The migrations of waterfowl through Iowa during the fall and spring provide a spectacle that is enjoyed by nearly all Iowan’s.  Waterfowl hunters, however, seem to be particularly attuned to the movements of these birds, especially during the fall when the duck and goose seasons are open.  In fact, their individual observations of these birds’ migrations often form the basis for their opinions as to when the waterfowl seasons should be opened or closed.  Whether you hunt or not, however, the migration of ducks and geese through the state is a special part of the changing of the seasons in Iowa.

For over 3 decades, the DNR’s Wildlife Bureau staff have been counting ducks and geese during the fall on most of the major waterfowl staging (gathering) areas in the state.  These counts are typically made once each week from mid September to mid December.  Most counts are made from the ground by experienced DNR staff.  The upper pools of the Mississippi River (Pools 4-13), however, are flown by the U.S. Fish and Wildlife Service.  On many areas, the majority of the birds are found inside the inviolate wildlife refuges on the state-owned wildlife management areas.  Inviolate refuges are closed to all public use during the fall to provide ducks and geese with undisturbed resting areas on their migration south. 

The migration of some species and groups of ducks and geese are illustrated in Figures 1-8.  When viewing these figures, it is important to keep in mind how the information is collected and analyzed.  For one thing, the graphs are based on a single count made on many wildlife areas across the state during each weekly period in the fall, in most cases on the same date.  So, it is a snapshot in time.  The migration is continuous and, in some respects, this survey is like dipping a cup into a stream.  Nevertheless, it provides a relative idea of when the ducks of various species are most numerous in the state.  Obviously, the ducks are on the area when the count is made – so they must have arrived anywhere from 1-6 days before the actual survey date.  In other words, the migration of the birds into the state often precedes the period in which they are counted. 

The survey dates were grouped into weekly periods as follows:
Sept 8-14 = Sep-2,  Sept 15-21 = Sep-3,  Sept 22 –28 = Sep-4,  Sept 29 - Oct 5 = Oct-1,  Oct 6-12 = Oct-2,  Oct 13-19 = Oct-3,  Oct 20-26 = Oct-4,  Oct 27 - Nov 2 = Oct-5,  Nov 3-9 = Nov-1,  Nov 10-16 = Nov-2,  Nov 17-23 = Nov-3,  Nov 24-30 = Nov-4,  Dec 1-7 = Dec-1,  Dec 8-14 = Dec-2,  Dec 15-21 = Dec-3.

The abundance of ducks or geese for each weekly period is represented as a percent of the total number observed during the entire fall season.  The 1995-99 average is the average of these percents.  This helps smooth-out some of the variability in numbers of ducks seen from year to year and more clearly illustrates the chronology of the migration, irrespective of the changes in the magnitude. 

These figures represent statewide totals of ducks observed each weekly period; the ducks seen on the northern areas are lumped with those seen on the southern areas.  This does not affect when certain species of ducks are first seen in the state, but it probably affects the tail-end of the curves (i.e., birds move out of the northern part of the state before they move out of the southern part). 

In the process of examining the migration chronology of various species, it became clear that some species of ducks could be grouped together for the purpose of illustrating their migration.  Mid-season dabblers include pintails, wigeon, gadwall, green-winged teal and shovelers.  Mid season divers include redhead, scaup and ring-necked ducks.  (Canvasbacks could also be included in this group but the numbers of cans observed on the Mississippi River had a disproportionate influence on the graphs so they were excluded.)  Late season divers are goldeneyes and buffleheads.

The numbers of mallards counted on the areas each week appears to have a disproportionate influence on the form of the migration curve for “all ducks” when mallards and other ducks are lumped together.  Mallards are conspicuous, refuge orientated, and relatively easy to count.  Each year, about half of all the ducks recorded on our refuges are mallards.  Mallards dominate the survey despite the fact that only about 15% of the ducks migrating through Iowa each fall are mallards (the proportion may actually be smaller if you throw ring-necked ducks, wood ducks, goldeneyes, buffleheads, and mergansers into the mix).  Of course, mallards are disproportionately important to hunters: 35-50% of the ducks harvested in Iowa each fall are mallards. 

Finally, 1999 was indeed a different year in terms of migrations, more so for mallards and snow geese than other waterfowl.  You don’t have to go too far back in time (1997), however, to see what a more typical migration looks like.  Unless the climate has changed dramatically, however, this year’s waterfowl season dates should encompass the primary migration period for the vast majority of the ducks that will be flying through Iowa.