Geese in Denver
Denver has two populations of geese. Year-round resident flocks are larger Canada Goose. Migrants come in several flavors, though most are the small Cackling Goose.
In late summer through winter, resident Canada Geese gather in loose flocks of 15-50; though multiple flocks may group together, especially on night roost sites. Canadas may join with the extensive flocks of Cackling & other Geese, or they may go their own way. Either way, they tend to stick close together. All adults (whether or not they have babies) molt all their flight feathers in June/July; during this time they are unable to fly.
Cackling Geese migrate just far enough south each winter to find areas with exposed grass and open water. A few snow days don’t seem to bother them much, though extended periods of cover may push them further south in search of grazing sites. Migrants start trickling in in mid-October, staying until late March or early April. Cackling Geese seem to have small ‘primary’ groups of 10-15 birds, likely family groups, and these small groups mix and match into larger flocks of up to 2,500 birds. These are the extensive flocks you see in golf courses, parks, cemeteries, and schoolyards.
Can geese become overpopulated?
Yes, but only resident birds, and only if there are so many as to overwhelm the ecosystem or if a large flock can’t or won’t leave a site. Any species that sustains a population far in excess of local carrying capacity will be functionally invasive. While geese numbers are high, it is unlikely that they currently reach these levels of over-saturation in the Denver area; however, a few more successful breeding seasons and they could.
Do geese frighten away other birds that would otherwise be seen in this area?
Geese are very tolerant of other waterfowl, with modest exceptions while their goslings are very young. They do not threaten or drive off other species, and observations suggest they may even boost diversity. The exception is if geese so overwhelm a location as to destroy the landscape and render it unusable by other species.
Do geese carry disease that humans are susceptible to?
Geese, like most animals, can carry E. coli, Listeria, Salmonella, & other diseases; human transmission rates are very low. The CDC lists a few instances of transmission each year; on average, perhaps 1 per 1,000 cases of transmittable diseases come from wildlife (and often less); most cases result from either pets or poor food handling technique. People who work with geese/wildlife on a regular basis rarely experience illness despite regular, hands-on contact.
Aren’t Geese supposed to migrate?
Historically, large Canadas were listed as nesting in the Great Plains, the Great Lakes, and the Midwest. In winter they would move regionally to find open water; water features in urban areas seem to be acceptable alternatives today. What we now call ‘Cackling’ Geese did and still do migrate.
Written and contributed by Kenyon Moon, edited by Christine Franck