The evolution of acoustic identity signals in birds

In colonially breeding birds, the ability to discriminate between individuals is often essential. During post-fledging care, parents have to recognize their own offspring among many other unrelated chicks in the breeding colony. It is well known that fledglings and food-provisioning parents of many bird species use contact calls to convey their identity. These calls are also often used as hunger-related signals of need in young birds. Here, we investigate how such calls incorporate signals of need and at the same time act as reliable indicators of each chick’s identity. In a field study, we experimentally manipulated the hunger level of colonially breeding Jackson’s goldenbacked weaver (Ploceus jacksoni) nestlings close to fledging and investigated its effects on acoustic call parameters. Some acoustic parameters that were related to the timefrequency pattern showed high individuality and were largely unaffected by a nestling’s state of hunger. However, the majority of call parameters were significantly affected by hunger. Interestingly, most of these acoustic parameters showed both consistent changes with hunger and high between-individual differences, i.e. potential for individual recognition. The results indicate that individual recognition processes can be based on static, hunger-independent call parameters, but also on dynamic hunger-related parameters that show high individuality. Furthermore, these signal properties suggest that the assessment of signals of need can be improved if the signal value is referenced to a chick’s vocal spectrum. Revised version published as: Reers H, Jacot A (2011) The effect of hunger on the acoustic individuality in begging calls of a colonially breeding weaver bird. BMC Ecology 11:3 The evolution of identity signals in birds 28 coustic signalling in birds is a popular system in which to ask questions about the evolution of identity signalling systems and the costs and benefits of reliably communicating a sender’s condition. These two different kinds of signals (i.e. identity and condition) require very different trait properties. Signals of identity need to be relatively consistent over time within individuals to reliably indicate the senders identity (for review see Tibbetts and Dale 2007). In contrast, condition signals, indicating short term changes in condition (e.g. hunger in food-dependent young), need to be plastic within an individual to reliably reflect the different conditions of a sender (e.g. Iacovides and Evans 1998; Leonard and Horn 2001). Both types of signals have been studied separately in parentoffspring communication. However, these two signals occur simultaneously in begging calls of young birds and the question arises: how can one acoustic signal manifest the need for simultaneous high and low intra-individual variation? So far, no study has investigated both types of signals and their contribution to the acoustic variability in a parent-offspring communication system. A In many colonially breeding species, parents need to discriminate their mobile chicks from other conspecific young (Beecher 1989). In most species, offspring still rely on parental care after a post-fledging phase and the accurate recognition of own offspring is important. Although the crucial time period for parent-offspring recognition is relatively short (i.e. ranging from a few days to several months), one expects selection for reliable recognition mechanism. In species in which misidentification is likely, failure of parents to recognize their offspring is prone to entail fitness costs for both, parents (i.e. reduced reproductive success) and offspring (i.e. starvation). Acoustic parent-offspring recognition has been shown in colonially breeding seabirds (e.g. Tschanz 1959; Lefevre et al. 1998; Charrier et al. 2001; Insley et al. 2003), with a special focus on penguins (e.g. Aubin and Jouventin 2002), and in fewer studies on songbirds (e.g. Draganoiu et al. 2006; Jacot et al. 2010). Frequency modulation (FM) has been found to be an important cue for acoustic individual recognition in birds. For example, king penguin (Aptenodytes patagonicus) chicks recognize their parents by FM patterns in their call (Jouventin et al. 1999) and zebra finches (Taeniopygia guttata) facilitate mate recognition by using FM cues (Vignal et al. 2008). Although these studies establish that FM is important for acoustic recognition, most likely a combination of different acoustic parameters is used by Chapter 1: The effect of hunger on individuality 29 the receiver to recognize the signaller. By using multiple auditory components, individuals may increase the information content of the call, which is expected to facilitate recognition (Beecher 1989; Candolin 2003). Many studies have also shown that begging calls incorporate information about a chick’s energy requirements or body condition (i.e. signals of need) in an effort to solicit food from parents or compete with siblings (e.g. Godfray 1991; Redondo and Castro 1992; Kilner and Johnstone 1997; Wright and Leonard 2002; Kilner and Drummond 2007). These ‘need’ signals are highly dynamic (i.e. variable with changes in state of need) and exhibit large intra-individual gradual variation; such variation has been correlated to a chick’s body condition or hunger level (e.g. Iacovides and Evans 1998; Kilner et al. 1999; Marques et al. 2009). An increase in begging intensity is often associated with an increase in call rate, call amplitude and begging bout length (reviewed in Kilner and Drummond 2007; but see Anderson et al. 2010). Compared to these characteristics of call delivery, the influence of hunger on acoustic parameters of individual calls has been studied in relatively few species (e.g. Leonard and Horn 2001; Sacchi et al. 2002; Gladbach et al. 2009; Marques et al. 2009; Anderson et al. 2010). Consequently, in species where selection favours individual recognition, begging calls are likely to incorporate both identity cues and signals of need. Therefore, begging calls represent an excellent system to investigate the expression pattern of static (Tibbetts and Dale 2007) and dynamic traits (Kilner et al. 1999) within the same acoustic signal. In a field study with Jackson’s golden-backed weavers (Ploceus jacksoni) we aim to identify how variation in begging calls simultaneously reflects changes in hunger and incorporates individual distinctiveness. This species is perfectly suited for studying the evolution of individual signatures in begging. It is a colony breeder with highly synchronized breeding at the start of the rainy season. This high synchronization leads to simultaneous fledging on a massive scale, in which parents must be able to locate their offspring in order to provide post-fledging parental care (e.g. food provisioning) (Fry and Keith 2004). In a first step, we experimentally manipulated hunger levels of nestlings close to fledging and predicted that this treatment would change acoustic parameters in relation to a nestling’s hunger level. The time point is important because we The evolution of identity signals in birds 30 predict individual signatures to be developed closely before fledging to enable parents to learn individual signatures while the nestling is still in the nest. Following this, we analyzed the variance components in the begging calls to apportion the observed variance to differences between hunger levels and individuals. Our prediction was that begging calls contain stable individual information over varying hunger levels to reliably indicate a nestling’s identity and dynamic information about a chicks hunger. In a last step, we used multivariate methods (i.e. discriminant function analyses) to demonstrate statistically if and how distinctiveness of individuals changes with hunger.