Ornithology and integrated pest management: Red-winged Blackbirds Agelaius phoeniceus and corn

Red-winged Blackbirds Agelaius phoeniceus , the most abundant bird in North America, annually destroy over 360 000 tonnes of field corn and substantial amounts of sweet corn. This loss, representing <1% of North American corn production, is localized within 8 km of wetlands where Red-winged Blackbirds roost in late summer. Red-winged Blackbirds are often initially attracted to corn fields to feed on insect pests during the 2–3 week period between silking and kernel development. An ideal resolution of the conflict between corn farmers and Red-winged Blackbirds is to couple the birds' beneficial feeding habits in silking-stage corn with management techniques to reduce subsequent damage to kernels. Once birds switch from feeding on insects to feeding on corn, protective measures to reduce damage are more critical in the early (milk) stages of kernel development than later because the damage potential is much higher at this time. Birds must eat three times more kernels at 20 days after silking than at 40 days in order to receive the same corn biomass. The use of bird-resistant cultivars of corn, the deployment of frightening devices during the critical damage period, and the provision of alternative feeding sites are important components of an integrated management programme. The goal is to accommodate Red-winged Blackbirds as a part of the wetland-agricultural complex in North America.     

Sound-pressure Levels and Song Preferences in Female Red-winged Blackbirds (Agelaius phoeniceus) (Aves,Emberizidae)

Female red-winged blackbirds (Agelaius phoeniceus) were tested for response to songs of male red-winged blackbirds differing in sound-pressure level (SPL) of playback. Subjects gave significantly more copulation-solicitation displays in response to playback of songs at 85 dB SPL than to playback of the same songs at 72 dB SPL. The strength of the preference, as judged by the ratio of response to high and low SPL playback (1.15:1), was lower than that of preferences for high SPL shown by insects and anurans. Female red-winged blackbirds responded preferentially to a conspecific song relative to a heterospecific imitation, even when the imitation was presented at an SPL 13 dB higher than that of the conspecific song. By contrast, female redwings did not maintain a preference for multiple conspecific song types over single types when the single song types were presented at the higher SPL. These results are compatible with Klump & Gerhardt's (1987) suggestion that the intensity independence of female preferences varies with the relative benefit females obtain from each preference.     

Microsatellite Variation in Red-Winged Blackbirds (Agelaius phoeniceus)

Territorial male red-winged blackbirds from five locations in the United States and Canada were genotyped using a suite of six microsatellite loci. Each population possessed unique alleles, but numbers of alleles per locus (range = 7.3-8.8) and expected multilocus heterozygosities (range = 0.76-0.80) were similar in all populations. Significant overall allele frequency differences were detected between some population pairs, and some pairwise Fst values were significant (but small). However, Fst among populations, although significant, was also small (0.009). Despite revealing low levels of population structure, the high multilocus polymorphism indicates these loci will be valuable in the genetic analysis of behavior and reproductive strategies in this species.     

Habituation,Recovery and the Similarity of Song Types within Repertoires in Red-winged Blackbirds (Agelaius phoeniceus) (Aves,Emberizidae)

Male red-winged blackbirds (Agelaius phoeniceus) respond to playback of conspecific song on their territories with the song spread, a graded aggressive display in which males extend their wings to expose their red epaulets while singing. We show that the intensity of song spread display declines with repeated presentation of one song type, and recovers when song types are switched. Recovery is greater for switches between song types that are acoustically dissimilar than for switches between song types that are acoustically similar. Recovery is no different for switches between two song types taken from the repertoires of different males than for switches between song types recorded from the same male. Analysis of acoustic features also indicates that song types recorded from different males are not more dissimilar than are song types from the same male. Our results do not support the idea that repertoires of red-winged blackbirds are composed of similar song types in order to facilitate individual recognition. Rather, repertoires may be constructed of dissimilar song types, so as to help maintain the response of listeners despite habituation.     

Aggressive and Pair-bond Maintenance Songs of Female Red-winged Blackbirds (Agelaius pboeniceus)

Female red-winged blackbirds sing two distinct song types. Field observations and playback experiments indicate that Type 1 female song is sung chiefly in the presence of mates and probably functions in pair-bond maintenance, while Type 2 song is sung chiefly in the presence of female conspecifics, and appears to be aggressive in nature.     

