Lack of ground nesting birds likely due to high nest predation rates
Many forested areas that are part of the Mississippi River corridor in east-central Minnesota currently lack ground nesting passerines, although records document ground nesting passerines, particularly ovenbirds, in the same forests in the past. In order to determine if high predation rates may be responsible for the absence of these birds in some areas, we placed a quail egg and a clay egg in artificial ground nests in sites with and without ovenbirds in summer 2012 and 2013. The purpose of the clay egg was to reveal clues (e.g., bill, teeth, claw marks) as to the identity of egg predators. In addition, we also used several infrared trail cameras to identify nest predators. Nests were checked every other day for 14 days and two sets of artificial nests were created and tracked, the first set implemented during the first two weeks of June and the second set implemented during the last two weeks in June. Nest survival probability was calculated using the Mayfield Method. Vegetation and litter cover were also measure at each site and around each nest. Regression and ANCOVA were used to analyze the data.
We found significant differences in nest predation rates between sites with and without ovenbirds. From the time an ovenbird egg is laid, it takes between 19 and 24 days for the fledgling to leave the nest. In sites without ovenbirds, the probability of nest survival for this period of time was near zero, and was significantly lower than the nest survival rates in sites with ovenbirds. We found a negative correlation between vegetation cover and nest survival rates. However, in sites without ovenbirds, the increased protection provided by increased vegetation cover only delayed nest predation; it did not prevent it. The cameras and clay eggs revealed that raccoons are likely the primary nest predator at high predation sites although small mammals (mice) are likely predators as well. Raccoon populations have increased substantially in east-central Minnesota in recent decades, and are likely contributing to the decline of ground nesting forest passerines. The increased predation by raccoons may be just part of the story however. By reducing the depth of the litter, earthworms have significantly reduced the abundance of litter invertebrates, the primary food of ovenbirds. Also, ovenbirds face a new competitor, recently introduced wild turkeys, which also feed on litter invertebrates.