Acorn Count
Where Red-headed Woodpeckers spend the winter is generally thought to depend on the availability of hard mast, principally acorns. In some years, all the Red-headed Woodpeckers at Cedar Creek Ecosystem Science Reserve (CCESR) migrate elsewhere. In other years, all or nearly all remain at CCESR for the fall and winter. In still other years, some of them migrate and some remain.
To see if acorn production at CCESR affected the number of Red-headed Woodpeckers that stayed at CCESR over the fall and winter, we began counting the acorns on northern pin and bur oaks there, starting in 2018. We followed the counting protocols of a previous acorn production study at CCESR (Koenig and Knops, 2013). We placed a metal numeric tag on 100 northern pin oaks and 100 bur oaks. In August or September, when the acorns had ripened, each of two observers would count as many acorns as possible in 15 seconds on each tree. So, our acorn count was the number of acorns counted in 30 seconds.
With seven years of data, we can see some relationships. Probably our most important result is that the number of northern pin oak acorns produced greatly affects the number of RHWOs that overwinter. Our two lowest northern acorn counts were in 2020 and 2022. In those years no RHWOs were at CCESR in the fall or winter. Our highest acorns counts were in 2019 and 2023, and all the marked RHWOs at CCESR were present in the fall and winter. Bur oak acorn production had no statistically significant effect on the number or RHWOs that would overwinter. However, RHWO fledgling production from late nests may have been better in years when bur oak production was higher.
Bur oak acorns ripen by late July or early August. RHWOs eat them and feed them to nestlings but do not store them for winter. These acorns evidently are relatively low in tannins and may rot rather quickly.
Northern pin oak acorns ripen by late August or early September. They are relatively high in tannins and become more digestible as the tannins are washed out over time by rain and melting snow. RHWOs store these acorns in early fall in cracks and crevices, holes, and on occasion in large caches. We think that these acorns are the principal source of nutrition for the RHWOs that winter here.
Until this year, the number of acorns produced by each species of oak appeared to be related, with both species having good years together and poor years together. This year we counted only 47 bur oak acorns but 22,571 northern pin oak acorns. So, the acorn counts within the same year appear unrelated. See chart below.
However, since northern pin oak acorns form from flowers the previous year, we looked for a relationship between acorns produced from flowers in the same year. We found a positive relationship. See chart below.
Since we tagged 100 northern pin and 100 bur oaks in fall 2017, 22 of the northern pin oaks and 5 of the bur oaks have died. The losses are due to storms, damage from prescribed burns, heart rot, and oak wilt. Fewer bur oaks than northern pin oaks have died because bur oaks are more resistant to oak wilt and to fire damage. These losses reflect the general decline in mature oak tree numbers at CCESR since the burning regimen began in the 1960’s.
Walter D. Koenig and Johannes M. H. Knops. 2013. Environmental correlates of acorn production by four species of Minnesota oaks. Population Ecology 56 (1): 63-71. https://doi.org/10.1007/s10144-013-0408-z