Environmental Constraints on Song Structure

1. Environmental Constraints on Song Structure • Forests: pattern in amplitude modulation and spacing of sounds • More attenuation of short wavelengths disrupts frequency modulation over distance • Examples: Northern Cardinal, Carolina Wren • Grasslands: pattern in frequency modulation • Examples: Eastern Meadowlark, Song Sparrow • Upward defraction of sound leads to early singing, singing from high perches, song flights

3. Duets • Occurs 240 species in 44 families • 20+ species each shrikes, wrens, honeyeaters • Antiphonal singing = duets in which sexes alternate notes in what sounds like one song • Associated with jointly-held year-round territories, especially in the tropics • Possible function = territory defense female (she starts song), mate-guarding male (he adds) • Local example = Carolina Wren

5. Elements of Song Learning • Innate template: neural image in brain determines what pay attention to • Sensitive period: modify innate template to produce permanent motor tape based auditory input during a particular period • Practice (song crystallization): during later period sing subsong, modify based on hearing self sing to produce adult song

7. Experimental Evidence • Birds isolated during sensitive period sing unmodified template, even if surrounded by other singing birds as adults • Birds deafened after sensitive period sing subsong (no auditory feedback for practice) • Birds deafened as adults sing normal song, do not need to hear self sing to produce perfect song

8. Variations in Song Learning • Specificity of template • Swamp Sparrow learns only syllables from own species, Song Sparrow sometimes learns syllables other species, mimics learn everything • Complexity of template • White-crowned Sparrow simple, differs adult song; Song Sparrow more complex, more like adult song

9. Timing of sensitive period • Early, before practice • Later, overlaps with practice • Both early and later • Annual, modify song every year • How well copy what heard • Copy song units exactly • Copy syllables exactly, make own song units • Improvise and invent as well as copy

10. Outcomes of Variation • Broad templates, inexact copying, long and late sensitive periods associated with large song repertoires • Narrow templates, exact copying, early sensitive periods associated with small song repertoires and stereotyped songs • Timing of sensitive periods often matches ecology to enable song matching

11. Many sedentary species have fixed songs, early sensitive periods (after dispersal) Example: Marsh Wren

12. Many nomadic species relearn songs yearlyExample: Sedge Wren

14. Dialects in Birds • Regional differences in song • Some are sharp, distinct, persistent • White-crowned Sparrow example • Some are gradual, so that populations far apart sound the most different • Some may function to promote song matching, others may be accidental byproducts of the song learning system

16. Elements Song Control System • HVC (High Vocal Center) in hyperstriatum and RA nucleus in corpus striatum are the song control centers • Connections here = innate template that is modified into motor program for song • New neurons, synapses form here during song learning, even in adults • TSM nucleus controls muscles of syrinx to produce song • TSM controlled by HVC and RA

17. More Elements • L nucleus receives auditory input, provides feedback to song control centers • Provides input to modify template during sensitive period • Provides input during practice • Not involved in singing by adult birds • ICo nucleus in midbrain controls motivation to sing (when to sing, not what to sing)

18. Hormonal Control • Hormones involved in seasonal, other longer term changes • Hormones bind to neurons in ICo to control seasonal changes in motivation to sing • Hormones involved in sensitive periods, control growth and other changes (based on learning) in song control centers