The evolution of acoustic identity signals in birds

In this dissertation I present several novel findings on the evolution of acoustic identity traits in birds, with a special focus on parent-offspring communication. I present novel insights on how vocalizations are influenced by condition, environment and genes, and which acoustic parameters are used to form unique identity signatures. Furthermore, I provide first experimental evidence for the role of acoustic similarity for recognition errors in birds, supporting the importance of negative-frequency dependent selection for the evolution and spread of individual signatures

The effect of hunger on the acoustic individuality in begging calls of a colonially breeding weaver bird

<p>Abstract</p> <p>Background</p> <p>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.</p> <p>Results</p> <p>In a field study, we experimentally manipulated the hunger level of colonially breeding Jackson's golden-backed weaver (<it>Ploceus jacksoni</it>) nestlings close to fledging and investigated its effects on acoustic call parameters. Some acoustic parameters that were related to the time-frequency 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.</p> <p>Conclusion</p> <p>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.</p

Do Zebra Finch Parents Fail to Recognise Their Own Offspring?

Individual recognition systems require the sender to be individually distinctive and the receiver to be able to perceive differences between individuals and react accordingly. Many studies have demonstrated that acoustic signals of almost any species contain individualized information. However, fewer studies have tested experimentally if those signals are used for individual recognition by potential receivers. While laboratory studies using zebra finches have shown that fledglings recognize their parents by their “distance call”, mutual recognition using the same call type has not been demonstrated yet. In a laboratory study with zebra finches, we first quantified between-individual acoustic variation in distance calls of fledglings. In a second step, we tested recognition of fledgling calls by parents using playback experiments. With a discriminant function analysis, we show that individuals are highly distinctive and most measured parameters show very high potential to encode for individuality. The response pattern of zebra finch parents shows that they do react to calls of fledglings, however they do not distinguish between own and unfamiliar offspring, despite individual distinctiveness. This finding is interesting in light of the observation of a high percentage of misdirected feedings in our communal breeding aviaries. Our results demonstrate the importance of adopting a receiver's perspective and suggest that variation in fledgling contact calls might not be used in individual recognition of offspring

Parent – environmental interactions shape acoustic signatures in tree swallows: a cross-fostering experiment.

Acoustic signatures are common components of avian vocalizations and are important for the recognition of individuals and groups. The proximate mechanisms by which these signatures develop are poorly understood, however. The development of acoustic signatures in nestling birds is of particular interest, because high rates of extra-pair paternity or egg dumping can cause nestlings to be unrelated to at least one of the adults that are caring for them. In such cases, nestlings might conceal their genetic origins, by developing acoustic signatures through environmental rather than genetic mechanisms. In a cross-fostering experiment with tree swallows Tachycineta bicolor, we investigated whether brood signatures of nestlings that were about to fledge were attributable to their genetic/maternal origins or to their rearing environment. We found that the calls of cross-fostered nestlings did not vary based on their genetic/maternal origin, but did show some variation based on their rearing environment. Control nestlings that were not swapped, however, showed stronger brood signatures than either experimental group, suggesting that acoustic signatures develop through an interaction between rearing environment and genetic/maternal effects

Histogram plots of single response calls in relation to latency of response to (A) own and (B) alien fledglings.

<p>Most responses are given shortly after the stimulus, which shows that calls are emitted in response to the stimulus. The dashed line indicates average response frequency of random response latency. Response frequencies for alien fledglings are three times higher, caused by a 1∶3 ratio of own versus alien stimuli per adult.</p

Three representative calls from different individuals showing inter-individual variability of fledgling distance calls.

<p>Three representative calls from different individuals showing inter-individual variability of fledgling distance calls.</p

Average number of response calls (± standard errors) for mothers and fathers to own and alien fledgling calls.

<p>Average number of response calls (± standard errors) for mothers and fathers to own and alien fledgling calls.</p

Playback design for fledgling stimulus.

<p>Each fledgling stimulus was used with both parents. The order of own and alien stimuli (1, 2, 3, 4) was randomized with the repeat unit.</p