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17 March, 2000

Giant Petrels on Humble Island Question 27: What is the most numerous marine mammal anywhere in the world? Still beautiful sunny weather with hardly a cloud in the sky. The glacier is calving all the time and it takes quite a while to maneuver the boat out through the brash ice this creates. This morning I am going out with Donna Patterson and Bill Fraser to help weigh and measure Giant Petrel chicks on Humble Island. The petrels on this island have been habituated to human presence over many years of research and can be handled with little stress on the part of chick and parents. A Giant Petrel chick's extremely effective main defense is to spew its oily, smelly stomach contents onto the potential predator. With a seabird, such as a skua, this saturation destroys the insulation value of the feathers and eventually kills the predator. Once petrel chicks are large enough to defend themselves (at around 10 days old), they are left alone on the nest in perfect safety while both parents are out searching for food. It is not only more pleasant for the scientists to work with a population that does not have this defensive reaction to humans, but it is beneficial to the chicks. They do not lose the nutritional value of the stomach contents if they do not feel the need to defend themselves. A chick that is stressed and regurgitates its food frequently might not gain enough body mass to be a successful adult. There are other islands in the area with Giant Petrel breeding colonies but Humble is the only one with habituated birds. Like all the other islands in the area it is infested with fur seals whose personal space must be avoided. There are also a dozen shedding elephant seals. Earlier in the season there was a colony of over a hundred of these large pinipeds wallowing in all the depressions. Humble has an Adelie Penguin rookery that was also home this breeding season to a lone pair of Chinstrap Penguins. The young penguins have already taken to sea and will not return this year. The adults go b their adult feathers. Then some adults return to do their own molting on the breeding grounds. There are now scattered Adelies on most islands in this area. Every two days the Giant Petrel chicks in numbered nests (around 20 of them) are weighed and have the upper side of their bills (the culmen) measured by Donna. Culmen length is a good indicator of chick age. Information is also recorded about activity around them--parents present, absent or feeding, nonbreeding adults in the area, and counts of fur seal and other animal abundances. Donna has been working here for the past ten years, and this population of birds has been studied for over 30 years. She knows almost every adult and its history. Like many large seabirds, Giant Petrels are long lived. There is at least one female that was breeding when the work started that is still raising chicks on Humble Island. Giant Petrels lay only one egg a season and do not start breeding until they are 7 or 8 years old. There is a strong pair bond so if one partner doesn't return at the start of a breeding season, the remaining adult will take one to three years to cement a bond with another mate before raising chicks again. Even though I have been watching the Giant Petrels cruise the water around our dive sites, am well aware that their wingspan is nearly a meter across, and know that this late in the season the chicks are well grown, I was still astounded by the huge fuzzy armful of a Giant Petrel (or GP, pronounced "jeep") chick. A few of their adult primary wing feathers are coming in. The rest of the bird is one huge pouff of thick soft white down with black beady eyes. The wind could blow sleet at 40 knots and they would still be cozy. An individual chick is about 30 cm wide and 50 cm long. They are cute but huge! At this stage some of them outweigh their parents, having been fed by two adults all summer and not moving much themselves. They are starting to get more active. Most made growley, bleating noises that basically say "feed me!" So one took a few steps. As we went from chick to chick, Donna greeted each one with soft talk and mimicked the noises that the chicks made. They seemed to recognize humans and didn't make any changes to their posture that would indicate they felt threatened. A parent continued feeding its chick just five feet from where we were measuring its neighbor. Each chick was picked up and set on Donna's lap to have its culmen measured with calipers. Then the chick went into the big red canvas weighing bag with a scale hooked to it. When you hold up the scale dangling the bag, you get a reading of how heavy the chick and bag are; most chicks are now above 5 kg. Then the chick was removed from the bag and returned to its cup-shaped nest of stones, moss and earth on the ground while we moved on to the next nest. Most of the nests were on high points of land. These locations could be preferred because it is easier for the parents and chick (eventually) to take off from them, or it could be that they have no other choice, since earlier in the year the lower areas of the island are covered by seals and penguins. Each individual was quite a character. One that I held spent part of the time looking curiously down at the ground, stretching out its neck. All chicks watched us, sizing up our intent. The skin of their legs and feet are silky soft. Like a puppy, if you don't support their feet when holding them, they start to struggle for purchase. Answer 26: No, there are no sharks, but there are other predators in the water such as leopard seals and orcas (killer whales).


Elephant seal shedding old skin.


Donna Patterson preparing to measure GP chick.


Measuring a GP chick's culmen with calipers.


Pair of Giant Petrels on Humble Island. Male is on the right.


GP chick beginning to fledge (dark primary feathers are visible in wings.)


Donna Patterson weighing GP chick.


Replacing GP chick in nest after weighing.


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