Age-Related Difference in Size of Brain Regions for Song Learning in Adult Male Dark-Eyed Juncos <i>(Junco hyemalis)</i>

Abstract

In seasonally breeding adult male songbirds, the volumes of several song control regions (SCRs) change seasonally in parallel with plasma testosterone (T) levels and decrease following gonadectomy. Testosterone treatment to castrates prevents this decrease, indicating T dependency. During the breeding season, second-year (SY: birds entering their first breeding season) free-ranging male Dark-eyed Juncos (Junco hyemalis) have smaller testes than older (after second-year, ASY: birds entering at least their second breeding season) birds. SY males also have lower plasma T concentrations than ASY males at the beginning of the breeding season. We investigated differences in song structure of the two age groups and the relationship between age differences in gonadal function and SCR sizes. The average number of syllables per song, syllable duration, trill rate, song duration, and variability in song duration were age-independent. Two brain regions that are thought to be involved primarily in song learning and perception were 13 and 18% larger, respectively, in SY than in ASY males, the opposite of what would be expected based solely on reproductive measures (testis mass and cloacal protuberance width). In contrast, the volumes of two regions that directly control song expression did not differ with age. The lack of age-related size differences in regions that are required for song production may indicate that male juncos of all ages have similar brain space requirements for motor production. Where there were size differences, they were restricted to regions primarily controlling vocal behavior acquisition/perception, suggesting that first time breeders need more brain space than experienced breeders to acquire crystallized song and/or acoustically perceive aspects of their environment.

Song Variation in a Recently Founded Population of the Dark‐Eyed Junco (Junco hyemalis)

“…In addition, among the 16 males considered fully recorded in 2002 and also re-recorded in 2003 [average 99 songs ⁄ male (range 27-248) recorded in 2003], we discovered no additional song types, implying that no learning goes on beyond the first year. These results are consistent with the findings of Williams & MacRoberts (1977) who studied a small sample of birds from 1 yr to the next, and Corbitt & Deviche (2005) who compared some known young males with older males.…”

Age- and Sex-Related Variation in Song-Control Regions in Cassin’s Finches, <i>Carpodacus cassinii</i>

“…This age-difference in X volume has not been reported for any species prior to our initial presentation of data on Cassin's finches [MacDougall-Shackleton et al, 2000] and does not occur in starlings [Bernard et al, 1996]. Recently a study of dark-eyed juncos (Junco hyemalis) has shown that SY males have larger area X than ASY males, although the effect size is smaller than that observed in Cassin's finches [Corbitt and Deviche, 2005]. Unlike Cassin's finches, however, the older juncos had longer songs than younger males, but did not sing a greater variety of song syllables.…”

No Correlation Between Three Selected Trade‐Offs in Birdsong Performance and Male Quality for a Species With Song Repertoires

“…It might be that the most pronounced longitudinal change in best song performance takes place between the second and third years of age (Corbitt & Deviche ; Ballentine ), rather than when the birds are older. Seven birds that were recorded in both years had an age assignment of second‐year in 2006 (plotted as triangles in Fig.…”