Premigratory Changes in the Hypothalamo-Hypophysial Neurosecretory System in the Red-winged Blackbird (Agelaius phoeniceus)*

Abstract

• 1 The hypothalamo-hypophysial neurosecretory system in the Red-winged Blackbird (Agelaius phoeniceus) has been studied with special reference to changes during the period prior to fall migration.
• 2 In September the supraoptic and paraventricular cells were found to be considerably larger with substantial amounts of aldehyde fuchsin positive neurosecretory material (NSM) accumulating in the perikaryon. Large amounts of the NSM were found also in the anterior median eminence and pars nervosa.
• 3 Towards migration time (November), the cells in the hypothalamic nuclei contained very little NSM. A marked decline in the amount of NSM was also seen in the anterior median eminence and probably in the pars nervosa too.
• 4 The release of NSM toward migration time has been discussed as an important event in the sequence of events that control and regulate migratory activity.

     

Red-winged blackbirds (Agelaius phoeniceus) with higher baseline glucocorticoids also invest less in incubation and clutch mass

Glucocorticoid hormones facilitate responses to environmental challenges by mediating diverse physiological and behavioral changes, including resource mobilization and altered reproductive effort. Elevated glucocorticoids might indicate that an individual is facing high levels of environmental challenges and thus, elevated concentrations might be associated with reduced fitness (CORT-fitness hypothesis). Alternatively, the energetic demands of reproduction might be a challenge that requires elevated glucocorticoids to mobilize resources to support reproductive effort, ultimately increasing reproductive investment and fitness (CORT-adaptation hypothesis). Investigations of glucocorticoid-fitness relationships have yielded mixed results. Variation in the direction of this relationship could be caused in part by differences in the contexts in which the relationship was assessed. Incorporating context, such as life history stage, could be key to understanding the role of glucocorticoids in influencing fitness outcomes. We investigated the relationship between corticosterone and reproductive effort and success within a single life history stage: incubation of eggs. In an observational study, we measured baseline corticosterone in incubating female red-winged blackbirds (Agelaius phoeniceus), monitored incubation behavior, and determined hatching success for each nest. Incubating birds with higher baseline corticosterone concentrations had more frequent, shorter incubation bouts and spent less time overall incubating their clutches of eggs than birds with lower corticosterone concentrations. Elevated corticosterone was also associated with lower clutch mass, but neither corticosterone nor incubation effort were correlated with hatching success. Although experimental tests are needed to establish causation, these results suggest that during the incubation period, corticosterone might shift resource investment towards self-maintenance, and away from current reproductive effort.     

Polygyny and extrapair fertilizations in eastern red-winged blackbirds (Agelaius phoeniceus)

Parentage of nestling red-winged blackbirds (Agelaius phoeniceus)from an eastern population was determinedusing DNA fingerprintingtechniques. Of 235 nestlings surveyed, 58 had fingerprints excludingthemale, but none excluded the female tending the nest. Data onpairing status during the female's fertilizable period was availablefor 232 offspring; 55 (25%of 1988 nestlings, 23% of 1989 nestlings)of those were sired through extrapair copulations. Of these55 offspring, 33 could be assigned to nearby territory holders;16 of the remaining nestlings may have been sired by nearbymales that were not captured. During both years, 44% of territorialmales had more than one female nesting simultaneously on theirterritory. The number of extrapair fertilizations gained bymales increased significandy with harem size in 1 year. Paternity(die proportion of nesdings on the territory sired by die territoryholder) showed a positive but nonsignificant increase widi haremsize in bodi years. There was no apparent cost in paternityfor males guarding two or more fertilizable females at the sametime. The broods of females that were fertilizable at die sametime anodier female was setding on die same territory tendedto have a greater proportion of extrapair fertilizations (0.37)than did die broods of odier females within harems (0.15). Establishedfertilizable females were chased significantly more by die territoryowner and by extrapair males when a new female was setding.There were no associations between a male's paternity or successat gaining extrapair fertilizations and his age or color-bandcombination. Overall, extrapair fertilizations had litde effecton die relationship between fledgling success and harem sizeand appeared to have a minimal impact on die overall intensityof sexual selection on males.     

