Female Blackbirds Research Articles

Female red-winged blackbirds (Agelaius phoeniceus) do not alter nest site selection, maternal programming, or hormone-mediated maternal effects in response to perceived nest predation or brood parasitism risk

Predation or brood parasitism risks can change the behaviors and reproductive decisions in many parental animals. For oviparous species, mothers can mitigate their reproductive success in at least three ways: (1) by avoiding nest sites with high predation or parasitism risks, (2) through hormonal maternal effects that developmentally prime offspring for survival in risky environments, or (3) by investing less in reproduction when predation or parasitism risks are high. Here, we tested if perceived predation and parasitism risks can induce any of these behavioral or physiological responses by exposing female red-winged blackbirds (Agelaius phoeniceus) to playbacks of two major nest threats, a predator (Cooper’s hawk, Accipiter cooperii) and an obligate brood parasite (brown-headed cowbird; Molothrus ater), as well as two controls (harmless Eastern meadowlark, Sturnella magna; and silence). We found that female blackbirds did not avoid nesting at sites treated with predator or brood parasite playbacks, nor were females more likely to abandon nesting attempts at these sites. Egg size and yolk hormone profiles, which are common proxies for maternal investment in oviparous species, were statistically similar across treatment sites. Instead, we found intraclutch variation in yolk steroid hormone profiles: concentrations of three progestogens (pregnanedione, 17α-hydroxypregnenolone, and deoxycorticosterone) and two androgens (testosterone and androstenedione) were higher in third-laid than first-laid eggs. Our study largely confirms previous findings of consistent intraclutch yolk hormone variation in this species, in birds in general, and in other oviparous lineages, but uniquely reports on several yolk steroid hormones largely overlooked in the literature on hormone-mediated maternal effects.

Male and female Blackbirds (Turdus merula) respond similarly to the risk of nest predation

Several studies have found that adult birds of altricial species adjust their parental care behaviour (i.e. nest visits) in response to the current risk of predation for their offspring. However, no experimental study has so far investigated whether there are sex differences in these passive anti-predator responses during the nestling period. Differences between males and females could arise because of several factors, including (1) probability of detection, (2) confidence in parenthood, and (3) different parental care behaviour associated with each sex. To test whether these anti-predator passive responses involve sex differences, we experimentally manipulated the perceived risk of nest predation of adult Common Blackbirds (Turdus merula), a sexually dimorphic species with a relatively high extra-pair paternity level. Our results showed that nest predation significantly reduced adult visits to the nest, but not differentially between males and females, which does not support our predictions. Our findings suggest (1) that sex differences in predator-induced behaviour could depend on the type of response (active vs. passive anti-predator strategies); (2) the potential existence of a minimum threshold in detectability between males and females for these behavioural changes to occur; and (3) the contrasting and opposite effects of several factors that might impede the selection of sex differences in these types of parental care behaviour.

Egg rejection in blackbirds Turdus merula: a by-product of conspecific parasitism or successful resistance against interspecific brood parasites?

BackgroundTraditional theory assumes that egg recognition and rejection abilities arise as a response against interspecific brood parasitism (IBP). However, rejection also appears in some species that are currently not exploited by interspecific parasites, such as Turdus thrushes. Recent evidences suggest that rejection abilities evolved in these species as a response to conspecific brood parasitism (CBP). To test these two alternative hypotheses, we performed an experimental study by parasitizing nests of the common blackbird (Turdus merula) with conspecifics or heterospecific eggs under different risk of parasitism (presence of interspecific or conspecific parasites near the nest). Common blackbird is a potential host of the common cuckoo (Cuculus canorus) but suffers low levels of CBP too.ResultsWe found that blackbirds were able to recognize and eject heterospecific eggs at high rates whereas most of conspecifics eggs were not recognized and, therefore, accepted. Ejection rates of conspecific eggs did not exceed 13 %, even in situations of high risk of CBP (blackbird female placed near the nest), which contradict the main prediction derived from the CBP hypothesis. Conversely, ejection rates of experimental eggs simulating IBP were much higher (80–100 %). Furthermore, female blackbirds were more aggressive towards cuckoos than towards blackbird dummies.ConclusionsOur results considered together support the IBP hypothesis, indicating that recognition and rejection of parasitic eggs in blackbirds have probably evolved due to previous cuckoo parasitism. The current absence of IBP in blackbirds may be due to the highly efficient rejection abilities in this species. Thus, these abilities have been retained in absence of brood parasitism as a consequence of the low costs involved for blackbirds, resulting in a successful resistance against interspecific brood parasitism.

