By Robin Rivet.
Have you completed “spring cleaning” your trees? Best practice timing for mature tree pruning is vague, and our bird nesting season complicates timetables. March through August is prime nesting season for many songbirds, while mid-January through mid-September may host raptor clutches. Many of our herons and egrets nest outside these seasonal ranges—well into our winter season—as do hummingbirds and owls. Since birds migrate north and south through San Diego County, gardeners should be aware that any month may be bird-nesting season. Amazingly, we host more bird species than almost any county in the nation.
Where do birds build their nests? Typically, in or around our trees, and although some nest elsewhere, careless urban tree pruning should be avoided. Federal and state laws are clear about penalties for interrupting wild bird nesting, but San Diego’s long growing season makes compliance especially difficult. Migratory rules state that once a nest is discovered, all disturbances should cease, with some guidelines suggesting hundreds of feet to protect breeding species. But how would you know that birds are nesting? Could you rely on your tree pruning contractor to tell you?
Of course, the unspoken rule remains the same: Do not prune a mature tree at all, unless you have a purposeful motive. Eliminating primarily dead, diseased, damaged, or deformed limbs provides reasonable, limited justification to prune healthy branches. However, in some cases, an active nest could warrant postponement of the removal of a small dead tree (unless it poses a clear risk to human safety). If feasible, leaving a backyard snag provides cavity nesting, while routine palm frond removals usually exacerbate oriole losses.
Although spring is when the majority of birds arrive here to nest, some may wait until summer, or even fall, to breed. So, is winter the best season for tree pruning? What about those hummingbirds crafting their intricate nests amidst your shrubbery in December?
There are always caveats:
Pruning in summer’s heat during the growing season is not advised, as trying to replace foliage during hot weather can stress most trees. One exception is eucalyptus, which need summer’s warmth to withstand limb cutting—unless, of course, something is nesting in their canopy. Lake Murray has heron nests most of the year in towering eucalypts—with oblivious summer picnickers seated below.
Avoid fall pruning, because we also have numerous broadleaf, semi-evergreen, sub-tropical trees which originate in the southern hemisphere and lose their leaves at odd times. Do we wait until spring for those species to go dormant?
Don’t prune in winter, because rainfall may infect open wounds. Also, Anna's hummingbirds—one of our most common resident breeding populations—are typically nesting. Many observant gardeners have seen their bold, but tiny, nests, which are often conspicuously constructed at eye height in evergreen foliage. I sense they use us as decoys to fend off worse predators, although these personal observations are anecdotal.
When it comes to making the avian-friendly pruning choice, there is no single right answer—except urban habitat needs everyone’s tree vigilance, and this “robin” agrees.
Member Robin Rivet is an ISA Certified Arborist, a San Diego Master Gardener and