Diane L. H. Neudorf

Elevated testosterone reduces choosiness in female dark-eyed juncos (Junco hyemalis): evidence for a hormonal constraint on sexual selection?

Because testosterone (T) often mediates the expression of attractive displays and ornaments, in the absence of constraints sexual selection should lead to an evolutionary increase in male T levels. One candidate constraint would be a genetic correlation between the sexes that leads to a correlated response in females. If increased T in females were to have deleterious effects on mate choice, the effect of sexual selection on male T would be weakened. Using female dark–eyed juncos (Junco hyemalis), we tested whether experimentally enhancing female T would lead to a decrease in discrimination between two classes of males, one treated with T (T–males) and one control (C–males). The two female treatments (T–implanted and C–females) spent equal amounts of time with both classes of males, but T–treated females failed to show a preference for either male treatment, whereas C–females showed a significant preference, albeit in an unexpected direction (for C–males). T–females were less discriminating than C–females, irrespective of the direction of their preference. To our knowledge, this is the first study to show that circulating hormones can alter female choosiness without reducing sexual motivation. Our results suggest that hormonal correlations between the sexes have the potential to constrain sexual selection on males.

Male Northern Orioles eject cowbird eggs: implications for the evolution of rejection behavior

We experimentally parasitized 16 nests of the Northern Oriole (Icterus galbula) to determine the method of rejection of Brown-headed Cowbird (Molothrus ater) eggs, i.e., removal by spiking or by breaking the egg into pieces, and whether males remove cowbird eggs. Cowbird eggs were ejected from all 16 nests. We observed egg ejection at 14 of the parasitized nests. At six nests, cowbird eggs were ejected by breakage. Egg breakage did not result in more damage to Northern Oriole eggs than removal by spiting. For every cowbird egg ejected 0.38 Northern Oriole eggs disappeared or were damaged. Males removed the cowbird egg at two nests, females removed the cowbird egg at 11 nests, and at one nest both the male and female participated in the removal of the cowbird egg. At 13 nests, ejection was carried out by the bird that first inspected the nest after experimental parasitism. Damage to Northern Oriole eggs occurred proportionately more often during male ejections, which suggests males are less experienced egg ejecters, but this requires further testing. Because males can eject cowbird eggs the evolution of the rejecter trait may occur at an even faster rate than previously modeled

A comparative analysis of laying times in passerine birds

Abstract We documented the time of day of laying and the total time spent on the nest during laying (laying bout) in 21 species of passerine birds and the brood-parasitic Brown-headed Cowbird (Molothrus ater) at Delta Marsh, Manitoba, between 1989 and 1995. Brown-headed Cowbirds laid on average 31.2 min before sunrise, whereas mean laying times of the other species ranged from 11.3 min after sunrise through early afternoon. Species that laid later in the day tended to spend more time on their nests during laying. Smaller species tended to lay earlier in the day and had significantly shorter laying bouts than larger species. Future studies accounting for variation in foraging and reproductive behavior and predation risk between taxonomic groups may explain additional variation in laying time and duration.

THE EFFECTS OF FOOD SUPPLEMENTATION ON EXTRATERRITORIAL BEHAVIOR IN FEMALE NORTHERN CARDINALS

Abstract We tested how food supplementation affects extraterritorial behavior in the socially monogamous Northern Cardinal (Cardinalis cardinalis). Assuming extraterritorial movements are energetically costly, we predicted that if these movements function for gaining extra-pair matings, females on supplemented territories would spend more time off territory, make more extraterritorial forays, and have larger home ranges in relation to territory size than those on unsupplemented territories. If extraterritorial movements function for foraging purposes, then supplemented females should spend less time off territory, make fewer forays, and have smaller home ranges than unsupplemented females as a result of plentiful food on their home territories. We radio-tracked seven females during their fertile periods and found a trend for supplemented females to make more extraterritorial forays and spend more time off territory than unsupplemented females; however, there was no significant difference in home range sizes between treatment groups. Our findings provide some support for the extra-pair copulation function of extraterritorial movements in female Northern Cardinals.

Parental Provisioning by Carolina Wrens

Abstract We studied nestling provisioning and male song behavior associated with provisioning by Carolina Wrens (Thryothorus ludovicianus) throughout the nestling period. Overall, males and females (n  =  14 pairs) provisioned at similar rates throughout four nestling stages although males tended to provision more at the earliest nest stage (1–3 days). Males and females increased their rate of provisioning progressively over the nestling period. Parents brought one food item at a time to the nest and load sizes were similar between males and females, and between early and late nest stages. Males sang 34% of the time on arrival at nests to provision and 49% on leaving nests after provisioning. There was no relationship between male song and age of the nestlings. Males sang more after provisioning nestlings when females were not present (55%) than when females were present (33%) at the nest. The function of male song during provisioning may be to communicate the males location to the female and to indicate he has recently provisioned the nestlings but this requires further study.

FEMALE SONG IN THE HOODED WARBLER

Abstract One female Hooded Warbler (Wilsonia citrina) was discovered singing in July 1993, and another in May 1996 among a color-banded breeding population under investigation. The first females song structurally resembled the typical male Hooded Warbler mixed-mode song in duration, frequency range, and number of syllables, although it had an atypical raspy quality. Males responded similarly to playbacks of the female song and a male song from the same population. We suggest that age and high breeding density may be explanatory factors for this rare behavior.

Predator recognition and nest defence by Carolina Wrens Thryothorus ludovicianus in urban and rural environments: does experience matter?

ABSTRACT Capsule: Carolina Wrens Thryothorus ludovicianus in urban and rural environments responded most intensely to predators common to their environments. Aims: To determine the role of experience in predator recognition and response among Carolina Wrens. We predicted that wrens in the urban environment, where domestic cats are common, would respond more intensely to mounts of cats than snakes (a less common nest predator) placed near their nests. In the rural (forested) environment, we predicted a greater response to snakes than cats, because snakes are the more common predator in that environment. Methods: We placed mounted specimens of a snake and a cat near wren nests at the late nestling stage and quantified responses. We used a Rock Dove Columba livia mount as the control because it is a non-threatening species to the nestlings, and should be familiar to the wrens. Results: Carolina Wrens in the urban area responded most intensively to the cat mounts, whereas those in the rural environment responded more to the snake, with indications of innate predator recognition and defence. Cats were more common in the urban environment. Wrens used different alarm calls in the two habitats, but further study is needed to understand the significance of this variation. Conclusions: Birds may have the ability to adapt their responses to local predators, both native and non-native, which may be especially important for their success in urbanized habitats.

Responses of Nesting Yellow-Headed Blackbirds and Yellow Warblers to Wrens

Abstract We studied nest defense behavior of Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) and Yellow Warblers (Dendroica petechia) in response to two species of common nest destroyers. We presented freeze-dried models of Marsh Wrens (Cistothorus palustris) at Yellow-headed Blackbird nests and House Wrens (Troglodytes aedon) at Yellow Warbler nests during the incubation stage. We presented a Clay-colored Sparrow (Spizella pallida) as a control for both species. Male Yellow-headed Blackbirds responded more intensively to the Marsh Wren model than the control and female blackbirds responded intensively to both models but were more aggressive toward the Marsh Wren. Most Yellow Warblers did not respond to the House Wren model with their typical predator responses (e.g., alarm calls). Some female warblers were aggressive toward the wren model, whereas others sat in their nest. Sitting in the nest as a defense to deter nest destruction by House Wren needs further investigation. Differences in response levels between blackbirds and warblers may be related to differences in levels of nest destruction experienced by the two species or differences in nest defense behaviors used by the two species.