Sex and seasonal differences in neurogenesis and volume of the song-control system are associated with song in brood-parasitic and non-brood-parasitic icterid songbirds.

The song-control system in the brain of songbirds is important for the production and acquisition of song and exhibits both remarkable seasonal plasticity and some of the largest neural sex differences observed in vertebrates. We measured sex and seasonal differences in two nuclei of the song-control system of brood-parasitic brown-headed cowbirds (Molothrus ater) and closely-related non-parasitic red-winged blackbirds (Agelaius phoeniceus). These species differ in both the development and function of song. Brown-headed cowbirds have a larger sex difference in song than red-winged blackbirds. Female cowbirds never sing, whereas female blackbirds do though much less than males. In cowbirds, song primarily functions in mate choice and males modify their song as they approach sexual maturity and interact with females. In red-winged blackbirds, song is used primarily in territorial defence and is crystalized earlier in life. We found that the HVC was more likely to be discernable in breeding female blackbirds than in breeding female cowbirds. Compared to males, females had a smaller HVC and a smaller robust nucleus of the arcopallium (RA). However, females had higher doublecortin immunoreactivity (DCX+) in HVC, a measure of neurogenesis. Consistent with sex differences in song, the sex difference in RA volume was greater in cowbirds than in blackbirds. Males of both species had a smaller HVC with higher DCX+ in post-breeding condition than in breeding condition when song is more plastic. Sex and seasonal differences in the song-control system were closely related to variation in song in these two icterid songbirds. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1226-1240, 2016.

Female Blackbirds’ Response to Stress during Breeding: Possible Implications for Future Management

Blackbirds are reported to cause between 1-2% crop damage per year, but the distribution of damage is not uniform, with some fields this number can be as high as 20%. With many consumers in today’s market concerned with animal welfare, nonlethal management techniques have become more important. Many of these techniques exploit natural predator-prey systems. One area of research that has not been previously addressed is the physiological response of birds to visual and auditory scare devices designed to imitate predators. The current project is part of a series of studies that aim to develop knowledge of both physiological and behavioral trade-offs of female red-winged blackbirds when exposed to predation risk as a chronic stressor. Breeding colonies were exposed to an avian predator, avian nest parasite, or a non-threatening avian effigy with corresponding bird call at the beginning of the breeding season. Behavioral responses were monitored across the season, including general response to the predators and reproductive trade-offs. We predicted that female response to perceived predation risk would be greater than response to parasites or control treatments, and that females would make a greater reproductive trade-off in favor of future breeding seasons when presented with the perceived risk and stress of predation. Results suggest that red-winged blackbirds do have a greater response to the perceived risk of predation than to the parasites or control treatments. In terms of nest success and lay date, females do not seem to have different reproductive behavioral trade-offs under different treatments. However, there is a trend for larger clutches in nests found within the predator treatment, suggesting that females may actually be making a trade-off for the current rather than future seasons. Future work will focus on analyzing the physiological trade-offs that females make during the breeding season, especially while under chronic stress of predation risk. Results will help provide a basis for applied research aimed at improving bird damage management.

Seasonal and urban effects on the endocrinology of a wild passerine.

In order to maximize their fitness, organisms in seasonal environments rely on external cues to optimally time their life‐history stages. One of the most important zeitgeber to time reproduction is the photoperiod, but further environmental cues are assessed to fine‐tune reproduction due to year‐to‐year variation in environmental conditions. However, in urbanized environments, the pervasive artificial light at night has altered the natural signal of light and darkness. Accordingly, artificial light at night was repeatedly shown to affect avian reproductive physiology and to advance seasonal reproduction in birds. However, these experiments were mainly conducted in the absence of further environmental cues to facilitate the investigation of the mechanisms which are still poorly understood. Here, we investigate whether the endocrine system of free‐ranging European blackbirds (Turdus merula) correlates with the amount of artificial light at night along a rural to urban gradient while the birds still encounter complementary environmental cues including seasonal variation in day length and temperature. Testosterone and estrone were assessed as metabolites in fecal samples and corticosterone in blood from mist‐netted blackbirds. We demonstrate that seasonal fluctuations in abiotic factors, individual conditions, but also light at night affect the reproductive and stress physiology of wild European blackbirds. Elevated artificial night light intensities were significantly positively correlated with corticosterone and negatively with female estrone levels. No effects of artificial light were found for testosterone levels. Our results suggest that female blackbirds in particular perceive even low levels of artificial light at night as a weak but chronic stressor that interacts with the hypothalamic‐pituitary‐gonadal axis and leads to a reduced secretion of reproductive hormones. These findings point out that the impacts of light pollution are diverse and we only slowly disentangle its multiple effects on physiology, ecology, and biodiversity.

