In the last post, we explored the phenomenon of habitat selection in birds. We saw that birds choose habitat at different scales. For instance, Yellow-rumped Warblers prefer coniferous forest but, at the level of individual trees, are typically in the lower parts of a spruce, firm or hemlock.
But, not so fast. Generalities about habitat preferences in birds don’t always hold. We’ll explore a couple of cases of changing habitat selection in birds in today’s column.
Let’s start with Red-winged Blackbirds. Males are polygynous; they may have more than one mate at the same time. Some males have five or more mates, meaning that some males remain unmated.
Red-wings nest in marshes. Although a marsh looks rather homogenous to our eyes, they are often quite patchy in terms of food resources. A male wants to grab one of those better patches for its breeding territory. The competition for the best territories is really intense in red-wings, probably explaining why males return to Maine in the spring a month ahead of the females.
During that month before potential mates return, the males duke it out with the top males getting the best territories. They aggressively proclaim their ownership by singing “konk-a-ree” while flashing their red epaulets.
When the females arrive, they began considering males as a possible mate. The females are not interested in the quality of a male’s song or the redness of his epaulets. Rather, they are interested in real estate.
How do we know that? Some researchers captured male red-wings and painted their red epaulets black. When they exposed their epaulets as they sang, the red signal was no longer there and they were attacked by other males. The lack of red epaulets didn’t weaken them; they repelled the attacks but had to endure more of them.
But the females didn’t care. A red-wing male with blackened epaulets was perfectly acceptable provided his territory was a rich one.
To keep the example simple, we’ll consider a marsh with four males on territory. The first female to arrive has an easy choice; she’ll become the partner of the male with the best territory.
Now, a second female arrives. She will either bond with the male with the second best territory or perhaps will become the second female on the best territory, if the top territory is particularly rich. Let’s say she chooses the male with the second best territory.
Now a third female arrives and the relative habitat quality has changed because of the presence of two females. She may decide to become the second partner of the top male, leaving two males with no mate yet.
And so it continues in this dynamic system where the relative value of each territory changes. Talk about complicated. So, we may end up with the fourth male in the hierarchy with no mate, male three with one female, male two with two females and male one with four partners. Birds are better at math than we think.
I did a study one winter in Maine on habitat selection in wintering birds. I divided the Maine landscape up into four categories: coniferous forest, deciduous forest, edge habitat (including suburban yards) and agricultural fields. I had 50 stops that I censused once a week. Nine of the 16 most common species showed a preference for one or more type of habitat. For instance, European Starlings preferred edge and agricultural habitats to forested habitats.
One of the deciduous forest sites had a feeding station and seven edge sites had feeding stations. When I excluded those sites from the analysis, four species showed different habitat preferences.
Downy Woodpeckers preferred edge habitats when feeders were present but switched to a preference for deciduous forest when feeders were absent.
Blue Jays, House Finches and American Goldfinches preferred edge habitats when feeders were present but that preference disappeared if feeders were absent.
The closer we look at biological interactions, the more complicated they become. It’s no wonder habitat selection is such a rich research topic.