Discussion
A subset of the robins at our tree-dominated study site exhibit
nest-placement flexibility, building their nests on the ground rather
than in shrubs or on trees in the early half of the breeding season. The
decision to use a ground nesting site could be the result of competition
for suitable tree sites. Limited sites are a known driver of nest
placement and a constraint on the number of breeding individuals,
although this is a pattern most often seen in cavity-nesting birds
rather than open-cup nesting birds (Newton 1994). Contrary to this
hypothesis, most of the robin ground nests we observed were located
directly under or within 5 m of seemingly suitable unoccupied trees,
suggesting that competition for nesting trees was not the primary driver
of the robins’ decision to build their nest on the ground at this site.
Because predation is the primary cause of nest failure in songbirds
(Martin 1995), avoiding predators can also drive nest-placement
decisions. While nesting on the ground may reduce the likelihood of
depredation by avian predators and arboreal mammals, nesting in trees
may make nests less accessible to mostly terrestrial predators like
raccoons, rat snakes, and deer observed at nearby nest monitoring sites
(Chiavacci et al. 2018), and nests placed higher are often safer overall
(Burhans et al. 2002). Here, we statistically overlapping daily survival
rates between ground- and tree-nests at the same study site, suggesting
that flexibility in nest placement does not impact predation-driven nest
failure overall.
While ground-nesting robins do not suffer increased predation risk
overall, it is possible that the relative predation risk for ground- and
tree-nesting birds varies across the season. Many robins at our site
initiate nesting before the planted trees have leafed out, making the
bulky robin nest structures built in barren trees clearly visible to
human observers (Figure 1 inset), which might cause some robin females
to show a seasonal preference for building nests on the ground. A
seasonal leaf-limitation hypothesis predicts that ground-nesting should
be beneficial relative to tree-nesting in the beginning of the breeding
season when the leaves are not present. Accordingly, all of our ground
nests had been initiated in April and early May, when tree leaves are
absent or still growing; we found no ground nests initiated after 12 May
despite our nest searching extending until July 4 each year. In
contrast, we regularly located many arboreal nests initiated from April
through early July (Figure 2). However, the ground nests in this early
portion of the season were not more likely to survive than the exposed
tree nests initiated during the same time period.
An alternative hypothesis is that nest placement may be an important
determinant of the immediate weather and microclimate conditions
experienced by the incubating/brooding parents and the developing
offspring (Ardia et al. 2006). Cup nests in trees in the open air
experience a different microclimate than nests partially in the ground.
The early half of the breeding season, when we found all of our ground
nests, is considerably colder (average for Champaign County in April and
May: low of 5oC, high of 23oC) than
the second half of the breeding season (average for June and July: low
of 17oC, high of 29oC, State
Climatologist Office for Illinois). It is possible that nests in
depressions on the ground retain heat better than nests in trees,
benefiting ground-nesters over tree-nesters when the temperatures are
cold, a prediction that could be readily tested with temperature
monitors in nests throughout the season. These conditions may present a
trade-off for the birds between choosing sites with a more suitable
microclimate or lower predation risk, as is the case in some desert
species (Tieleman et al. 2008), although the equivalent survival rates
between tree and ground nests in our study site argue against this
trade-off.
While it is not immediately clear why some of the otherwise
overwhelmingly arboreal robins in our system are nesting on the ground
within a tree farm, such behavioral plasticity in life history traits
could be consequential for the robins’ ability to adjust to changing
environments, including ongoing anthropogenic and climate change
(Mainwaring et al. 2017). For example, dark-eyed juncos (Junco
hyemalis ) colonizing novel, urban habitats have adjusted both their
nest placement and nest reuse patterns, producing more successful
offspring compared to forest dwelling populations (Yeh et al. 2007).
Although our study sites are inside a strictly agricultural matrix, they
are also within 10 km of dense urban robin population in
Urbana-Champaign, suggesting that more flexibly-nesting urban robins may
be the source of variation in nest site choice in our study. Future work
should elucidate not only the environmental correlates of the
ground-nesting behavior but also the fitness consequences of nest
placement in this still common species. Indeed, ground nesting at our
site may not be an adaptive response to local environmental conditions,
but, for example, due to immigration of robins with preference for
ground nesting due to previous experience, due to the breeding adult age
or inexperience, and/or vertical transmission of nest-site preference
from ground-nesting parents (Slagsvold et al. 2013).
Data availability: All data will be made publicly available
(currently archived for confidential review during peer-critique at
https://figshare.com/s/6c997b0afc2cbd9bca41 and
https://figshare.com/s/b2360ecb5302c0fff734)
Acknowledgements: We are grateful for the permission of local
landowners to work on their property. For funding we thank the Illinois
Distinguished Fellowship program (to SKW), the BRIDGE program of the
University of Illinois at Urbana-Champaign and the University of
Birmingham (to MEH), and the Harley Jones Van Cleave Professorship, the
University of Illinois’ Center for Advanced Study, and the
Wissenschaftskolleg zu Berlin, Germany (to MEH). For assistance and
discussions, we thank A. Luro, J. Enos, C. Goethe, C. Kim, J. Reynolds,
and O. Rhodes.