Globally, migration phenologies of numerous avian species have shifted over the past half-century. Despite North American waterfowl being well researched, published data on shifts in waterfowl migration phenologies remain scarce. Understanding shifts in waterfowl migration phenologies along with potential drivers is critical for guiding future conservation efforts. Therefore, we utilized historical (1955–2008) nonbreeding waterfowl survey data collected at 21 National Wildlife Refuges in the mid- to lower portion of the Central Flyway to summarize changes in spring and autumn migration phenology. We examined changes in the timing of peak abundance from survey data at monthly intervals for each refuge and species (or species group; n = 22) by year and site-specific temperature for spring (Jan–Mar) and autumn (Oct–Dec) migration periods. For spring (n = 187) and autumn (n = 194) data sets, 13% and 9% exhibited statistically significant changes in the timing of peak migration across years, respectively, while the corresponding numbers for increasing temperatures were 4% and 9%. During spring migration, ≥80% of significant changes in the timing of spring peak indicated advancements, while 67% of significant changes in autumn peak timing indicated delays both across years and with increasing temperatures. Four refuges showed a consistent pattern across species of advancing spring migration peaks over time. Advancements in spring peak across years became proportionally less common among species with increasing latitude, while delays in autumn peak with increasing temperature became proportionally more common. Our study represents the first comprehensive summary of changes in spring and autumn migration phenology for Central Flyway waterfowl and demonstrates significant phenological changes during the latter part of the twentieth century.

Waterfowl survey data were collected by refuge personnel at the respective National Wildlife Refuges.

The file ‘AllCountData.xlsx’ contains all raw count data from waterfowl surveys at 21 National Wildlife Refuges in the mid- to lower portion of the Central Flyway, USA, 1955–2008. Survey data consisted of both aerial and ground counts. Canada geese includes Canada Goose (Branta canadensis) and Cackling Geese (B. hutchinsii) and Light geese includes Snow Geese (Anser caerulescens) and Ross's Geese (A. rossii). Separate counts for Snow and Ross’s geese are included for the only refuge (Bitter Lake) that separated them during surveys. Total geese, swans, and ducks represent the sum of all goose, swan, and duck species, respectively. Missing values were propagated through summations so that any sum based upon a missing count value resulted in a missing value. The Comment field contains any pertinent information to the respective survey. Missing data are indicated by n/a.

The file ‘PeakAbundanceMonth.xlsx’ contains the phenological parameter ‘peak abundance month’ for spring (January–March) and autumn (October–December) migration seasons by year and average daily maximum temperature (Mean Daily Max Temp) for waterfowl at 21 National Wildlife Refuges in the mid- to lower portion of the Central Flyway, USA, 1955–2008. Peak abundance month was defined as the month with the highest monthly count average within the respective migration season (spring or autumn) and year for each species and refuge. Within each survey, all counts belonging to a waterfowl group for which the proportion of unidentified birds was indeterminable or exceeded 10% were excluded from the data set. Only species-refuge combinations with a total of at least 5 non-zero counts, fewer than 25% zero counts, and an average peak count of 200 birds or more for the migration season considered were included. Canada geese includes Canada Geese (Branta canadensis) and Cackling Geese (B. hutchinsii), Light geese includes Snow Geese (Anser caerulescens) and Ross's Geese (A. rossii), Goldeneyes includes Common and Barrow's Goldeneyes (Bucephala clangula and B. islandica), and Scaup includes Greater and Lesser Scaup (Aythya marila and A. affinis). Separate phenological data for Snow and Ross’s geese are included for the only refuge (Bitter Lake) that separated them during surveys. Missing data are indicated by n/a.