By SFBBO Director or Waterbird Science Dr. Nathan Van Schmidt
Why do Black-crowned Night Herons love city life? That was the question of one of SFBBO’s recent research efforts. Downtown areas in several cities of the Bay Area, including Oakland and Santa Rosa, have become unexpected homes for Black-crowned Night Herons. These striking birds once nested around Oakland’s Jack London Square and Lake Merritt parks, but tree and shrub trimming there in the past decades have pushed them to form nesting colonies in the trees of the Chinatown neighborhood.
However, this has led to ongoing challenges for both the community and the birds, with their droppings covering sidewalks, businesses, and parked cars, and young herons sometimes falling onto busy streets. Efforts to relocate them back to Lake Merritt have had limited success, raising the question: what makes these urban trees so appealing to the herons?
Last summer, SFBBO intern Bella Nunez-Garro sought to answer this by examining the herons’ nesting preferences, focusing on tree species, canopy cover, and surrounding environmental factors. We hypothesized they might be drawn in by particular tree characteristics, or potentially easier access to food from human refuse – more than a few herons have been observed going after discarded fast food! Bella found that the birds almost exclusively favored two non-native trees, the Chinese banyan and Brazilian peppertree.
Despite being non-native, the herons seem to love these trees for nesting because they offer wide canopies with thick leaf coverage, which may provide stable nesting spots that are safer from predators. There was also evidence the birds preferred shorter trees. The presence of trash near nesting sites was linked to higher nest occupancy, confirming our suspicion that waste food may be providing an unintended food source, but this wasn’t nearly as important as tree species. These insights have major implications for urban planning and wildlife management.
By better understanding what draws these birds to urban spaces, the city has a chance to create a future where both people and herons can coexist more harmoniously. Unfortunately, several of the nesting colony trees were removed this winter due to a BART station redevelopment project. Tree removal rarely has been observed to help this issue, as the birds simply nest in other nearby trees – after all, the removal of their nesting colonies is what made them move downtown in the first place! If Oakland wants to reduce conflicts while protecting the herons, our results indicate a strategic approach to tree planting and maintenance could help. Planting trees with heron-friendly canopies near Lake Merritt could encourage them to nest in more suitable areas, while choosing different tree species for Chinatown might make those locations less attractive for nesting.
Despite being non-native, the herons seem to love these trees for nesting because they offer wide canopies with thick leaf coverage, which may provide stable nesting spots that are safer from predators. There was also evidence the birds preferred shorter trees. The presence of trash near nesting sites was linked to higher nest occupancy, confirming our suspicion that waste food may be providing an unintended food source, but this wasn’t nearly as important as tree species. These insights have major implications for urban planning and wildlife management.
By better understanding what draws these birds to urban spaces, the city has a chance to create a future where both people and herons can coexist more harmoniously. Unfortunately, several of the nesting colony trees were removed this winter due to a BART station redevelopment project. Tree removal rarely has been observed to help this issue, as the birds simply nest in other nearby trees – after all, the removal of their nesting colonies is what made them move downtown in the first place! If Oakland wants to reduce conflicts while protecting the herons, our results indicate a strategic approach to tree planting and maintenance could help. Planting trees with heron-friendly canopies near Lake Merritt could encourage them to nest in more suitable areas, while choosing different tree species for Chinatown might make those locations less attractive for nesting.
SFBBO partnered with other organizations wanting to help these birds – Oakland Zoo, International Bird Rescue, Golden Gate Audubon, and California Fish & Game – and brought these concerns to the City of Oakland’s arborists to help devise a plan to encourage these birds to move into a better home. At that meeting (meeting participants shown above) we devised a plan to plant the Chinese banyan and Brazilian peppertree tree species herons love in a neglected nearby strip of green space by the slough. The planned planting site (photo below) is just three blocks from current colony sites and adjacent to water, both of which we hope will make it attractive to the birds – especially if we plant trees they’re already familiar with nesting in! It is also directly between Laney and Peralta Community Colleges, providing a potentially great spot to engage students in wildlife classroom activities.
The tree planting will begin in earnest next fall, during the optimal season for planting. In the meantime, SFBBO biologists will continue this spring and summer to refine our analysis with a second season of data collection. We are particularly curious to see if the birds that had nested in the BART station’s Brazilian peppertrees that were removed will renest in adjacent trees, or find the same species elsewhere in the city. Our hard-working community volunteers are already out tracking the new nests across Oakland as the birds begin their breeding season!
The tree planting will begin in earnest next fall, during the optimal season for planting. In the meantime, SFBBO biologists will continue this spring and summer to refine our analysis with a second season of data collection. We are particularly curious to see if the birds that had nested in the BART station’s Brazilian peppertrees that were removed will renest in adjacent trees, or find the same species elsewhere in the city. Our hard-working community volunteers are already out tracking the new nests across Oakland as the birds begin their breeding season!
