Dawn Chorus Variation in East-Asian Tropical Montane Forest Birds and Its Ecological and Morphological Correlates
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Contributions to Zoology, 84 (3) 255-265 (2015) Dawn chorus variation in East-Asian tropical montane forest birds and its ecological and morphological correlates 1 1, 5 1 2 3 4 1 Wei-Ming Chen , Ya-Fu Lee , Chi-Feng Tsai , Cheng-Te Yao , Yung-Hsiu Chen , Shou-Hsien Li , Yen-Min Kuo 1 Department of Life Sciences, National Cheng Kung University, Tainan 701, Taiwan 2 Taiwan Endemic Species Research Institute, Nantou 552, Taiwan 3 Liouguei Research Center, Taiwan Forestry Research Institute, Kaohsiung 84442, Taiwan 4 Department of Life Sciences, National Taiwan Normal University, Taipei 106, Taiwan 5 E-mail: [email protected] Key words: breeding, canopy, diets, eye size, foraging, perch, Taiwan, tropical forests Abstract Dawn chorus sampling .......................................................... 257 Eye size and morphometric measurements ....................... 257 Foraging perch heights and food habits Many birds in breeding seasons engage in vigorous dawn sing- ........................... 258 ing that often turns to a prominent chorus. We examined dawn Phylogenetic independent contrasts .................................. 258 chorus variation of avian assemblages in a tropical montane Data analyses .......................................................................... 258 forest in Taiwan and tested the hypothesis that onset sequence Results ............................................................................................. 259 is affected by eye sizes, foraging heights, and diet of birds. Cho- Dawn singing pattern and temporal variation ............... 260 rus onset and duration varied between sampling months, but Independent contrasts ........................................................... 261 generally peaked in the prime breeding season. Overall dawn The effects of eye size, foraging height, and diet ............ 261 chorus length increased with, but mean duration per species Discussion ...................................................................................... 262 was negatively correlated with, species richness. The inter-cor- Acknowledgements ...................................................................... 264 relation among trait variables in phylogenetic independent con- References ...................................................................................... 264 trasts was calculated and examined. Both foraging height and relative eye size were selected as positively explanatory factors, but no dietary effects were detected, for chorus onsets. Dawn Introduction singing onsets also tended to delay as decreasing log (eye sizes) that, however, was not selected with enough explanatory power. The positive relationship of chorus onsets versus relative eye Many bird species, notably but not exclusively the sizes contradicts our prediction, yet the negative correlation of songbirds (Passeriformes), use vocal sounds to fulfill relative eye sizes with log (eye sizes) indicates an allometric intersexual interactions, such as self-advertising and constraint on eye sizes along with increasing body sizes. Lower- stimulating mates (e.g., Kroodsma, 1976; Otter and layer species initiated singing earlier than upper-layer species, et al which complies with the positive correlation of onsets with for- Ratcliffe, 1993; Welling ., 1995), or intrasexual in- aging heights and supports our prediction. This pattern may be teractions like social communication and acquiring or condition-specific and more likely occurs in forests lacking a defending territory (Amrhein and Erne, 2006; Foote sufficient canopy height and layering and a distinct light differ- et al., 2008), or both. This behaviour is particularly ence among forest layers. Foraging heights were additionally found negatively correlated with canopy coverage and slightly prominent at dawn and for males in the breeding sea- positively correlated with ground coverage of perches. Our re- sons (Slagsvold, 1996; Kunc et al., 2005; McNeil et al., sults concord that foraging height is a relevant factor as eye size 2005; Amrhein and Erne, 2006), and often turns into a in determining the onset and sequence of dawn chorus, yet, sug- collective phenomenon known as dawn chorus. gest that forest settings and vegetation structure may likely While the association of dawn chorus with the complicate the prediction. breeding seasons offers ontogenetic (i.e., sexual matu- rity) and mechanistic (i.e., hormone) explanations, the Contents reasons why male birds engage in vocal activity vigor- ously at dawn appear to be due to, at least in part, cer- Introduction ................................................................................... 