Val Nolan Jr. - Selected Abstracts

Back to Val Nolan's main page

Val Nolan Jr. and Ellen D. Ketterson. 1990. Timing of autumn migration and its relation to winter distribution in dark-eyed juncos. Ecology 71:1267-1278.

ABSTRACT : Migratory Dark-eyed Juncos in eastern North America migrate southward in autumn and tend to segregate in the winter range according to sex and age. North to south, the most abundant classes are young males, adult males, young females, and adult females. Because adults tend to dominate young at winter feeding sites, this distribution appears at first to conflict with the view that dominance interactions are responsible for differential avian migrations and that dominant interactions are responsible for differential avian migration and that dominant individuals remain nearest the breeding range. However, if young juncos establish winter residency at earlier dates than adults, a prior residence effect might make them dominant. This hypothesis requires that young arrive at wintering sites earlier than adults and that residents of all classes arrive earlier than their transient counterparts. We tested these predictions by comparing first-capture dates of juncos at Bloomington, Indiana, which is north of the latitudinal midline of the junco's winter range. During 13 autumns, individuals were classed as local residents or as transients and were sexed and aged. Contrary to predictions, residents were not caught earlier than transients. Further, adults were caught earlier, not later, than young among transients and probably also among residents. Thus, settlement of the winter range does not proceed from north to south, and dominance established through prior residence cannot account for the concentration of 1st-yr males in the northern part of the winter range. The fact that some sex-age classes tend to winter south of others predicts that at a northern capture site the classes that migrate farthest should be commoner among transients than among residents. This expectation was fulfilled. In addition, the median capture dates of the sex-age classes were arranged approximately according to the north-to-south order of their distribution, indicating that classes with the farthest to travel passed through (transients) or settled (residents) earliest in autumn. We conclude that comparison of autumn migration schedules of transient and resident passerine birds at a single location can yield considerable information about the dynamics of settlement of the entire winter range, including possible information about differences in destination of subsets of transients.
© 1990 by the Ecological Society of America

Val Nolan Jr., Ellen D. Ketterson, Charles Ziegenfus, Daniel P. Cullen, and C. Ray Chandler. 1992. Testosterone and avian life histories: Effects of experimentally elevated testosterone on prebasic molt and survival in male dark-eyed juncos. The Condor 94:364-370.

ABSTRACT : Male Dark-eyed Juncos (Junco hyemalis) that breed in Virginia begin their prebasic molt after breeding has ended, usually in August. Almost all males caught in late October have completed the molt. In 1989, we obtained anecdotal evidence that males whose testosterone (T) we maintained at artificially elevated levels beyond the end of the breeding season postponed or suppressed prebasic molt. To test the effect of T experimentally, in spring 1990 we implanted some males (T-males) with testosterone and others (C-males) with empty implants, and we released both groups to breed. We caught some members of both treatment groups in October and removed their implants. The T-males had delayed their prebasic molt, while the C-males were molting on schedule. Other implanted T- and C-males were not caught; these carried their implants into winter. Next spring we examined surviving males whose implants we had removed in October as well as males whose T- and C- implants had not been removed. T-males whose implants we had removed had molted completely, despite their delayed start, whereas T-males whose implants we had not removed had not molted. Still-implanted C-males had molted. We compared the minimum over-winter survival (i.e., return rates in spring) of the treatment groups. T- and C- males whose implants we had removed in October returned at the same rate, but among males whose implants we had not removed, significantly fewer T-males than C-males returned. The transition between reproduction and molt of male juncos apparently can be blocked by preventing the normal seasonal decline in T. This suggests a physiological basis for a possible trade-off between time allocated to reproduction and time allocated to molt. Our results indicate that males could maintain high T and prolong breeding, possibly into October, and still molt completely with no adverse effects. We consider why such a modification of schedule has not occurred. However, postponement of molt beyond some date in autumn, possibly late October, suppresses it altogether, as indicated by the failure to molt of the returning T-males whose implants we did not remove. This treatment group apparently suffered higher overwinter mortality, and we consider possible reasons.
© 1992 The Cooper Ornithological Society

Torgeir S. Johnsen, James D. Hengeveld, James L. Blank, Ken Yasukawa & Val Nolan Jr. 1996. Epaulet brightness and condition in female red-winged blackbirds. The Auk 113:356-362.

ABSTRACT : Epaulets of female Red-winged Blackbirds (Agelaius phoeniceus) vary in brightness from brown to bright red-orange. We test predictions of the hypothesis that condition at the time of molt determines female epaulet brightness and that females in superior condition produce brighter epaulets. We compared each female's epaulet with a series of color photographs ranked from 1 (dull brown) to 12 (bright red-orange) and considered females to have increased in brightness between years if their color increased by at least two ranks. More first-year females than older females increased in brightness, more older females increased in brightness after a year of superabundant food (an emergence of periodical cicadas, Magicicada spp.) than after other years. In each case, we expected an improvement in condition. We estimated female condition and regressed condition index on day of the breeding season. During the breeding season, females that later increased in brightness previously reported correlation between age and epaulet brightness, but age does not explain the relationship between change in brightness and either reproductive effort or the emergence of cicadas. We conclude that the most likely explanation for our results is that condition, at least in part, determines epaulet brightness in female Red-winged Blackbirds and that superior signaling hypothesis and mate-choice hypotheses for the evolution of variable plumage. The best explanation for the evolution of variable plumage among female Red-winged Blackbirds is that brightness signals status in female-female aggressive encounters.

C. Ray Chandler, Ellen D. Ketterson, and Val Nolan Jr. 1997. Effects of testosterone on use of space by male dark-eyed juncos when their mates are fertile. Animal Behaviour 54:543-549.

ABSTRACT : Testosterone is an important determinant of spatial activity in male birds. Using radio-telemetry, male dark-eyed juncos, Junco hyemalis, were followed during the period when their mates were fertile to investigate the relationship between testosterone and behaviours (territoriality, consorting with female) through which males might guard paternity. Males with experimentally elevated testosterone levels (T-males) and control males (C-males) did not differ significantly in their use of space during this period. T-males and C-males occupied similar home ranges, used similar-sized core areas (areas enclosing 95% of all activity), spent similar amounts of time close to their mates, and experienced similar intrusion rates onto their territories. Experimentally elevating testosterone above control levels did not detectably affect the spatial activity of male juncos during the female fertile period, although earlier studies have shown that its effects are pronounced during other stages of the nesting cycle.
© 1997 The Association for the Study of Animal Behaviour

Russell C. Titus, C. Ray Chandler, Ellen D. Ketterson, and Val Nolan Jr. 1997. Song rates of dark-eyed juncos do not increase when females are fertile. Behav. Ecology & Sociobiology 41:165-169.

ABSTRACT : Frequency of singing by birds may vary with reproductive stage in ways that reflect variation in the functions of song in intersexual and intrasexual communication. In dark-eyed juncos (Junco hyemalis) high-amplitude song is produced only by males. To investigate the function of this song, we tested whether fertility of females affected singling by their mates or by neighboring males. Using focal observations, song censuses, and radiotracking data, we determined whether song production varied between and among periods when females were fertile and non-fertile. Our findings show that males do not increase song production when their mates are fertile, nor do they increase song production when neighboring females are fertile. These results suggest that male juncos do not signal their intent to defend territories (or mates) more when females are fertile and that they do not use song to advertise to specific potential participants in extra-pair fertilizations.
© Springer-Verlag 1997