Habitat quality affects early physiology and subsequent neuromotor development of juvenile black-capped chickadees.

In songbirds, the ability to learn and render the species-specific song is influenced by the development of both the song nuclei in the brain and the syrinx (bird's vocal apparatus) early in the bird's life. In black-capped chickadees (Poecille atricapillus), habitat quality is known to affect song structure, with birds in high-quality habitat (mature forest) having a higher song consistency than birds in low-quality habitat (young forest). Although this difference is suspected to stem from differences in development, the developmental status of juvenile birds in either habitat remains unexplored. In this study, we used ptilochronology and feather corticosterone to compare the conditional state of juvenile chickadees in young and mature forest during two distinct periods of song learning - the sensory phase, which occurs prior to settlement, and the sensorimotor phase, which occurs post-settlement. A sample of juvenile males was captured and euthanized several weeks prior to their first breeding season to compare the development of song center nuclei and syrinx in both habitats. The corticosterone levels of natally-grown feathers were greater among birds that settled in mature than young forests - as these feathers were grown pre-settlement, they reflect differences in physiology during the sensory phase. This difference in conditional state is reflected by differences in syrinx and song center nuclei development later during the sensorimotor phase - birds in young forest have smaller syrinx, and moderately-larger RA, than birds in mature forest. Those differences could be responsible for the difference in consistency in song structure observed across habitats. The difference in physiological state across habitats, combined with potential compounding effect of differences in winter resources between habitats, could influence the difference in syrinx and neural development seen in juvenile males during the early spring, and influence the male's ability to learn and render their species-specific song

Comparing CORT levels in juvenile birds from young and mature forest during the sensory phase.

<p>CORT levels (mean ±SEM) in natal feathers are higher in birds that settled in mature forests compared to birds settling in young forests (General linear model, F_1,37 = 4.71, N = 46).</p

Dominance and geographic information contained within black-capped chickadee (Poecile atricapillus) song

In songbirds, male song is an acoustic signal used to attract mates and defend territories. Typically, song is an acoustically complex signal; however, the fee-bee song of the black-capped chickadee is relatively simple. Despite this relative simplicity, two previous studies (Christie et al., 2004b; Hoeschele et al., 2010) found acoustic features within the fee-bee song that contain information regarding an individual\u27s dominance rank; however each of these studies reported a different dominance-related acoustic cue. Specifically, the relative amplitude of the two notes differed between the songs of dominant and subordinate males from northern British Columbia, while the interval pitch ratio differed between the songs of dominant and subordinate males from eastern Ontario. In the current study, we examined six acoustic features within songs from both of the chickadee populations (northern British Columbia and eastern Ontario) examined in these previ- ous studies and used bioacoustic analyses and discriminant function analyses to determine whether there is a consistent dominance-related acoustic cue across both, or in each of these populations. Consistent with the previous findings, the current results indicate that relative amplitude differs based on dominance status in the songs from British Columbia; however, our results failed to reach significance with songs from Ontario. These results suggest that acoustic cues that signal a male\u27s dominance in this species vary with geographic location. Furthermore, examining songs from these two locations and one additional location in northern British Columbia, we found that discriminant function analyses could correctly classify songs based on geographic location. Considering the broad extent of the species\u27 range, black-capped chickadee song is considered relatively invariant; however, our results suggest that there is geographic variation in songs, although the differences are subtle compared to geographic song variation in other species. © 2013 Koninklijke Brill NV, Leiden

Comparing the development of syrinx, RA and HVC across habitat.

<p>(a) Birds from young forests have a lighter syrinx than birds in mature forest (F_1,17 = 5.86, N = 20, p = 0.02). (b) An intermediate effect (based on power analysis) was found between RA volume in young vs. mature forests; birds in young forests tend to have a bigger RA volume than birds in mature forest (F_1,17 = 0.81, N = 20, p = 0.38; d = 0.38). (c) HVC volume did not differ significantly across habitats (F_1,17<0.01, N = 20, p = 0.99; d = 0.13). Values presented are Means ±SE.</p

Correlation between CORT levels during sensory phase and syrinx weight.

<p>Syrinx weight of black-capped chickadee in their first spring is correlated with CORT levels of their natal feathers (regression analysis, R² = 0.1, N = 20, p = 0.03).Circles represent birds from young forest; squares represent birds from mature forest.</p

Black-capped chickadees categorize songs based on features that vary geographically

The songs of many songbird species vary geographically, yet, the songs of black-capped chickadees, Poecile atricapillus, show remarkable consistency across most of the species\u27 North American range. Previous research has described subtle variations in the song of this species by comparing songs produced by males at distant parts of the species\u27 range (British Columbia and Ontario). In the current study, we used an operant discrimination task to examine whether birds classify the songs produced by males in these two previously studied locations as belonging to distinct open-ended categories. In both experiments, when birds were presented with new songs, they continued to respond to songs from the same geographical location as the songs that were reinforced during initial discrimination training, suggesting that birds were using open-ended categorization. We also presented birds with songs in which we manipulated acoustic features in order to examine the acoustic mechanisms used during discrimination; results provide support that birds use the duration of the song when discriminating, but the results also suggest that birds used additional acoustic features. Taken together, these experiments show that black-capped chickadees classify songs into open-ended, geography-based categories, and provide compelling evidence that perceptible acoustic differences exist in a vocalization that is seemingly consistent across the species\u27 range. © 2015 The Association for the Study of Animal Behaviour