The results of this study demonstrate that, by
identifying the origins of local breeding geese using an airport and removing
them, the bird-aircraft strike hazard can be reduced dramatically. The repeated
use of the ASA by some geese and not others supports the rigid summer-fall
movement described by Zicus (1981a) and corroborated by Schultz et al. (1988).
The same geese were consistently observed in ASA in both 1984 and 1985. The
frequency at which individual geese from Wood Lake Nature Interpretive Center
were observed in 1984 and 1985 (Fig. 3) suggests that goose movement patterns in
summer and fall are consistent for at least a 2-year period. Neckbanded geese
from Wood Lake Nature Interpretive Center and Lake Nokomis used the airport area
during the summer and fall and at a far greater intensity than geese from the
other brood-rearing marshes, and the individual geese had similar patterns
between years. Koerner et al. (1974), Zicus (1981a), and Schultz et al. (1988)
also observed that the majority of individual geese used the same general
locations day after day, and in some cases, the same individuals were observed
in the same locations as long as the food supply remained. The success of goose
removal from specific brood-rearing areas in reducing goose numbers at the
airport supports our assumption that subflocks also existed. For example, Penn
Lake birds used the airport, whereas nearby Miller's Playlot birds did not.
Because local subflocks exhibited these patterns in a predictable manner during
the summer and fall, "hazard" birds from distinct brood-rearing areas can be
identified, and selective rather than blanket control measures used.
Individual geese varied in their use of the ASA. Not
all birds from a specific brood-rearing marsh followed identical patterns, but
the patterns were consistent from year to year. This variability could allow
managers to work on an even finer level than that of the subflock. For example,
Wood Lake pairs that were not resighted in the ASA could be left, and the pairs
with a high percentage of reobservations at the airport removed. This would
provide the nature center with geese for public enjoyment and lessen the hazard
at the airport.
Removal and translocation is an effective and efficient
method for reducing local subflocks of Canada geese. The removal of the
airport's "problem birds" significantly reduced the population of local geese
and decreased the number of goose flights through the airport operations
airspace. In other urban locations such as golf courses, city parks, and
residential neighborhoods removal could provide an immediate reduction in the
number of geese, whereas, other control options such as adult sterilization or
egg removal-destruction will reduce the population in the first year, but at a
reduced rate compared with tremoval and translocation. Sterilization and egg
removal affect offspring produced that year, whereas bird removal in June and
July reduces the number of offspring in addition to the breeding adults. Any
non-breeding birds molting in the control area can also be removed.
Unfortunately, removal and translocation have
drawbacks. Cooper (1987) found that translocation was not as effective at some
sites within the Twin Cities compared to that reported in Michigan by Martz et
al. (1983), and in New York and Connecticut by Converse (1985). Cooper (1987)
reported that adult birds translocated to Oklahoma returned at an average annual
rate of 21% (range 12-28%), and that 50% of the breeding females captured at
Lake of the Isles and other sites in the Twin Cities area in 1985 were
translocated birds. Ten percent of the birds captured in this study returned
from Oklahoma in 1985. Because of the importance of population reduction at the
airport, all adult geese caught at control locations were rendered permanently
flightless after 1984. The major limitation of translocation is that ultimately
it will be self-limiting as managers using translocated geese for restoration
programs meet their goals.
The rate at which new geese will begin using the
airport is unknown and needs to be studied. The restricted subflock movement
that limited the use of the airport is not so fixed that no new geese used the
ASA. For example, geese from Penn Lake in Bloomington did not use the ASA in
1984 but did so in 1985. This could have resulted from variation in the fall
movement patterns or from the reduction in competing groups of geese in ASA in
1985. It is important that additional monitoring be done to ascertain the
longer-term (5-10 years) effectiveness of selective removal.
Migrant geese, present in the ASA from late October
until the end of November or mid-December, are a potential problem that cannot
be controlled by brood-rearing removal and translocation. Decoy flocks of
captive geese attracted migrant geese during reintroduction programs at Crex
Meadows, Wisconsin (Kooiker 1982), Michigan, Minnesota, Missouri, and Ohio
(Nelson 1962). Local breeding flocks may function similarly. Thus, a decrease
in the local goose numbers could lead to a decline in migrant geese using a
site. The results of this study do not support this logic; migrant use of the
ASA continued after a substantial reduction of local geese. Alternative
techniques such as intensive harassment of migrant flocks, rocket-netting and
removal, or special hunts where possible, are needed to control the number of
migrants at the airport.
Acknowledgments.--I thank Tim Anderson and Arlis
Olson, Metropolitan Airports Commission, and Oren Burckhardt and Les Case,
Federal Aviation Administration for their coordination of funding and providing
access to the airport facilities. Roger Johnson, Lloyd Knudson, Jon Parker, Tim
Wallace, and others with the Minnesota Department of Natural Resources provided
invaluable assistance, as did Nancy Burgstahler and Mary Mitchell, U.S. Fish and
Wildlife Service, and Richard Wetzel, U.S. Department of Agriculture. Ann
Sigford, Tim Anderson, and volunteers from the Wood Lake Nature Interpretive
Center assisted with a public information meeting, goose capture, and surveys.
Mari Smaby, Wayne Winkleman, Terry Birkenstock, Lori Hawkins, Tom Roster, Judi
Mikolai, Llew Wright, Tom Buhl, Caroline Yineman, and Mike Palanuk, University
of Minnesota, cheerfully assisted with banding, population surveys, tower
observations, translocation, and data processing. Norma Essex typed and
proof-read the manuscript.