Selected abstracts on Northern Mockingbirds

FLUCTUATIONS IN INTRA-PAIR CALLING ACROSS BREEDING PHASES IN NORTHERN MOCKINGBIRDS (MIMUS POLYGLOTTOS )

[Cheryl A. Logan, Behaviour, 1994, 130: 123-141]
Abstract. When in close proximity, mated mockingbirds (Mimus polyglottos ) often exchange a broad band "hew" call. The call is also produced by lone males and females. I observed single hew production and intra-pair hew exchange across three breeding conditions: when adults were only feeding young, when the male was nest building with no young present, and when the male was nest building in the presence of young still being fed by the pair. Fewer calls of either type were produced during nest construction, and the relative frequency of each call pattern differed among breeding conditions. The percentage of pair interactions with hew exchange dropped while pairs were nest building, while the percentage of interactions accompanied by single hews increased during nest construction. Moreover, when only one member of the pair called (single hews), it was almost always the female. During nest building with young present, more pair interactions were accompanied by hew exchange than when pairs built in the absence of young, and in pairs nest building with young present, the number of hew exchanges per 60 min was positively correlated with brood size.

Patterns of behavioural interaction between males and females also differed across the three breeding conditions. While feeding young, males approached females more than females approached males, and during nest building, males departed from females more than the reverse. Moreover, the responses of each sex were affected by breeding phase. Females departed first more while feeding than while nest building, and males approached first more while feeding than while nest building. More follows occurred during nest construction than during feeding, and females followed males somewhat more during nest building than during feeding. These data suggest that 1) hews directed within the pair reflect intra-pair aggression; 2) a drop in hew exchange may signal increased readiness to renest; 3) males may be ready to renest before females, who continue to call after males stop; and 4) readiness to renest may be affected by the presence of dependent young.

HORMONAL CORRELATES OF BREEDING STATUS, NEST CONSTRUCTION AND PARENTAL CARE IN MULTIPLE-BROODED NORTHERN MOCKINGBIRDS, MIMUS POLYGLOTTOS

[Cheryl A. Logan and John C. Wingfield, Hormones and Behavior, 1995, 29: 12-30]
Abstract. Blood samples were taken from free-living male and female mockingbirds to determine correlations between plasma steroids and breeding behavior. Plasma levels of testosterone (T), estradiol (E2), progesterone (P) and corticosterone (B) were assessed during pre-breeding courtship, while males built nests, while females incubated, while parents fed young, and while males built new nests as pairs continued to care for dependent fledglings. Samples were also taken from unmated males holding spring territories. In males, T and P fluctuated with breeding phase, and in all groups, male T levels exceeded basal values measured in autumn. But, when mated males built nests in the absence of young, T levels were higher than in all other conditions, though only for early broods. In females too, T and P levels changed with breeding phase; T levels rose during courtship and while males built first nests and were intermediate during nest construction for later broods. Male and female P levels rose during courtship and when males built nests with young present. While feeding young in the nest, males showed significantly higher corticosterone levels than females, though in females, both P and B levels increased during incubation for replacement broods. These findings strengthen an earlier finding that T may activate male nest construction, and they suggest that breeding behavior may be related to T in females as well. Progesterone data raise the possibility that P too, may be linked to breeding behavior in male and female mockingbirds. Sex differences in corticosterone levels during the nestling period may reveal subtle costs associated with male mockingbirds' highly aggressive defense against predators.

AGGRESSIVE HARASSMENT BY MALE NORTHERN MOCKINGBIRDS (MIMUS POLYGLOTTOS) DIRECTED AT THEIR INCUBATING MATES

[Cheryl A. Logan & Kim C. Derrickson, Bird Behavior , 1996, 11, 71-80.]
Abstract. Fourteen of 21 mated male mockingbirds from three different study populations were observed attacking or chasing their mates in a highly aggressive fashion predominantly during the incubation phase of breeding. In one population, harassment also occurred during nest construction, when the female is fertile, but more instances were seen during incubation. Overall, most attacks (70.2%) occurred during the incubation period.

We analyzed a total of 112 chases in detail. Males almost always initiated the interactions, and over 95% of the cases of harassment were accompanied by aggressive "hew" calls produced by one or both members of the pair. When only one member of the pair called it was always the female.

Individual males varied considerably in the frequency of aggressive harassment. Analyses of this variability revealed no correlation between the frequency of aggressive chases and either the male's territorial activity or the amount of incubation shown by the female, though most chases occurred after the female left the nest following a period of sitting. Male song never occurred during a chase. However, singing was common in the period immediately before the chase, and in one population the frequency of aggressive chases was positively correlated with the mean amount of song/h produced by a male. Moreover, harassing males sang more in samples with chases than they did in samples in which no chases were observed. We discuss several hypotheses that may explain the adaptive value for males of what appears to be a very costly strategy of intra-pair aggression during incubation.

Fledgling Age Affects Female Reactions to Mate Song in
Free-living Northern Mockingbirds ( Mimus polyglottos )

[Logan, C. A. and Donaghey, B. A. 1997. Bird Behavior, 12, 1-6.]
Abstract. The responses of mated female Northern mockingbirds to their mates' songs, to strange mockingbird song, and to brown thrasher song were tested with fledglings of varying ages present on territory. Males who had resumed singing in preparation for a subsequent brood, were temporarily removed from their territories, and females were tested with no male present. When responses were compared against a baseline measured just prior to playback, females showed evidence of responding to both mate and strange mockingbird song, but not to control brown thrasher song.

In addition, two measures of females' reactions to their mates' songs were positively associated with fledgling age. As fledgling age increased, so did time spent near the speaker playing mate song and the number of approaches to the song. However, no variation in response to stranger or brown thrasher song was associated with the age of dependent young, though there were trends indicating a negative correlation between fledgling age and latency to approach both mate and strange mockingbird song. Females' species specific response to song confirms other work indicating that males direct their songs to females. In addition, these data indicate that fledgling age affects females' reactions to male song. They suggest that the effect of fledgling age on re-nesting intervals in double and multiple brooded species may be mediated in part by the impact of fledgling age on females' reactions to male vocal signals.

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