Food Intake and Circadian Rhythms of Activity of Red-Winged Blackbirds (Agelaius phoeniceus). A Time-Course Study on the Effects of Alpha-Chloralose and Secobarbital

Anesthetic drugs like alpha-chloralose and secobarbital are used to capture wild birds. The doses are usually chosen on the basis of low mortality rates and high capture success. However, little information is available on the time necessary for the birds to fully recover from the drugs' effects after they have regained consciousness. In this work, we used circadian rhythms of activities to study the long-term effects of a mixture of alpha-chloralose and secobarbital on adult male Red-winged Blackbirds. The birds were housed individually in sound-proof boxes for 30 days, during which they received alpha-chloralose and secobarbital mixed with corn, either on day 3 or on day 15. On the day of treatment, the birds experienced a significant decrease in activity level, including feeding activities, which lasted more than 22 days for the birds treated on day 3, compared to about 8 days for the birds treated on day 15. Moreover, typical circadian distribution of activity was lost and many birds became aperiodic. Two weeks were needed for the birds treated on day 3 to recover normal rhythms, compared to less than 9 days for those treated on day 15. These results indicate that circadian parameters can be used to detect after-effects of intoxication long after the birds have regained consciousness.     

Food Intake and Circadian Rhythms of Activity of Red-Winged Blackbirds (Agelaius phoeniceus). A Time-Course Study on the Effects of Alpha-Chloralose and Secobarbital

Anesthetic drugs like alpha-chloralose and secobarbital are used to capture wild birds. The doses are usually chosen on the basis of low mortality rates and high capture success. However, little information is available on the time necessary for the birds to fully recover from the drugs' effects after they have regained consciousness. In this work, we used circadian rhythms of activities to study the long-term effects of a mixture of alpha-chloralose and secobarbital on adult male Red-winged Blackbirds. The birds were housed individually in sound-proof boxes for 30 days, during which they received alpha-chloralose and secobarbital mixed with corn, either on day 3 or on day 15. On the day of treatment, the birds experienced a significant decrease in activity level, including feeding activities, which lasted more than 22 days for the birds treated on day 3, compared to about 8 days for the birds treated on day 15. Moreover, typical circadian distribution of activity was lost and many birds became aperiodic. Two weeks were needed for the birds treated on day 3 to recover normal rhythms, compared to less than 9 days for those treated on day 15. These results indicate that circadian parameters can be used to detect after-effects of intoxication long after the birds have regained consciousness.     

Effects of Exogenous Androgen and Antiandrogen on Territorial and Nonterritorial Red-winged Blackbirds (Aves: Icterinae)

We investigated the role of testosterone (T) in territory establishment and maintenance in male red-winged blackbirds (Agelaius phoeniceus) by implanting exogenous T or pharmacological agents that block the action of T in nonterritorial floaters and territory owners. Floaters with artificially elevated plasma T levels were unable to obtain territories. Territory owners implanted with T did not expand their territories, although they engaged in more aggressive behavior than did control males. Flutamide, which binds to T receptors in target areas, had no effect on territoriality. However, most territory owners given a combination of flutamide and ATD, an aromatization blocker, lost portions of their territories even though they actively defended them. Our results 1) suggest that, although T influences aggressive behavior, elevated plasma levels alone are insufficient to overcome previously-established social relationships between territory owners or between owners and floaters; 2) indicate that impairing the action of T leads to reduced abilities of territory owners to maintain territories against vigorous challenges: and 3) support recent findings that T acts on reproductive behavior in birds through both androgenic and estrogenic metabolites.     

Patterns of Vocalization Use by Female Red-winged Blackbirds (Aves: Emberizidae,Icterinae): Variation and Context

Variation within communication systems can be examined at several levels: variation among individual notes, variation in the arrangement of notes, and variation in delivery. Here, variation in the arrangement and delivery of notes used in female red-winged-blackbird vocalizations is described, and this variation is examined for correlations with the context in which the vocalizations were given. Within a vocalization, notes were normally arranged in a series of similar notes. Of the 75 different note arrangements recorded from the population, 12 arrangements accounted for 84 % of vocalizations. Provisional results indicate that the arrangement of notes was influence by the following contexts: 1. The stage of the breeding season; 2. The individual female; 3. The breeding status of the female; 4. Certain female behaviour patterns; 5. Other red-winged-blackbird vocalizations; and 6. The presence or absence of a female's mate in the territory. Variation in delivery was measured by the number of notes within a vocalization; delivery was influenced by the stage of the breeding season, the individual female, and the breeding status of a female. Most note arrangements were used in a wide variety of contexts and did not appear to be functionally distinct.     

