Honest advertisement and song output during the dawn chorus of black-capped chickadees

Costly signals can evolve under sexual selection, as only thosesignals that are difficult to produce and reflect the relativequality of individuals should be important in mate choice. Onesuch signal may be dawn singing behavior in birds. We assessedwhether the song output at dawn of breeding male black-cappedchickadees Parus atricapilhis honestly reflects quality, whererelative quality is assessed by relative dominance rank in winterflocks. Dawn choruses were recorded from 20 male chickadeesfrom 10 flocks during the fertile period of their mates in 1992,1994, and 1995. Dominance ranks of males were assessed by tabulatinginteractions at winter feeders from 1993 to 1995. A comparisonof the dawn singing behavior of the high-ranking and the low-rankingmales from each of the 10 flocks showed that high-ranking malesbegan singing earlier, sang longer, and sang at higher averageand maximum rates than low-ranking flockmates. Age of the maleshad less effect on song output at dawn than rank; older malestended to sing longer dawn choruses, but there was no differencein onset of singing, average song rate, or maximum song rateat dawn between hatch year and after-hatch year males. Our findingssuggest the dawn chorus can provide an accurate signal to femalesof the relative quality of their mate compared to neighboringmales     

Parental aggression in black-capped chickadees

Black-capped chickadee (Parus atricapillus) adults are aggressivetoward their young after the young have fledged. We examinetwo hypotheses to explain the function of this aggression. Eightfamilies were observed throughout the period after fledgingto see if aggressionfunctions in discouraging the youngs' dependenceon their parents for feedings or in encouraging dispersal ofyoung from the natal territory. Aggression consisted mostlyof chases, supplantings, and attacks. Most aggressive interactionsoccurred when young were following a parent closely or begging.After an aggressive act, fledglings tended to move away fromthe parent and forage on their own. The frequency of aggressiveacts peaked at 13 days after fledging, then decreased. Thispeak occurred when fledglings were making the transition fromdependence on parental feedings to independent foraging. Mostfledglings had not dispersed by 22 days after fledging. Ourresults support the hypothesis that parental aggression encouragesfledglings to forage independently, rather than to dispersefrom the natal territory.     

Sex allocation in black-capped chickadees Poecile atricapilla

Optimal sex allocation for individuals can be predicted from a number of different hypotheses. Fisherian models of sex allocation predict equal investment in males and females up to the end of parental care and predict brood compositions based on the relative costs of producing males and females. The Trivers-Willard hypothesis predicts that individual females should alter the sex ratio of their broods based on their own condition if it has a differential impact on the lifetime reproductive success of their sons and daughters. The Charnov model of sex allocation predicts that females should alter sex allocation based on paternal attributes that may differentially benefit sons versus daughters. Because females are the heterogametic sex in birds, many recent studies have focussed on primary sex ratio biases. In black-capped chickadees Poecile atricapilla , males are larger than females suggesting they may be more costly to raise than females. Female condition affects competitive ability in contests for mates, and thus may be related to variance in fecundity. Females prefer high-ranking males as both social and extrapair partners. These observations suggest that females might vary the sex ratio of their broods based on the predictions of any of the above models. Here, we report on the results of PCR based sex determination of 1093 nestlings in 175 broods sampled from 1992 to 2001. Population-wide, we found a mean brood sex ratio of 0.525±0.016, with no significant deviation from a predicted binomial distribution. We found no effect of clutch size, female condition, hatch date, parental rank or paternity. Our results reject the idea that female black-capped chickadees systematically vary sex allocation in their broods.     

Pitch-related cues in the songs of sympatric mountain and black-capped chickadees

Acoustic frequency (pitch) cues are known to be important in the recognition of conspecific song in a number of songbird species. Mountain chickadees (Poecile gambeli) and black-capped chickadees (Poecile atricapillus) are sympatric over parts of their ranges and their species-typical songs share many features. I examined the acoustic characteristics of song of these two congeners in a region of sympatry in southern Alberta, Canada. As reported for other populations in allopatry, black-capped chickadees emphasized relative frequency cues in song production. In particular, variation in the ratios between note frequencies was significantly less than variation in the note frequencies themselves. In contrast, songs of mountain chickadees did not have constant frequency ratios and contained an introductory acoustic element absent in black-capped chickadee song. Both species may rely on song note frequency or the presence of this introductory acoustic element when differentiating between conspecific song and heterospecific song. Song measures for chickadees in sympatry were similar to measures in allopatry, providing little evidence for character displacement in song production.     