255 tain proximate physical factors that are not mutually Material and methods .................................................................. 257 exclusive, and also carry functional perspectives. Low- Study sites ................................................................................ 257 er temperatures and high humidity at dawn may restrict 256 Chen et al. – Dawn chorus variation tropical montane forest birds activity of both birds and their prey (Avery and Krebs, even at dim light (Hutchinson, 2002; Thomas et al., 1984), but also that of predators that otherwise would 2002; McNeil et al., 2005), which may be determined be attracted to bird calls. In contrast, lower wind speed by a bird’s visual anatomical features, but also associ- and background noise levels in this period are more ated with the surrounding settings and light levels energetic-efficient and favourable for sound travel with where a bird is situated. consistency (i.e., the acoustic transmission hypothesis; Birds with larger eyes tend to have earlier dawn Henwood and Fabrick, 1979; Cuthill and Macdonald, singing (Thomas et al., 2002; McNeil et al., 2005). 1990; Brown and Handford, 2003; but see Dabelsteen This is supported by anatomical relationships that and Mathevon, 2002). The limited light level at dawn larger-eyed birds own higher numbers of light-sensi- prohibits diurnal birds from many daytime activities, tive visual cells on the retina, a thicker inner retinal such as foraging (i.e., the inefficient foraging hypothe- layer, and more neuron synapses, all these indicating a sis; Kacelnik, 1979; Kacelnik and Krebs, 1982) or social better visual sensitivity and higher resolution for low contacts with one another (Cuthill and Macdonald, light conditions (Hart, 2001a, 2001b; Garamszegi et al., 1990), but the latter can be achieved vocally (Kacel- 2002; McNeil et al., 2005). Forest layers, due to the nik, 1979; Amrhein and Erne, 2006; Foote et al., amount of foliage overhead, can have different light 2008). levels, and birds perching at a higher place usually re- Temporal variation in dawn chorus may also be ceive a richer light availability. Yet, the position cho- caused by proximate factors such as light availability sen by a bird for signaling (i.e., songposts) may not be that is usually, but not completely (see Kempenaers et the same as, and often is higher than, that used for al., 2010; Da Silva et al., 2015), related to time, weath- roosting and foraging (Hunter, 1980; Barker and Men- er, and vegetation (Berg et al., 2006). Overnight birds nill, 2009). Most forest birds foraging at lower-layers may have surplus energy from previous feeding to sus- or on the ground also tend to roost at higher places or tain the night time fast and a lower temperature, which even up to the canopy at night. In contrast, species helps to explain why birds can afford to delay foraging adapted to constantly foraging at lower heights or on and instead engage in singing at dawn (i.e., energy the ground may have evolved a higher visual sensitiv- storage stochasticity hypothesis; Hutchinson, 2002). ity and acuity that allows them to sense better in dim This reasoning concerns more on the variability of light earlier (McNeil et al., 2005; Thomas et al., 2006). participation, so individual variation in quality or sta- Further, avian foraging relies heavily on visual tus can be assessed (i.e., state-dependent), but does not cues, so different visual sensitivities may have evolved directly address the sequence and variation at dawn in species adapted to different diets (Hart, 2001b; Ga- singing among species (Cuthill and Macdonald, 1990; ramszegi et al., 2002; Fernández-Juricic et al., 2004). Berg et al., 2006). Masking of signals, such as by Compared to plant resources, prey are often vigilant, noise, carry additional cost and negatively impact requiring rapid detection by birds, or cryptic in colora- avian reproductive success (Halfwerk et al., 2011). Si- tion or pattern, both require substantial light levels to multaneous songs by various species may also inter- detect. In case of insect swarms, aerial insectivores are fere with each other and affect the transmission effec- generally equipped with a larger degree of movement tiveness, thus a temporal asynchrony to avoid signal sensitivity and resolution-related double cone cells for overlapping should be favored (Cody and Brown, detecting prey in movement at fast speeds (Hart, 1969; Ficken et al., 1974; Popp et al., 1985). 2001a). Thus, predatory birds should have better visual Species may differ in their sensitivity to light, there- capacity and light sensitivity to effectively sesarch for fore sense the light level effectively at slightly different prey, particularly for foraging within a forest setting time or place (Thomas et al., 2002; McNeil et al., (Thomas et al., 2002; McNeil et al., 2005).