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The Carletonian

The Carletonian

The Carletonian

    Arb Notes

    <u’ve spent any time in the Arb this winter, you’ve likely seen a dark-eyed junco (Junco hyemalis). If not, you’ve undoubtedly heard one—a call often described as a high, smacking ‘stip’ emerging from the ground below stands of dense vegetation. One of the most common birds during the winter months in Minnesota, juncos are at first glance the least worthy of a column in the Carletonian—common, drab, begging for an yawn-inspiring article.

    With segregated migration, the story becomes much more interesting. Chances are any junco that you’ve seen in the Arb is male—females migrate further south than males. Northfield is near the northernmost extreme of the winter range and therefore is nearly female-less this time of year. Population surveys often reveal that junco populations in northern states like Minnesota and Michigan are 80% male whereas populations in southern states like Alabama are only 25% male. This difference appears to be caused by variation between males and females in the hormonal cues that regulate pre-migration restlessness. Such a marked difference between the sexes (sexual dimorphism) likely occurs because of the advantage that males gain by over-wintering near their breeding range. In a northern clime where resource-rich territories are few, males that weather the winter have first access to the best territories in the spring, and thus are the most attractive to returning females.

    Beginning in March and continuing into April, expect to see dark-eyed juncos returning and congregating in flocks of 20-30 individuals in the Arb and on campus. These ground-dwelling flocks will be predominantly female and will disappear after several weeks—all of the Arb’s juncos breed in the Canadian boreal forest and parts of northern Minnesota. When breeding they are ‘socially monogamous,’ meaning that each individual tends to have only one mate with which they nest and defend a territory. However, recent genetic evidence reveals that juncos (and many other birds) secretly copulate with individuals in neighboring territories.

    Get out into the Arb and junco-watch before they move north and become hopelessly lost in the task of making more juncos!

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