Achromatic plumage reflectance predicts reproductive success in male black-capped chickadees

The size of achromatic (black, white, gray) plumage patchesserves as a male status signal in many species of birds, butvariation in the colors of these patches has received littleattention. We assessed the relation between achromatic plumagereflectance, dominance rank, body condition, and reproductivesuccess in male black-capped chickadees, Poecile atricapillus.We measured plumage reflectance for five body regions of 40male chickadees in late winter and monitored these males throughoutthe following breeding season to determine whether they survivedto breed, whether they successfully paired, whether their partnerlaid eggs, and both their apparent and realized reproductivesuccess. As expected from past studies, a male's dominance ranksignificantly predicted whether his partner laid eggs. However,only achromatic plumage reflectance significantly predictedother measures of male reproductive performance. Among maleswho fledged at least one offspring, both the brightness of whiteplumage regions and the UV-chroma of melanin-based plumage regionswere significant predictors of the proportion of within-pairyoung in their nests. When we consider all males we measured,assigning zero values to males who failed to sire any offspring,the UV-chroma of melanin-based plumage regions was a significantpredictor of realized reproductive success. Bib size was alsorelated to male realized reproductive success. Our findingssuggest that individual variation in achromatic plumage mayplay an important role in sexual signaling in chickadees.     

Food storage by black-capped chickadees: Memory for the location and contents of caches

Black-capped chickadees (Parus atricapillus) store food in a scattered distribution in their winter home range. Several hundred food items may be stored in a day, each in a separate cache site. Previous studies of marsh tits (Parus palustris) and nutcrackers (Nucifraga spp.) have shown that spatial memory is used to relocate caches. Memory for storage sites, if used by black-capped chickadees, is predicted to have four properties. Birds should be able to: (1) accurately relocate cache sites, (2) recall which caches they have previously emptied, (3) recall which sites they have discovered empty (as a result of loss to other animals) and (4) recall what type of food is stored at a cache site. Laboratory experiments show that chickadees do incorporate these kinds of information in memory for cache sites.     

The effects of cache loss on choice of cache sites in black-capped chickadees

Food-storing birds lose a great deal of their stored food toother animals. We examined whether blackcapped chickadees (Parusairicapillus) modify their choice of cache sites using informationthat predicts cache loss. In experiment 1, birds learned toavoid caching at spatial locations where cache loss had previouslyoccurred, but they did not avoid caching near local color cuesthat predicted cache loss. Birds did not modify their generaluse of space in the aviary. Birds also learned to reduce searchingfor caches where spatial location predicted cache loss. Experiment2 confirmed the birds’ ability to discriminate among thespatial locations and the local color cues used in experiment1. In experiment 3, learning a food-rewarded approach to potentialcache sites occurred without any change in the choice of sitesfor caching. We discuss how chickadees selectively associatethe choice of cache site with its consequences, even over delaysof several hours between caching and cache recovery.     

Alternative forms of song matching in song sparrows

Song matching, replying to a song with a similar song, occurs in many songbird species. Almost all investigations of song matching have been of type matching, where one bird's reply is unambiguously similar to the other's song (i.e. the same song type). In many populations, however, neighbours do not share song types, and therefore cannot type-match. We hypothesized that a bird lacking a true type match could still song-match a stimulus song with a song from his repertoire that was similar in some way the birds recognized. We tested this hypothesis in song sparrows, Melospiza melodia, in two playback experiments. We played the subject a stranger song that was similar to one or more of his songs, but a type match to none of them. In the first experiment, we used playback songs that began with two buzzes (‘double-buzz’ songs). In the second experiment, we used songs that began with a slow trill that increased in tempo ('speed-up' songs). Birds replied at rates significantly above chance with their own double-buzz, or speed-up song match to the respective types of playback. The results suggest that birds who do not share true song types, can still song-match each other. This broad-sense form of song matching may also occur in populations with low song type sharing. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Seasonal variability in habitat structure may have shaped acoustic signals and repertoires in the black-capped and boreal chickadees