Now you see it, now you don't: flushing hosts prior to experimentation can predict their responses to brood parasitism.

Brood parasitic birds lay their eggs in other birds' nests, leaving hosts to raise their offspring. To understand parasite-host coevolutionary arms races, many studies have examined host responses to experimentally introduced eggs. However, attending parents often need to be flushed from their nests to add experimental eggs. If these birds witness parasitism events, they may recognize and reject foreign eggs more readily than parents who did not. We found that, after being flushed, female blackbirds, Turdus merula, remained close to their nests. Flushed females were more likely to eject foreign eggs and did so more quickly than females that were not flushed during experimentation. In contrast, flushing did not predict responses and latency to responses to parasitism by song thrush, Turdus philomelos, which flew farther from their nests and likely did not witness experimental parasitism. When statistically considering flushing, previously published conclusions regarding both species' response to experimental parasitism did not change. Nevertheless, we recommend that researchers record and statistically control for whether hosts were flushed prior to experimental parasitism. Our results have broad implications because more vigilant and/or bolder parents can gain more information about parasitism events and therefore have better chances of successfully defending against brood parasitism.

Helminth community structure study on urban and forest blackbird (Turdus merula L.) populations in relation to seasonal bird migration on the south Baltic Sea coast (NW Poland)

Abstract The aim of the study was to compare helminth community structure of urban and forest blackbird populations. 24 helminth species in 98 blackbirds were found. Higher species richness was noted in the forest population of the blackbird (23 species) in comparison to the urban population (14 species). The response of the helminth fauna to a synanthropic habitat, contrary to a natural habitat, consists in a significant reduction in most parasitological parameters. Higher species richness has been noted in spring (17 species) than in autumn (14 species). Urban habitat, in contrast to the forest, may cause changes in the abundance of helminth communities in male and female blackbirds. The helminth fauna of nestlings, in spite of low species richness is characterized by a higher prevalence and intensity of infection in comparison to blackbirds feeding on their own. Helminth fauna of the blackbird seems to be a good indicator of environmental quality.

Predator-induced female behavior in the absence of male incubation feeding: an experimental study

Parental care plasticity is critical to understanding the ecological and evolutionary influence of nest predation on life history strategies. In birds, incubation imposes a trade-off between the requirements of females (i.e., food) and egg requirements (i.e., heat and protection from predators). However, studies on this topic are rare and usually restricted to species where the male feeds the incubating female, relaxing her incubation costs. Males and females can reduce their activity at the nest to avoid detection by predators. However, females could follow two alternative antipredator strategies: to delay their return to the nest to avoid attracting attention from the potential predator or to return to the nest as soon as possible to enhance nest concealment. In this study, we manipulated the perceived risk of nest predation of incubating common blackbirds (Turdus merula), a species without incubation feeding, to study female behavioral changes induced by nest predation risk. We show experimentally that female blackbirds can reduce their nest visits in the situation with higher nest predation risk. In addition, we confirm that females significantly delay their return to the nest in the presence of a nest predator, contradicting the nest concealment hypothesis. However, our results could be interpreted as a passive antipredator response (to minimize clues given to predators) or as an active antipredator response (to search for predators to expel them from their territories).

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 respons...

Vertical Transmission of Feather Lice Between Adult Blackbirds Turdus merula and Their Nestlings: A Lousy Perspective

There is limited information about the natural history of the transmission of feather lice (Phthiraptera) from parent birds to their young. This article therefore examines the transmission of 4 species of feather lice from parent blackbirds to their nestlings in an English population, and addresses questions formulated from the perspective of the lice. The lice that disperse onto the several young in the nest were mostly found on the larger chicks, those with higher survival prospects. The lice dispersing to chicks were overwhelmingly nymphs, which cannot be sexed morphologically, and so the prediction that the adult lice dispersing would be disproportionately female, potential founders of a new population, was only supported for the most numerous species, Brueelia merulensis. There was no evidence that louse dispersal to chicks was density dependent and more likely when the parents were more heavily infested. Finally, I predicted that lice might aggregate on female blackbirds, which undertake more brooding, to increase their chance of transmission to nestlings. For 1 louse species, B. merulensis, prevalence, but not louse intensity, was higher on female than male blackbirds. For 2 other louse species, Philopterus turdi and Menacanthus eurysternus, no differences between male and female blackbirds were detected.