Ontogeny of catabolic and morphological properties of skeletal muscle of the red-winged blackbird (Agelaius phoeniceus)

Thermogenic capabilities of red-winged blackbirds improve markedly during their 10-12-day nestling period, especially between day 5 and day 8. The time course of improvements may be determined by the maturation of skeletal muscles involved in shivering thermogenesis, particularly the pectoralis muscles. To test this hypothesis, morphological and biochemical changes in pectoral and leg muscles were measured in young and adult blackbirds. Both muscles grew disproportionately relative to body mass. The pectoralis consisted entirely of fast-twitch fibers, predominantly fast oxidative glycolytic. In contrast, the gastrocnemius muscle consisted of a mixture of slow and fast fibers (predominantly fast glycolytic). Although fiber composition was constant, both cross-sectional area and density of fibers increased with age in both muscles. Catabolic capacities of the pectoralis increased significantly (approximately 7-8-fold) throughout the nestling period, most abruptly after day 3 (citrate synthase, CS) or day 4 (3-hydroxacyl-CoA-dehydrogenase, HOAD). Myofibrillar ATPase activities in the pectoralis were initially low, but increased after day 5. Further increases in CS and myofibrillar ATPase activities occurred in the pectoralis after fledging. CS and HOAD activities in the leg were much lower, but myofibrillar ATPase activities were remarkably similar in the two muscles, differing only in adults. These results are consistent with the hypothesis that the development of endothermy is dependent on the morphological and biochemical maturation of skeletal muscles important in thermogenesis.     

Frequencies of mutagen-induced coincident mitotic recombination at unlinked loci in Saccharomyces cerevisiae

Frequencies of coincident genetic events were measured in strain D7 of Saccharomyces cerevisiae. This diploid strain permits the detection of mitotic gene conversion involving the trp5-12 and trp5-27 alleles, mitotic crossing-over and gene conversion leading to the expression of the ade2-40 and ade2-119 alleles as red and pink colonies, and reversion of the ilv1-92 allele. The three genes are on different chromosomes, and one might expect that coincident (simultaneous) genetic alterations at two loci would occur at frequencies predicted by those of the single alterations acting as independent events. Contrary to this expectation, we observed that ade2 recombinants induced by bleomycin, beta-propiolactone, and ultraviolet radiation occur more frequently among trp5 convertants than among total colonies. This excess among trp5 recombinants indicates that double recombinants are more common than expected for independent events. No similar enrichment was found among Ilv(+) revertants. The possibility of an artifact in which haploid yeasts that mimic mitotic recombinants are generated by a low frequency of cryptic meiosis has been excluded. Several hypotheses that can explain the elevated incidence of coincident mitotic recombination have been evaluated, but the cause remains uncertain. Most evidence suggests that the excess is ascribable to a subset of the population being in a recombination-prone state.     

Plasma LH and androgen levels in the red-winged blackbird (Agelaius phoeniceus) treated with a potent GnRH analogue.

1. Low doses of GnRH-A (0.01-0.10-1.0 micrograms) given during the annual testes growth period did not clearly affect plasma LH and androgen levels 10 min following the injection. 2. The first injection of high doses of GnRH-A (2.0-10.0-20.0 micrograms) markedly increased plasma LH and androgen levels measured 10 min following the injection. The increase in plasma LH level was dose-dependent and the maximal LH level was obtained with 10.0 micrograms of GnRH-A. 3. Impairment of the LH response to GnRH-A was assessed by comparing the first and the fourteenth injection of high doses of GnRH-A. Evidences of pituitary gland desensitization are reported since plasma LH levels were reduced following the fourteenth injection in all groups. 4. Plasma androgen levels following high doses of GnRH-A were not clearly affected in red-winged blackbirds.