Many songbird species have evolved multiple vocalizations, or repertoires, that function to communicate various biological signals. More diverse repertoires may have evolved in response to the effects of seasonal variation in habitat structure on signal transmission. Such changes in habitat necessarily occur for migrating species, but they also occur for resident species that occupy deciduous forests. The black-capped chickadee (Poecile atricapillus) possesses a chick-a-dee call and a fee-bee song, but the closely related boreal chickadee (P. borealis) lacks a song. Consistent with the habitat variability hypothesis, the black-capped chickadee possesses a larger repertoire and primarily occupies deciduous forests, whereas the songless boreal chickadee occurs more often in coniferous forests. We explored the ecological basis of this hypothesis by recording audio playbacks of two species in two habitat types during two seasons. Specifically, we played both songs and calls of the black-capped chickadee and calls of the boreal chickadee in deciduous and coniferous habitats, prior to and after leaf-out. We measured attenuation and degradation in re-recorded vocalizations. For black-capped chickadees, the song was less degraded than the call in post-leaf, deciduous forests. The boreal chickadee call attenuated more quickly in all treatments, but maintained its acoustic structure better than both black-capped chickadee vocalizations in coniferous forests. Our results support the hypothesis that variable habitats provided a seasonal transmission benefit for both song and call in the black-capped chickadee, but that the transmission benefit of song is lost in the less variant coniferous forests, which may underlie the absence of a song in the boreal chickadee.     

Photoperiodic regulation of food storing and hippocampus volume in black-capped chickadees, Poecile atricapillus

In seasonal environments animals organize their behaviour around annual cycles of resource availability. Wild black-capped chickadees are most likely to hoard food in autumn. At this time of year chickadees are also reported to have a larger hippocampus, a brain area important for spatial memory. This study examined how photoperiodic condition affects these seasonal changes. Captive chickadees were exposed to one of three treatments. Photorefractory birds were held on long days (19:5 h light:dark) and had small gonads. Photosensitive birds were held on short days (LD 9:15 h) and also had small gonads. Photostimulated birds were switched from short to long days and quickly entered breeding condition with large gonads. Photosensitive birds (on short days) stored more seeds than photorefractory birds (on long days). Photostimulated birds stored seeds at a high rate when on short days, but reduced storing when transferred to long days. These results indicate that long days inhibit storing regardless of gonadal condition. There were no differences between groups in hippocampal volume, indicating that photoperiod can produce changes in food-storing behaviour without affecting hippocampal size. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.     

Food hoarding behaviour of black-capped chickadees (Poecile atricapillus ) in relation to forest edges

In temperate forests, small birds avoid the use of forest edges in adverse winter weather suggesting high foraging costs in terms of energetic requirements. Since hoarding species will often retrieve caches during adverse winter weather, they may perceive forest edges, especially exposed ones, as low quality hoarding sites. We tested whether black-capped chickadees ( Poecile atricapillus) inhabiting fragmented forests modify and reduce hoarding activity near forest edges. We also tested whether hoarding behaviour will be most affected in sites with forest edges more exposed to extreme weather. Black-capped chickadees taking food from a feeder 30 m from the nearest forest edges hoarded items mostly towards the forest interior, whereas no preference in hoarding location was observed with birds taking food from a feeder placed >100 m from the edge. Furthermore, birds avoided direct flights towards forest edges and, at sites exposed to prevailing winds, hoarding trips were shorter than at other locations. These results suggest that individuals avoid hoarding near forest edges and there, they lower their investment in terms of hoarding effort. The observed difference in hoarding behaviour was more evident near forest edges delimiting wide unforested areas than in edges delimiting narrower unforested areas. Edge exposure to prevailing winds influenced hoarding behaviour much less. We suggest that hoarding birds may partially overcome the ecological costs of habitat loss and fragmentation due to abiotic edge effects. By hoarding food away from forest edges in good weather, they may use forest interiors as low-cost retrieval sites under adverse weather.     

