeniopygia guttata and the Bengalese Finch Lonchura © The Ornithological Society striata var. domestica, and we know comparatively little about other closely related of Japan 2011 species. Therefore, we investigated sexual dimorphism and female choice in the Java Sparrow Padda oryzivora, which also belongs to the family Estrildidae, to open up the potential for comparative research and to better understand sexual selection in this family. First, we took measurements of eight morphological traits of six male and six female Java Sparrows: natural wing length, maximum wing length, tarsus length, tail length, exposed culmen length, bill width, bill depth and body mass. We quantitatively confirmed that Java Sparrows show sexual dimorphism in bill depth, with males having deeper bills than females. Second, we examined individual variation in male courtship songs by analyzing their acoustic and syntactical structure. After collecting data on male morphological and song-related traits, we conducted two-way choice tests to ask which kinds of male traits predicted which males were preferred by female Java Sparrows. In the choice tests, we put a cage with a female in between two cages each containing one male and recorded the position of the female every 30 seconds by point sampling. We performed a stepwise regression analysis to assess the relationship between the time females spent in front of each male and male morphological and song-related traits. The results indicated that females base their preference upon large body size, which is likely to act as a good indicator of male quality. However, no preference was observed for song-related traits or sexually dimorphic bill depth. Perhaps the sexual dimorphism in bill size is instead the evolutionary outcome of male-male competition. Key words Bill size, Courtship song, Female choice, Java Sparrows, Sexual dimorphism Since Darwin (1871) proposed that male morphol- between the sexes, because sexual dimorphism is ogy and behaviour have evolved through female mat- considered to be the outcome of selective pressures ing preferences, many studies have been done to that act differentially on males and females. investigate which male trait is the target of female For the study of female choice, several bird species preference. In the evolutionary context, a preference belonging to the family Estrildidae have long been is defined as any trait that biases the probabilities that used as model species. Regarding the ‘sexual dimor- females mate with different kinds of males (Kirkpatrick phism’ in behaviour, song behaviour of Zebra Finches & Ryan 1991). To study how female preferences Taeniopygia guttata and Bengalese Finches Lonchura work in a certain species, it is essential to discern striata var. domestica has been extensively studied. which trait is sexually dimorphic, that is, different Only male Zebra Finches and Bengalese Finches sing. Their songs consist of discrete song elements (Received 24 August 2010; Accepted 13 February 2011) called ‘notes,’ whose ordering follows a finite-state # Corresponding author, E-mail: [email protected] syntax (Honda & Okanoya 1999). Song features pre- 73 A. HASEGAWA et al. ferred by Estrildine females can be divided into two understand sexual selection in this family. Therefore, trait types: performance-related and elaboration- we aimed to investigate female preference for male related (Nowicki & Searcy 2005; Soma et al. 2006a). sexual traits in the Java Sparrow Padda oryzivora, Features of the former are associated with song pro- which belongs to the family Estrildidae. Java Spar- duction, such as song rate or song duration, whereas rows do not exhibit sexual differences in plumage the latter features represent song complexity, such as (Goodwin 1982; Restall 1996). However, it is reported repertoire size, note type repertoire size, or song lin- that the base of the male’s bill becomes swollen and earity. It has been reported that females of Estrildine redder in breeding condition, whereas the female’s finches show preference for both types of male song does not (Goodwin 1982). This description suggests features (Okanoya 2004a; Okanoya 2004b; Soma et the possible existence of sexual dimorphism in the al. 2006a). Of features related to song production, bill of Java Sparrows, but there is no quantitative evi- song rate seems to play an important role in female dence to support this view. The songs of male Java mate choice. Most studies that have investigated the Sparrows are likely to have evolved through female female preference for song rate in Zebra Finches have mate preference as is the case in Zebra Finches and come to the same conclusion: females preferentially Bengalese Finches, but almost nothing is known choose males that sing more frequently (Houtman about them because of the lack of research. 1992; Collins et al. 1994; De Kogel & Prijs 1996; In this study, we first took measurements of mor- Forstmeier 2004). On the other hand, studies of phological traits, including bill depth, to investigate female preference in Bengalese Finches have focused sexual dimorphism in Java Sparrows quantitatively. on song complexity. It has been reported that female We also collected data of male song-related traits of Bengalese Finches have a preference for more com- this species. Then we conducted a female choice plex songs over simpler ones. For example, when experiment to find out which of these traits contribute they hear conspecific songs of large note type reper- to female mate preference. toire size from a speaker, they show copulation solic- itation displays more often than they do when they MATERIALS AND METHODS hear songs of smaller note type repertoire size (Clayton & Pröve 1989). According to Morisaka et 1) Subjects al. (2008), female Bengalese Finches tend to choose Twelve adult Java Sparrows (six males and six more complex songs in an operant experiment using females) were obtained from local pet suppliers and conspecific songs as stimuli. kept in an aviary at the University of Tokyo. All birds While there are numerous reports on male song had pied wild-like plumage except one white male. behaviour and female choice in estrildid finches, rel- None had experienced breeding. Males were kept atively few studies have focused on female preference individually in small wooden cages (17×34.5×23 cm for male morphological traits in this family, and there high or 14.5×29.5×19 cm high) to prevent aggres- is no clear evidence that any male morphological trait sion between birds, and females were housed together functions as the target of female preference. One of in a metal cage (37×41.5×44 cm high). Males and the few well-studied sexually dimorphic traits in females were visually isolated, but they could hear estrildid finches is the bill colour in Zebra Finches, in each other. The birds had constant access to food, which males have redder bills than females. Research- water and crushed oyster shell. Each bird was marked ers have attempted to demonstrate that female Zebra with individual coloured leg bands, with red excluded Finches preferentially mate with males with a redder to minimize a potential effect on mate preference bill. However, most recent studies have failed to pro- (Burley et al. 1982). All birds were kept under a fixed vide evidence for female preference in bill colour 14:10 hr light:dark cycle, under which it is reported (Collins et al. 1994; Vos 1995; Collins & ten Cate that most Java Sparrows stay in breeding condition 1996; Forstmeier 2004). throughout the year (Kato et al. 1995), at an ambient In contrast with the two Estrildine species, in which temperature of 25±2°C. song behaviour, morphology and female choice have been well investigated, other estrildid finches have 2) Morphological measurements received little attention. However, studying female Before choice tests, eight external morphological choice in multiple Estrildine species will allow us to traits were measured for six males and six females. make interspecies comparisons and thus to better The measurements taken included body mass in 74 Female choice in Java Sparrows grams on an electric balance; natural wing length, level for all males in at least three out of the four tarsus length, exposed culmen length, bill width and acoustic characteristics (data not shown). Therefore, bill depth in millimetres using callipers; and maxi- we concluded that the note type categorization per- mum wing length and tail length in millimetres using formed in this research is reliable. For later analysis, a ruler. The Mann-Whitney U test was used to inves- the number of different note types each male had and tigate whether there is a sexual difference in morphol- the mean values of the duration of each song bout ogy in Java Sparrows. were computed. The syntactical complexity of songs was measured 3) Song recordings and song variables using a linearity index score (Scharff & Nottebohm The songs of six male Java Sparrows were recorded 1991). We assigned a letter to each note type and using a microphone (M-AUDIO NOVA) and a pre- described each song as a string of letters. We then amplifier (M-AUDIO MobilePre USB). Songs were analyzed the string and calculated transition probabil- recorded when males sang spontaneously. During ities from one note type to others to obtain a transition recording, males were visually isolated by cardboard matrix (see Fig.

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