An effective nest trap for female Yellow-headed Blackbirds

Capturing breeding female blackbirds using mist nets and food-baited traps can be challenging because nets are difficult to set in wetland habitats and females feed preferentially on insects. Given the problems associated with other trapping methods, we designed a highly effective nest trap that allowed us to successfully capture targeted female Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) at our wetland study sites in North Dakota. From 15 May to 16 July 2004, we captured 56 of 59 (95%) targeted females during incubation and eight of 18 (44%) targeted females with nestlings, and were also able to recapture 12 of 16 females (75%). Based on these results, we recommend using our trap to capture female blackbirds during incubation rather than during the nestling period. The nest trap is easy and inexpensive to construct and is light and easy to carry through dense wetland vegetation. In addition, the trap's large diameter and wire construction provide the flexibility needed to trap at sites where the density of vegetation varies. This trap has also been used to capture female Red-winged Blackbirds (Agelaius phoeniceus) and could potentially be used to trap females of other species of wetland-breeding icterids, including Common Grackles (Quiscalus quiscula). SINOPSIS El capturar hembras en estado reproductivo de individuos de Xanthocephalus xanthocephalus utilizando redes de niebla y trampas con cebo, pudiera ser un ejercicio de gran reto, dado el caso de que las aves se alimentan de insectos y las redes son dificiles de montar y manejar en anegados. Dados los problemas asociados con otros metodos disenamos una trampa efectiva que nos permitio capturar con exito individuos particulares en un estudio que llevamos a cabo en Dakota del Norte. Del 15 de mayo al 16 de julio de 2004, capturamos 56 de 59 individuos deseados (95%) durante el periodo de incubacion, ademas a 8 de 18 hembras (44%) con pichones. Tambien recapturamos 12 de 16 (75%) de las hembras. Basandonos en estos resultados, recomendamos el uso de nuestra trampa durante la fase de incubacion, en vez de la epoca de cria. La trampa es facil de construir, liviana, facil de transportar (a traves de un anegado denso) y de bajo costo. Ademas, el diametro de la trampa y su construccion en alambre ofrecen la flexibilidad necesaria para colocarla en sitios en donde la densidad de la vegetacion resulte variable. Este tipo de trampa tambien ha sido utilizada para atrapar hembras de Agelaius phoeniceus y puede ser, potencialmente, utilizada para atrapar hembras de otras especies de icteridos que anidan en anegados, incluyendo a Quiscalus quiscula.

First Report of Fishing in the European BlackbirdTurdus merula

During two hours of observation, a female Blackbird made approximately 30 attempts to catch fish, 20 of which were successful. The bird watched and followed a minnow shoal in shallow parts of the river, hopping from stone to stone in a way that resembled its normal behaviour when catching earthworms. The bird only caught small individuals (5–7 cm). Larger individuals were present in the river but were not seen within the shoal and were not attacked.

Nest Re-Use by Blackbirds — The Way for Safe Breeding?

The biology of individually colour-ringed European Blackbirds inhabiting two city parks in Szczecin (NW Poland) was studied in 1997–2003. In each park one to three observers watched the behaviour of Blackbirds every other day from dawn till afternoon (6–8 h a day); in this way almost all of their nests were discovered. 35 cases were recorded where pairs re-used their own nests, as well as two cases where the nest of another Blackbird pair and one Fieldfare nest were re-used. In 33 out of 81 cases observed, the re-use occurred after the brood had been successfully reared, while in two cases out of 378, female Blackbirds initiated the second breeding attempt in the same nest after the loss of the first one. The re-used nests were better concealed (80 ± 19% and 69 ± 18%, respectively) and had been built at greater heights than those abandoned after breeding (9.0 ± 5.0 m and 6.5 ± 4.2 m, respectively). No shortening of the interval between successive clutches was noted in the case of nest re-use (re-used nests 36.8 ± 4.9 days, newly-built nests 37.7 ± 8.0). The more and more frequent re-use of nests where breeding had been successful, their better concealment and higher sites, and also the lack of any differences in the intervals between successive clutches of pairs occupying old nests and those building new ones before the next breeding attempt, suggest that in the investigated population the basic reason for nest re-use was the insufficient number of safe nesting sites.