Patterns of extrapair mating in relation to male dominance status and female nest placement in black-capped chickadees

In sexually promiscuous animals, females may benefit by nestingclose to the edge of their partner's territory to facilitateextrapair copulations. In the present study, we describe theextrapair mating system of black-capped chickadees, Poecileatricapillus, and test whether nest locations are influencedby conspecific attraction to extrapair partners. We conducteda spatial analysis of female mating strategies by using microsatellitepaternity analysis in conjunction with geographic informationsystem (GIS) analysis of nest and territory locations. Extrapairoffspring comprised 52 of 351 offspring (14.8%) and were presentin 19 of 57 broods (33.3%). Females paired to males with lowdominance status in the previous winter's flock hierarchy weremore likely to engage in a mixed reproductive strategy thanwere females paired to males with high dominance status. Femaleshad extrapair copulations and extrapair fertilizations withhigh-ranking males more often than with low-ranking males. Notall extrapair copulations resulted in extrapair fertilizations.Females constructed their nests within 16.8 ± 1.0 m ofthe edge of their partner's territory, significantly closerto the edge of their nearest neighbor's territory than to thecenter of their own partner's territory. Extrapair males usuallyshared territory boundaries with cuckolded males. Females pairedto low-ranking males constructed nests near the territory edgesof neighboring high-ranking males. However, females did nothave extrapair copulations with the neighbor nearest to theirnest or even with the high-ranking neighbor nearest to theirnest. We conclude that conspecific attraction to neighbors mayinfluence nesting location in black-capped chickadees; however,it does not operate by facilitating extrapair copulations.     

Evidence of accelerated beak growth associated with avian keratin disorder in black-capped chickadees (Poecile atricapillus)

We recently documented an epizootic of beak deformities in more than 2,000 Blackcapped Chickadees (Poecile atricapillus) and other wild bird species in North America. This emerging avian disease, which has been termed avian keratin disorder, results in gross overgrowth of the rhamphotheca, the outer, keratinized layer of the beak. To test the hypothesis that the beak deformities characteristic of this disorder are associated with accelerated keratin production, we measured rates of beak growth and wear in affected Black-capped Chickadees (n=16) and a control sample of unaffected chickadees (n=14) collected from south-central (61°09'-61°38'N, 149°11' -149°48'W) and interior Alaska (64°51' -64°53'N, 147°49' -147°59'W). Rates of absolute growth were 50-100% higher in affected birds than they were in control birds and exceeded records from other passerine species. These results suggest that abnormally rapid epidermal growth is the primary physical mechanism by which beak deformities develop and are maintained in affected chickadees. Although beak overgrowth typically worsened over time, differential patterns of wear influenced the severity and morphology of deformities. In some cases, the effects of accelerated keratin growth were partially mitigated by frequent breakage of rhamphothecal tips. However, mortalities occurred in 9 of 16 birds (56%) with beak deformities during the study, suggesting that avian keratin disorder results in severe health consequences for affected birds. Additional study of factors that control beak keratin production is needed to understand the pathogenesis of this debilitating disease in wild birds.     

Failure to detect seasonal changes in the song system nuclei of the black-capped chickadee (Poecile atricapillus)

Most temperate songbird species sing seasonally, and the brain areas involved in producing song (the song system) vary in size alongside the changes in behavior. Black-capped chickadees (Poecile atricapillus) also sing seasonally, and we find that there are changes in the stereotypy and the length of the fee-bee song from the nonbreeding to the breeding season. Yet despite these changes, we fail to find any evidence of seasonal changes in the song system. The song system of males is larger than that of females, as is typical in songbirds, but the ratio between the sexes is small compared to other species. We suggest three hypotheses to explain our failure to find seasonal variation in the chickadee song system.