Exotic shrubs as ephemeral ecological traps for nesting birds

Certain exotic plants may increase risk of nest predation, and, in this way, may act as ecological traps. We hypothesized that the greater vulnerability to predation was a consequence of either (1) reduced nest height due to architectural differences among plant species or (2) seasonal changes in the distribution of nests among forest strata. To test this, we examined temporal variation in nest survival of 888 nests of Northern Cardinal (Cardinalis cardinalis) in native substrates and two exotic shrubs (Lonicera maackii and Rosa multiflora) in Ohio, USA, 2001-2006. We evaluated evidence for an ecological trap by monitoring the annual reproductive productivity of 245 breeding pairs of cardinals. Only nests in Rosa experienced relatively constant survival rates across the season, whereas probability of survival increased over the season for nests in other substrates. Interestingly, the relative vulnerability of nests in different substrates varied across the season. Most strikingly, nests in Lonicera in early spring showed the lowest survival rates but exceeded survival rates of nests in native substrates late in the season. Nest height failed to explain seasonal changes in nest survival, as only nests in native plants significantly increased in height as the season progressed. Rather, predation risk seemed to be a function of the proportion of nests within each substrate, as illustrated by the decreased predation in Lonicera as the relative proportion of nests in native substrates increased. The patterns of temporal variation in predation risk that we detected show that impacts of Lonicera are not a function of plant architecture alone and may be related to leaf phenology, changes in nest density, nest site location, and/or nest synchrony. Examination of the reproductive productivity of cardinals showed that pairs that made their first nest attempt in Lonicera fledged 20% fewer cardinal young than birds that began the season using other substrates. Thus, we suggest that exotic plants may represent an ephemeral ecological trap for certain nesting birds, where negative effects persist only during certain periods.