Mate acquisition in the European blackbird and its implications for sexual strategies

Female European blackbirds, Turdus merula have been shown to engage in mixed reproductive strategies and females do not seem to gain any material benefit from their extra-pair copulation behaviour. To test the hypothesis that such behaviour arises out of constraint on their choice of social mate, I investigate if and how these females are constrained in their choice at mate acquisition. I recorded patterns of territorial distribution and interactions between resident birds both before pairing and during the breeding season. Patterns of observed behaviour agreed with earlier descriptions of winter and breeding territoriality and provided clues to the underlying mechanism of mate acquisition. I conclude that the constraint on breeding success imposed by high nest predation promotes intrasexual competition for high quality nesting habitat between females prior to pair formation. Female distribution promotes intrasexual competition between males for territorial area overlapping females or for habitat likely to be occupied by females. Mate choice during pair formation is constrained by the outcome of this intrasexual competition and, typically, overlapping territorial males and females pair into social monogamy on a shared breeding territory. Although this ultimately leads to assortitive mating by competitive ability, low quality females paired to low quality males may improve upon the quality of their genetic mate by engaging in extra-pair copulations. These data suggest that the process of mate acquisition in urban European blackbirds constrains the choice of social mate and so generates sexual selection pressures to drive mixed reproductive strategies during the breeding season.

Field-Testing the Suitability of a New Back-Pack Harness for Radio-Tagging Passerines

Attaching radio-tags to highly dynamic organisms can bias the data collected. The nature and extent of any bias should be understood and quantified. This is vital with an untested attachment method or when tagging a new species or age-class. The suitability of a new back-pack harness design was field tested on 39 adult thrushes (33 Blackbird Turdus merula and 6 Song Thrush T. philomelos) at a study site in England. Direct chronic effects were assessed by examination of recovered corpses and recaptured birds. The harness caused the development of a small subcutaneous lipid mass on the patagium but no epidermal abrasion or inter-scapular blistering or bruising. Indirect measures of possible chronic effects came from behavioural observation and breeding performance measures. For female birds, the attachment of a back-pack tag did not significantly reduce clutch size or egg volume compared with untagged females. Nest survival rates were also unaffected. The number and quality of young fledged by tagged females were as high as those for untagged females. Video data showed that provisioning rates of the nestlings did not differ significantly between tagged and untagged parents. The proportion of time spent brooding by tagged female Blackbirds was non-significantly higher than for untagged birds.

Diurnal and seasonal mass variation in blackbirds Turdus merula: consequences for mass‐dependent predation risk

1. A bird’s mass in winter should reflect the trade‐off between the benefits of an increased energy store to reduce starvation risk, and its costs in terms of increasing mass‐dependent predation risk. State‐dependent models of this trade‐off predict that as starvation risk increases then energy reserves should (i), increase, and (ii) be acquired earlier in the day. 2. Blackbirds increased their minimum weight by 25. ±. 1·5% in midwinter; adults weighed significantly more than juveniles in midwinter, but weighed less in late winter. Weight for both sexes, and particularly adults, decreased with increasing daylength, but weight increases with respect to temperature decreases were most pronounced in males. Female blackbirds showed relatively small seasonal weight changes so that their midwinter mass‐dependent predation risk relative to males was independent of diurnal weight changes. 3. Blackbirds lost at least 1–9% of their body weight overnight depending on temperature. Over 60% of the weight lost was then regained in the first 3 hours after dawn, with only 5% being regained after 12.00 h. The pattern of early morning weight gain was maintained throughout the winter, with the ratio of weight gain rates in the first 4. h of dawn compared to the rest of the day being c.. 3:1 in midwinter (when individuals maintained high weights) and in early spring (when individuals were losing weight seasonally). 4. The results agree with the theoretical prediction that energy reserves should increase as risk of starvation increased (as measured by seasonal factors such as cold, short midwinter days). There was, however, little change in the seasonal pattern of diurnal weight gain, with weight always being gained early in the day. In the likely absence of foraging constraints within the study system, this result fits the predictions of state‐dependent foraging models where animals can escape from predation after achieving weight gains, by either using refuges or adopting low‐risk foraging options.

