Age-related phenotypic changes (maturation–senescence) commonly occur during an organism’s lifetime. While maturation is usually considered as a measure of individual quality, senescence is associated with loss of function and physiological deterioration. To date, little is known about age-dependent expression of acoustic sexually selected traits over a lifetime. Using a free-living population of great tits, Parus major , we recorded the song of individually marked males in a standardized way during a 4-year period, in a longitudinal design. The study focused on two previously identified, acoustic, sexually selected signals: song consistency and repertoire size. Young (1–2 years old) and old (5–6 years old) individuals expressed lower song consistency than individuals at intermediate age (3–4 years old), suggesting song tuning during the first 3 years of life, and deterioration after the peak is reached. Repertoire size, in contrast, did not vary with age. To the best of our knowledge, our study is the first to evaluate song changes at advanced age and to report senescence effects on song expression in free-living songbirds. Our results suggest that, similarly to speech in humans, different aspects of birdsong are differentially affected by age: while motor performance traits (song consistency) deteriorate with age, language skills (repertoire size) may not be affected during the life span.

Males often express traits that improve competitive ability, such as aggressiveness. Females also express such traits but our understanding about why is limited. Intraspecific aggression between females might be used to gain access to reproductive resources but simultaneously incurs costs in terms of energy and time available for reproductive activities, resulting in a trade-off. Although consistent individual differences in female behaviour (i.e. personality) like aggressiveness are likely to influence these reproductive trade-offs, little is known about the consistency of aggressiveness in females. To quantify aggression we presented a female decoy to free-living female great tits (Parus major) during the egg-laying period, and assessed whether they were consistent in their response towards this decoy. Moreover, we assessed whether female aggression related to consistent individual differences in exploration behaviour in a novel environment. We found that females consistently differed in aggressiveness, although first-year females were on average more aggressive than older females. Moreover, conform life history theory predictions, ‘fast’ exploring females were more aggressive towards the decoy than ‘slow’ exploring females. Given that personality traits are often heritable, and correlations between behaviours can constrain short term adaptive evolution, our findings highlight the importance of studying female aggression within a multivariate behavioural framework.

Acoustic signals during intrasexual interactions may help receivers to establish the cost and benefits of engaging in a confrontation versus avoiding the cost of escalation. Although birdsong repertoires have been previously suggested as providing information during agonistic encounters, the cost (time/neural resources) of assessing large repertoires may decrease the efficiency of the signal for mutual assessment. Acoustic-structural features may, therefore, be used to enable a fast and accurate assessment during this kind of encounters. Recently, it has been suggested that the consistency of songs may play a key role during intrasexual interactions in bird species. Using a playback experiment in a colour-ringed great tit population, we tested the hypothesis that songs differing in consistency may elicit a differential response, indicating that the signal is salient for the receivers. Great tit males clearly responded more aggressively towards highly consistent songs. Our findings, together with previous evidence of increased song consistency with age in the great tit, suggest that song consistency provides information on experience or dominance in this species, and this phenomenon may be more widespread than currently acknowledged.

Colombia is the country with the highest bird diversity in the world. Despite active research in ornithology, compelling morphological information of most bird species is still sparse. However, morphological information is the baseline to understand how species respond to environmental variation and how ecosystems respond to species loss. As part of a national initiative, the Instituto Alexander von Humboldt in collaboration with 12 Colombian institutions and seven biological collections, measured up to 15 morphological traits of 9,892 individuals corresponding to 606 species: 3,492 from individuals captured in field and 6,400 from museum specimens. Species measured are mainly distributed in high Andean forest, páramo, and wetland ecosystems. Seven ornithological collections in Colombia and 18 páramo complexes throughout Colombia were visited from 2013 to 2015. The morphological traits involved measurements from bill (total and exposed culmen, bill width and depth), wing (length, area, wingspan, and the distance between longest primary and longest secondary), tail (length and shape), tarsus (length), hallux (length and claw hallux), and mass. The number of measured specimens per species was variable, ranging from 1 to 321 individuals with a median of four individuals per species. Overall, this database gathered morphological information for >30% of Colombian bird diversity. No copyright, proprietary, or cost restrictions apply; the data should be cited appropriately when used.