Sunday, May 19, 2013
By HERB WILSON
Have you been awakened this March by a loud con-ka-ree? The song of the male red-winged blackbirds is ringing through the air again. Male redwings have been arriving back in Maine since the first of March. Females will not appear for a month or more. The reason for this striking difference in arrival between males and females can be understood as the result of an unequal partnership.
To understand what I mean, let's start with a human example. Susan and James have decided to open a business together. They know that they will need $100,000 to get the business launched. Susan has $90,000 she can contribute to the start-up and James can only contribute $10,000. Their partnership is essential to create the business, but Susan is clearly the partner with the most to lose. So she rightly should have a greater voice in the operation of the business.
Now, let's switch back to birds. The business of birds is to make more birds. The egg of a female is a huge investment relative to the modest cost to a male of making sperm. Unequal partners again. Both partners are necessary for the production of viable eggs and ultimately new generations. The female, however, because of her far greater investment in a fertilized egg, runs the show. She gets to choose her mate. The males may have to sing, display, dance and even bring food for the female to entice her to choose him as her business partner. In some species, the female chooses a male as her mate based on the qualities of his territory.
Red-winged blackbirds are good examples in which the female chooses a mate by inspection of the quality of his territory. In anticipation of the arrival of the females, males vie for territories in marshes, fields and even roadside ditches throughout the state. Territories are relatively small, averaging about half an acre.
Not all territories are equal, however. Male and female red-winged blackbirds forage extensively outside the territory. The territory is used primarily for nesting. A high-quality territory therefore will have dense vegetation allowing nests to be effectively hidden from predators.
For the next few weeks, territorial skirmishes will be frequent among the male red-winged blackbirds. Each male tries to claim the best territory he can defend. Some males are stronger than others; the dominant birds are usually older, experienced males. The dominants will claim the best territories and the weakest birds may have none at all.
Females only arrive after the territorial battles have finished. There is no reason for them to come early so they can dally a little longer in their more hospitable wintering areas to the south.
When a female arrives on the breeding ground, she assesses the habitats of the various territorial males. The first female will usually choose the male with the best territory. Subsequent females have interesting choices. Is it better to be the second female on a high quality territory or the only female on a lower quality territory?
Quality of territory is so variable that it's often better for a female to bond with an already paired male who commands a particularly fine territory. The high variability of territories gives rise to this mating system in which a male may have multiple mates.
The scientific term of this phenomenon is polygyny, meaning many females. One dominant male may have up to a dozen females on his territory while other males may have only one mate. Some males may be too subordinate to even have a territory, and therefore no mates.
Twelve mates or no mates? You can see how a junior partner seeks to ensure his success by arriving early on the breeding grounds.
Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at: