Lisa Jones took this short video of an albino American Robin in her backyard in Clinton. This bird is a pure albino with no feather coloration at all and with pink eyes.
June 2014
Satellite Males, Infidelity and Delayed Plumage Maturation
Spring migration is winding down. The arrival of Blackpoll Warblers, Black-billed Cuckoos, Saltmarsh Sparrow and Nelson’s Sparrow signifies the end of the spring spectacle.
Having returned to Maine from the south, male birds are setting up territories and trying to attract a mate with songs and displays. We have a tendency to project an idyllic image of a happy bird couple raising a family. That image is often inaccurate.
For starters, not every bird will be able to find a mate. Some of the best evidence for this statement comes from an experiment that was done in Maine over 50 years ago. The methodology of the study will be reprehensible to some. Nevertheless, we learned much from this experiment.
The researchers mapped out a 40-acre forest plot. In early June, they determined that 154 territorial birds (males) were present. Then the removals began. The researchers shot as many of the singing males as they could. Within two weeks, the density of male birds was reduced to 21% and kept at that level until July 11. By July 11, 528 adult birds had been killed. That’s 3.5 times the original density of birds!
This removal experiment tells us there are lots of unmated males that are lurking around, hoping for a chance to acquire a territory and a mate. These non-territorial birds are called satellite males in the ornithological literature. The experiment shows that there must be many satellite males waiting for an opportunity.
As in mammals, birds show a 50:50 proportion of females:males. With so many satellite males, there must be unmated females present as well who never come into the vicinity of an unmated male. The failure of some birds to find a mate challenges our fanciful notion of wedded bliss in birds.
An even greater challenge to that notion lies in the fact that avian social life has soap opera aspects. Cheating on a mate occurs frequently. Thanks to the development of DNA fingerprinting techniques, we can determine the paternity of nestlings. Although 90% of bird species are classified as monogamous, 30% of nestlings are sired by a male other than the female’s mate.
It’s a two-way street. Females seek multiple partners to fertilize their eggs and males seek as many female partners outside of the pair-bond as they can find. These dalliances are referred to as extra-pair copulations and their importance in the field is indicated by the fact that every ornithologist knows the initialized version, EPC, of this behavior.
The male incentive for EPCs is clear: to father as many baby birds as possible. What’s the advantage for a female who can produce only a few eggs? The best argument is that the female is seeking to increase the variability of her nestlings. The environment is always changing and having greater variation in her offspring increases the chances that one or two of those offspring will better fit the demands of the environment in the future.
The reproductive life of a territorial male is pretty good. He fathers at least some of the eggs laid by his mate and perhaps has some EPCs with females on neighboring territories. But what about those unmated satellite males? Do they get to reproduce at all? Satellite males do not have a territory because they are outcompeted by the stronger males who can defend a territory. Weaker males are usually younger as well.
Some males engage in a type of trickery called delayed plumage maturation. In the second year of their life, their plumage resembles that of a female. This disguise allows them to slip into the territory of a male (he’s got cheating on his mind) and sneak a quick mating with the resident female. Check your field guide to see the second-year plumage of American Redstarts, Baltimore Orioles and Red-winged Blackbirds.
[Originally published on May 25, 2014]
Human-related Mortality of Birds – IV
The last three columns were devoted to a consideration of the various human-related sources of bird deaths. Perhaps because some readers read only one or two of the columns, I have gotten many emails that indicate I failed to get across the point I wished to make. The columns were timed to lead up to Earth Week. But we should not try to tread lightly on our planet for only a week a year so I will provide this overview column.
Let’s consider loggerhead turtles as an apt analogy for the importance of understanding the impact of different sources of death for a species. Loggerhead turtles are an endangered species. Some are killed illegally for food, others are trapped in trawl nets by commercial fishing boats, and predators get some hatchlings are they stumble down the beach after hatching for their first swim in the ocean. Nests on sandy beaches are often lost to predators including dogs.
For decades, conservationists have monitored the arrival of loggerhead females on nesting beaches. These people may cordon off the nest site to keep egg predators away. Hatching turtles may be accompanied by humans as the turtles head to the water for the first time. These efforts have saved many turtle lives.
However, modeling the population dynamics of loggerhead turtles revealed some intriguing truths. First, even if every egg were to hatch and every hatchling could make it to the water, the loggerhead turtle would still go extinct under present conditions. The model further showed that the critical stage of the life cycle is the 5-7 year-old turtles. Many of these turtles died from becoming entangled in trawl nets; the turtles drown when trapped in a net.
To reduce these deaths, the federal government is mandating that trawlers, fishing in areas where loggerhead turtles occur, must have turtle excluder devices (TEDs) installed on their nets. When a turtle is captured in a net, the TED opens up to allow the sea turtle to escape. The power of the model lies in showing environmental managers which stage of the life cycle should be targeted for conservation efforts. Protecting the 5-7 year old juveniles is more effective than protecting eggs.
Reducing human impacts on bird deaths requires a similar approach. We need to understand the magnitude of the different types of human-related mortality. The last column described by far the two most potent sources of bird deaths related to humans: building collisions and cats. My argument is that we should be trying to reduce these hazards first because of the sheer magnitude of the effects. As an unabashed cat lover, I know that keeping cats as indoor pets is the way to go for the safety of many birds and the safety of the cats. Proper placement of bird feeders and improving the visibility of glass in our houses can reduce collision-related bird deaths.
Do I therefore disregard deaths from wind turbine collisions? Of course not. As indicated earlier, I am a long-time opponent of mountain-based wind farms. Any bird death from human causes should be of concern. Collectively, wind farms result in far fewer deaths than cats or building collisions. However, we need to realize that wind turbines pose threats to some species like cranes and eagles that are not likely to die from a cat attack (!) or a window collision.
We all need to take energy conservation more seriously. Better yet, we should be practicing energy avoidance. We can thereby reduce the need for more coal-burning power plants pumping carbon dioxide into the atmosphere as well as wind farms and hydroelectric dams that cause the loss of habitat.
Birding locally reduces bird deaths from car collisions and cuts down on carbon dioxide emissions. Buy a carbon dioxide offset to mitigate the carbon dioxide released from your car or plane travel.
[Originally published on May 11, 2014]