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Photo of Dave Anderson Dave Anderson as seen on Voyage to the Galapagos: Evolving Beaks and Masked Killers

Click on Dave's photo to read a brief bio.

q Do the offspring of the blue and red footed boobies force their siblings out like the masked boobies? What about other birds like the finches? (Question from Terry)

A Some booby species have only one chick in the nest at a time; that is the case with red-footed boobies. Blue-footed booby chicks fight with one another, but it only gets serious during food shortages when only one chick can be supported by the available chow. One other kind of booby, the brown booby, shows "obligate" (=always) siblicide like the masked booby.

Some other kinds of birds are also siblicidal, like some herons, eagles, and pelicans. Finches aren't siblicidal though.

q Saw the show on the Galapagos last night…great stuff. I was particularly fascinated by the study you mentioned on Daphne Major regarding the evolution of the birds. Where can I gain access to the data being collected? Thanks. (Question from Chris, Environmental Science Teacher, Sachem HS, Long Island, NY)

A The principal investigator for that study is Peter Grant, and he is at Princeton University. He and his wife and collaborator Rosemary have written two excellent books summarizing the group's work on the finches:

Ecology and Evolution of Darwin's Finches, Princeton University Press, 1986, by PR Grant


Evolutionary Dynamics of a Natural Population, University of Chicago Press, 1989 by BR Grant and PR Grant

q I am a student at EMHS in NY. I watched the show about the Galapagos and it was really interesting. I have a worksheet given out by my Biology teacher for extra credit. One of the questions on the Masked Killers I did not understand, so I decided to get online to see if I could find out more. I found you instead which is much better. The question is: why is "siblicide" a successful adaptive behavior for the masked boobies? I think the answer is: "It's very rare that both of the boobies that are hatched survive because there's so many dangers, so usually only one of the boobies survives." Is this correct? I appreciate your time and assistance. (Question from Aleesha)

A Aleesha, after 15 years of study, we are now convinced that what you have suggested is correct. Broods of one chick yield higher reproductive success (the currency of natural selection) than broods of two do. When two chicks hatch, it is in the parents' and the older chick's best interests to dispatch with the younger chick; we know this from experiments in which we suppress the siblicide and then measure reproductive success compared to normal controls.
However, clutches of two yield higher reproductive success than clutches of one do. This is because masked boobies have lousy hatching success. To have a good chance of hatching at least one chick they have to lay two eggs, and the chance of getting at least one chick is greater than if they lay only one egg. However, sometimes both eggs hatch (this would be good in most species but in this case it is a "mistake" of sorts), and a mechanism is needed to bring the brood size down to the optimum number of one.

q Peter Grant et. al studied the Galapagos finches over a 20-year period and observed that finch survivors of a 1977 drought had slightly larger bodies and beaks. However in 1983, food was once again abundant, and beak size quickly reverted to the pre-1977 size.

1. Does Grant's study do anything to confirm *macro* evolution?

2. What fossil evidence is there for transitional forms leading up to finches, or to birds in general?
(Questions from David, Kelowna, British Columbia)

A 1. Does Grant's study do anything to confirm *macro* evolution?

The Grant study is in the area of microevolution, evolution within a population. Very likely the same processes that cause microevolution cause the genetic divergence leading to macroevolution (production of separate species) when one population is separated into two. Galapagos is a likely place for this to happen, because a subgroup from one population might colonize a separate island and experience a different set of selective influences. After a period of responses to the separate sets of influences, the two populations can be expected to have evolved differently. However, the Grant study has not documented macroevolution in progress; macroevolution is a difficult phenomenon to identify as it is happening in the wild. In laboratory situations, however, it is easy to cause macroevolution to happen and it has been done many times.

2. What fossil evidence is there for transitional forms leading up to finches, or to birds in general?

Sadly, not much evidence has been found so far in the fossil record that helps in figuring out the finch situation. However, some great fossils have been found and continue to turn up (lately from China) having to do with the early evolution of birds and their ancestors. For good recent reviews see Alan Feduccia's book "The Origin and Evolution of Birds" from Yale University Press and the article by Kevin Padian and Luis Chiappe in the February 1998 Scientific American magazine "The Origin of Birds and Their Flight".

q Our class was interested in the mockingbirds found on the islands. We have a native species in East Texas. How is the Galapagos bird different and what is it called? (Question from Denise, Middle School Teacher)

A Four species of mockingbird are recognized from the Galapagos. Three are found on only one island each: the Hood mockingbird on Espanola Island, the Charles mockingbird from Floreana Island, and the Chatham mockingbird from San Cristobal Island. (Hood, Charles, and Chatham are old, now unused, names for Espanola, Floreana, and San Cristobal islands, respectively. The mockingbirds got their names back when the old island names were still in use.) The last species is found on almost all of the other islands, and is just called the Galapagos mockingbird. These four species are all larger than the northern mockingbird that you see in North America. In many respects they are similar, though. For example, northern mockingbirds and the Galapagos species all can be seen running along the ground, then stopping abruptly and flashing their wings forward as they look at the ground. Scientists think that the birds are possibly trying to flush bugs out of the grass by flicking their wings.

q Professor Anderson, I know that the offspring of Darwin's Finches are born with beaks that are either smaller or larger than the year before. This comes from the resources available at that time. I was wondering... Have the beaks of offspring been generally getting larger or smaller in the past years? (Question from Randi)

A The study on Daphne Island has not shown any consistent long-term trend. In some environmental conditions the larger size is favored and those birds have more babies, so large size is more common in the next generation, and sometimes the conditions favor the reverse. So bill size bounces around over time but to the present, no consistent trend has appeared.

q I realize that there must be laws dealing with your contact with the wildlife and plant life on the Galapagos Islands. What are some of these guidelines, and how do they help or hinder research on the islands? (Question from Megan, High School Student)

AThe Galapagos National Park Service has a set of regulations that visiting scientists must adhere to. Some have to do with what scientific work is permitted, and you can only do what has been approved for your project by the Park Service. Some have to do with how you live when you are camping; basically you have to leave the site the way you found it. Much of it is common sense for a wilderness area.

The most important regulation that scientists must pay attention to involves the danger of transporting organisms between islands. A number of damaging creatures have been introduced to the Galapagos since humans arrived, and we are extremely careful when we move from one island to another to clean all of our equipment, the bottoms of our shoes, the nooks and crannies in our tents, and everything else. If we moved an introduced plant or fire ant or mouse from, say, an island with a town to a pristine wilderness island then that organism and its descendents might have a bad effect on the native biota on the pristine island and that would be terrible.

q In the teaching guide for this show, there is a photo on page 10 of a masked boobie sitting on three eggs. How often do they lay three eggs? If three eggs hatch, will one of the hatchlings push the other two out into the sun? (Question from Mark, Middle School Teacher)

A In the many (>10,000) masked booby nests that we have studied we have never seen a reliable case of one female laying three eggs in a single clutch. However, we do sometimes see a nest with three eggs, and sometimes with two eggs and a sealion vertebra, or one egg and a much larger albatross egg! Masked boobies are happy to pull anything into their nest that is vaguely similar to one of their own eggs, and we think that is how three egg clutches come to be.


Scientific American Frontiers
Fall 1990 to Spring 2000
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