Methods and Results
As part of our ongoing studies of robins and their anti-parasitic egg rejection behaviors (e.g. Abolins-Abols and Hauber 2020a, Hauber 2020), from 2018-2020 we located 1,170 robin nests in a commercial deciduous tree farm from April until July in Champaign County, Illinois, U.S.A, including 24 robin nests built directly on the ground. Notably, we only found ground nests early in the breeding season (initiated between 22 April and 12 May) while we found tree nests throughout the spring and summer (initiated between 8 April and 30 June). While most of the nests were built in trees and shrubs and the ground nests represented a minority (2%) of nests we found, it is noteworthy that ground nests were present in all three years and that multiple ground nests were active simultaneously, indicating that this ground-nesting behavior was not limited to a single individual female robin within and across years.
The ground nests were located near deciduous tree saplings or at the bottom of ~0.5 m-deep holes in the ground left by commercial tree-and-root removal at our commercially active site. The substrates around the nests included bare dirt and mud, thick dead plant litter, or growing grasses and forbs. The nests consisted of a depression in the ground and a low (<5 cm tall) circular rim of dried mud and plant material above the ground and were lined with fine plant materials, similar to arboreal robin nests (Figure 1).
The clutches in the ground nests contained 2–4 eggs (mean + st. dev.: 3.00 + 0.71), comparable to tree-nesting robin nests (3–4; Vanderhoff et al. 2020) and our local data (range: 1–5, mean +st. dev.: 2.78 + 1.02). Of the 18 ground nests with known outcome, at least eight (44%) survived until the eggs hatched but only two (11%) successfully produced fledglings compared to 25% of the tree nests that made it to hatch and 17% survived to produce fledglings. Tree and ground nests were statistically similar in in their likelihoods of depredation: the daily survival rates (calculated using RMark(Laake 2013)) were 0.891 (95% confidence interval (CI95%): 0.869–0.909) for tree nests in across the entire breeding season in 2019 (the year for which we have the most such data from experimentally unmanipulated tree nests) and 0.868 (CI95%: 0.795–0.917) for ground nest across all three years. Tree nests that were active at the same time as ground nests (initiated prior to 12 May, which was the latest initiation date for ground nests) were not more likely to be depredated than ground nests (daily survival rate for tree nests in the early season: 0.877, CI95%: 0.840–0.907), nor were they more likely to be depredated compared to tree nests initiated after 12 May (daily survival rate: 0.900, CI95%: 0.873–0.922).