Loon Magic/Loon Magic - Wayzata Technology (8011) (1993).iso/mac/pc_text/10raisne.txt

Text File | 1993-07-25 | 6KB | 105 lines

   Some Notes on Productivity

   Everyone wants "their" loons to hatch a pair of chicks, raise them
successfully and go on to do it all over again every summer for the
next twenty or thirty years. One biologist/mathematician determined
that robins producing their typical two broods of four young each
year would be responsible, counting the work of their offspring, for
nineteen million robins over a ten year period. That's a lot of robins.

    Using the same concept, but plugging in a different formula
because loons only produce two young per year and the offspring
don't breed for at least three years, a loon mathematician, assuming
reproductive maximums, can project a total of 1,180 new loons over
fifteen years. That total assumes several things: the first pair and all
their offspring produce two chicks per year, every year; all young
mature and reproduce at three years of age (their fourth summer);
and none of the loons die.

   While there are many factors which prevent populations from
expanding in such geometric proportions, the most significant
limitation in loon productivity is delayed breeding. Most birds and
small mammals will breed within their first year and even large
mammals like the whitetail deer will breed in their second fall.
Although the breeding age of loons is not known with absolute
certainty, most biologists believe loons are sexually mature in their
fourth summer; six years must pass before the first grandchloons
appear. Many biologists contend this delayed breeding period allows
immature loons time to develop the fishing skills necessary to
provide proper parental care later, but it certainly affects
productivity. To continue their species, loons have to survive three
winters in coastal waters where severe stress, disease and pollution
cause significant mortality.

   The delayed breeding in loons is not uncommon. Albatrosses do not
breed until they are five years old and lay only one egg. Many
seabirds follow the same principles. It could well be that the low
productivity of loons is an evolutionary protection of the loons' food
supply.

    In addition, the significant number of loons which return north in
breeding plumage but don't breed must be considered. There are
quite a few territorial pairs that do not attempt nesting and a larger
number of unpaired single birds, possibly very young or very old
loons. Surveying Wisconsin's loons, Gary Zimmer classified twenty-
eight percent of the entire population as non-breeding birds. Studies
by Scott Sutcliffe in New Hampshire suggest that loons occupying the
same territories each year will attempt nesting only three years in
four, but will defend the territory in the off-year. Data from other
studies support this theory. Studies in Minnesota and Alberta
estimated that about twenty percent of the territorial loon pairs did
not nest. Since the original calculation assumed that all three-year-
old loons would reproduce, subtraction of at least twenty percent is
in order.

   Then there are egg loss and chick mortality; eggs and chicks face
many dangers from predators. Another ten to twenty percent should
be subtracted from the theoretical productivity.

   Loons live in a real world, not a mathematical one. The annual
productivity of loons is measured as the number of surviving chicks
per territorial pair. Obviously that number is not 2.0 with each pair
contributing their fullest to the preservation of the species, but is
typically within a range of 0.30 to 0.50, or thirty to fifty chicks per
100 pairs of adults. Considering historical data, one may expect 100
pairs of loons in the Boundary Waters Wilderness Area of Minnesota
to produce forty-seven chicks; 100 loon pairs in New Hampshire to
produce fifty-one loons and 100 pairs in Wisconsin to produce forty-
eight chicks. For some areas the productivity is extremely low. In her
1978 study in north central Minnesota, Judy McIntyre found a
productivity rate of 0.27, or twenty-seven chicks per 100 pairs.

   When all the data on nesting success is reviewed, areas of special
interest stand out. If loons are nesting in raccoon country, island
locations are clearly more successful. In wilderness areas, however,
location is not a factor. Titus found that smaller lakes produced
relatively more young than larger lakes. He believed that the effort
required to defend larger territories typical of large lakes might help
explain the lower success rate. Smaller lakes in his Minnesota study
area also had a considerably better ratio of loons to surface area of
water: an average of one loon per forty-nine acres in small lakes,
compared with one loon per 121 acres in large lakes.

   According to Titus' data, the lucky loons are those in two-egg
clutches. In his northern Minnesota study, eggs from one-egg nests
had a mean of 0.21 chicks surviving per egg while eggs from two-egg
clutches had a 0.40 mean rate of survival. So an egg has double the
chance of producing a loon if it happens to have another egg next to
it. While common sense might suggest loons could provide better
incubation and care for a single egg and chick, common sense doesn't
work well on this point. Olson's results were even more dramatic. Of
nineteen one-egg nests only one succeeded, while fifteen of twenty-
two of his two-egg nests produced chicks. He speculated that there
might be less attachment, and therefore, less care for one egg than
two.

   While different studies show slightly different loon nesting success
percentages, the nesting success average is about forty percent,
which isn't bad when compared to other birds. In a 1979 study,
Margaret M. Nice estimated a forty-nine percent success rate for all
birds with open nests and a sixty-six percent success rate for cavity
nesting birds. Common loons have a system that works. They have
maintained viable populations for thousands of years. Given a little
room, they will probably continue their survival game for thousands
more.