Are Conspicuous Birds Unprofitable Prey? Field Experiments with Hawks and Stuffed Prey Species

-The unprofitable-prey hypothesis predicts that birds of prey, when given a choice, should more often attack a cryptic than a conspicuous bird if conspicuousness is a signal of unprofitability. We tested this prediction by exposing pairs of stuffed birds to migrating birds of prey. In experiment 1, we paired the conspicuous White Wagtail (Motacilla alba) with the cryptic Meadow Pipit (Anthus pratensis). In experiment 2, we paired the conspicuous female Great Spotted Woodpecker (Dendrocopos major) with the cryptic female Blackbird (Turdus merula). Automatic cameras documented attacks by birds of prey, mostly accipiters, on the mounts. The numbers of attacks on wagtails and pipits were similar, whereas Blackbirds were attacked much more often than woodpeckers. Experiment 2 thus supports the prediction of the unprofitable-prey hypothesis. Using photographs of the experimental situation, we examined detectability of mounts by human observers. Wagtails and pipits were equally easy to detect from a distance, but woodpeckers usually were easier to detect than Blackbirds. Thus, wagtails and pipits did not differ in long-distance conspicuousness, which may explain the result of experiment 1. To analyze reasons for unprofitability, we presented flesh from woodpeckers and Blackbirds to captive falcons, but found no evidence that woodpeckers are distasteful. Other explanations for why woodpeckers were rarely attacked are discussed. Received 26 April 1993, accepted 17 August 1993. THE PLUMAGE of diurnal birds exhibits striking variation in color patterns, from dull, cryptic patterns in some species to bright red, yellow, or black-and-white in others. The evolution of conspicuous colors in birds has been debated since Darwin (1871) proposed his theory of sexual selection to explain the bright colors of many male birds. Darwin's idea of selection through female choice of bright males was discussed by Hingston (1933), who argued that contest competition among males primarily selected for conspicuous colors (review in Butcher and Rohwer 1989). Several studies have supported both the idea of female choice (e.g. Hoglund et al. 1990, Norris 1990, Hill 1991) and male-male competion (Roskaft and Rohwer 1987, Rohwer and Roskaft 1989). Avian coloration is also related to risk of predation. Cryptic plumages in birds such as nightjars and female ducks suggest an important role for predation. Conspicuous plumages may be antipredator adaptations as well. Certain brightly colored birds are distasteful to hornets, cats, and people and, therefore, may be aposematic (Cott 1947, Cott and Benson 1970, Gotmark 1994a). Recently, two or three apparently

Geographic Variation in Yellow-Headed Blackbirds from the Northern Great Plains

We assessed geographic variability in morphology among Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) collected from breeding populations in Minnesota, North Dakota, and South Dakota in the United States and Alberta, Manitoba, and Saskatchewan in Canada during spring. Both male and female Yellow-headed Blackbirds tended to be larger in the more northern breeding populations. Males collected in Manitoba and southeast Saskatchewan tended to be more like populations from the United States, whereas other populations in Saskatchewan and Alberta populations were morphologically similar to each other. For females, however, only the Manitoba population was similar to the U. S. populations, and the females from the southeastern Saskatchewan were similar to other Saskatchewan birds. Southeastern Saskatchewan appears to represent a transitional region between northwestern and southeastern breeding populations of Yellow-headed Blackbirds.

Song-based species discrimination and behaviour assessment by female blackbirds, Turdus merula

Abstract. The ability of female blackbirds to integrate information about a singer's species and possible ensuing behaviour from different parameters of the introductory part (i.e. the motif part) of the blackbird's song was tested. Females were made receptive to song stimulation by oestradiol injections and tested with tapes with modified motif parts in an anechoic chamber using a broad spectrum of female reproductive behaviour as response measures. Females, like males, attended to the natural upper and/or lower limits of variation in seven parameters in the process of species discrimination: (1) the frequency level and (2) the duration of sound figures (i.e. sounds separated by pauses greater than 50 ms); variation across the motif part in the sound figures' (3) frequency level, (4) duration and (5) amplitude; (6) the ratio of the sound pressure of the fundamentals to that of the first and second overtones; and (7) the relative proportions of the three types of sound figures (i.e. (a) whistles; (b) trills, gurgles and squeaks; and (c) compounds of a and b). The results suggest that females, unlike males, disregard both the upper limit of variation in the frequency level in the process of species discrimination, and the degree of modulation of the sound figures' amplitude functions (i.e. envelopes) and, perhaps, the pauses between the sound figures in the process of assessing the ensuing behaviour of the singer. The validity of such sexual differences in song decoding and possible factors leading to their evolution are discussed. The present differences all concern parameters that degrade more when passing through dense vegetation to females than when passing through the more open communication channels to males. This is consistent with the 'song-degradation' hypothesis that one of the sexes may pay less attention than the other to certain song features if these features are degraded to a different extent on their way to the two sexes.