SHORT HILLS, NJ — Male Wood Thrushes were heard singing in the Cora Hartshorn Arboretum (CHA) forest in late May and into June.  This was unusual because in the past six years the male thrushes at the Arboretum had always stopped singing by mid-May.  Typically, the first Wood Thrushes, along with their thrush cousins, Swainson’s Thrush and Veery, would arrive from their spring migration during the first week of May, announcing their arrivals with their songs.  But in all previous years the Arboretum’s woods were thrush free by mid-May.

Cora Hartshorn Arboretum Forest Background:

For almost 20 years the Arboretum has invested considerable effort to repair the CHA forest’s understory to increase the likelihood of neo-tropical migrant nesters like the Wood Thrush.  One of the primary culprits of damage done to the Arboretum’s forest were White-tailed Deer.  Ironically, the deer’s presence was the result of a conservation initiative that had started in the early 1900’s to maintain the deer populations that had dropped to about 250,000 east of the Mississippi River.  By 2005, estimates put the deer population in the United States at around 30 million.  Conservation practices have proved so successful that, in many parts of their range, deer populations far exceed their carrying capacity and the animals are considered, at best, a pest.  High densities of deer have had severe negative impacts on native plant and animal populations in parks and natural areas.

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One of the places that showed the negative impact of the deer was the CHA woodlands, bequeathed to the Township of Millburn by Cora Hartshorn in 1958 with the provision that the property be maintained as a wildlife preserve for the use and enjoyment of the public and wildlife.   The 16 plus acres of the Arboretum had a herd of deer that ranged from 20 to 40 individuals.  Forests can sustain themselves from deer browsing as long as the deer population remains at about 1 deer per 30 acres.  The CHA property could only sustain about ½ a deer.  The deer herd completely stripped away most of the Arboretum’s herbaceous and understory layers, leaving much of the forest with only adult trees and leaf litter or exposed soil.  This left the Arboretum vulnerable to invasive plants that the deer did not prefer to eat and that had few native species to compete with.  By 2007, the Arboretum’s Board of Trustees and Millburn Township recognized that immediate action was needed to protect the remains of the Arboretum’s woodlands and a decision was made to enclose almost the entirety of the Arboretum’s woodlands with a deer fence in 2008. This decision was bolstered by an experimental fencing of approximately 2 acres in 2002 that had produced a promising regeneration of native plant species within the enclosed area.

Now, as part of the Cora Hartshorn Arboretum mission, we are restoring the woodlands so its structure can sustain a higher level of biological diversity.  This effort is focused on the restoration of the herbaceous layer, the understory, and the development of a cohort of sapling trees which will eventually replace the current crop of mature trees as they age. The planting and cultivation of native plants combined with the continual detection, eradication, and/or management of invasive plant species formed the backbone of the Arboretum’s 2007 40-year Forestry Management Plan.

Wood Thrush Natural History:

The Wood Thrush is one of the quintessential neo-tropical nesting birds in the forests of New Jersey and most east coast forests, from Florida to Maine and west to Minnesota, Iowa, and Louisiana.  Their summer diet is predominantly invertebrates, including adult and larval insects, spiders, and worms.  Fruits like spicebush, holly, elderberry, jack-in-the-pulpit, Virginia creeper, pokeweed, and dogwood make up the rest of their diet.  Parents feed chicks soft invertebrates and pre-softened fruits.  In late summer and fall, after breeding season, Wood Thrushes shift their diet toward fruits (particularly fatty fruits) in preparation for the demands of migration.

The Wood Thrush is usually one of the first song birds to be heard in the woods in the morning and among the last in the evening.  To establish a territory, the male sings his evocative song from an exposed perch in the understory or lower canopy.  Within a few days a female hopefully initiates pairing by enticing him to chase her in circular flights.  After pairing, the female helps defend the territory from intruders as she begins nest building.

The nest is usually in the lower branches of a sapling or shrub, where a fork provides good support and foliage provide shade and cover. The male may draw attention to a spot by calling or by placing nest materials nearby, but the final decision is hers.  The female builds the nest by laying down a platform of grass, leaves, stems, and sometimes paper or plastic.  She weaves walls 2–6 inches high, ending up with a cup that’s 4–6 inches across, and uses the weight of her body to mold a 3-inch inner cup.  She lines the cup with mud and adds a covering of rootlets to bed the eggs.  The process takes 3–6 days.  An average clutch is 3-4 eggs, with the female doing all the incubation.  Incubation lasts 12-15 days.  Once the eggs have hatched both parents will feed the chicks until they fledge after another 12-15 days.  If the pair have a successful first brood they will often try for a second brood in a new nest. 

The Wood Thrush is one of the most prominent examples of declining forest songbirds in North America.  Though still common throughout deciduous forests of eastern North America, the North American Breeding Bird Survey has noted that from 1966 to 2015 Wood Thrush populations have declined by 62% percent.  The Wood Thrush was placed on the 2016 State of North America's Birds' Watch List, which includes bird species at most risk of extinction without significant conservation actions to reverse declines and reduce threats.  Some of the steepest declines have been in Atlantic Coast and New England states.  The primary reasons for the declines include habitat destruction, habitat fragmentation, and habitat degradation.  In the late 1990’s avian ecologists considered the CHA forest a badly degraded forest fragment, at least in the eyes of a Wood Thrush.

Present:

The Wood Thrush is an invaluable indicator of successful woodlands management; its very presence signifies a healthy forest.  When choosing a nest site, thrushes are looking for enough understory leaf cover to help hide nests and enough plants in the herbaceous and understory layers to support the robust invertebrate and berry/fruit populations that provide the protein demands of a growing bird family.  Growing chicks need to be fed their body weight every day for most of their development. 

Male thrushes singing past May 15 meant that the males were finding females who were considering CHA as a potential site for nesting.  On May 27 we found the first nest with a brooding female.  We think there was a second nest with a female who is even better at hiding her nest.  This was a very exciting moment.  If the pairs can hatch eggs and raise fledglings there is a high probability that they will return for more nesting to raise future clutches.  Furthermore, the offspring will also likely return to the area for their own nesting opportunities should they survive to breeding age.  

On June 11 we heard peeping from the nest and on June 13 we had visual confirmation of 4 chicks.  Both parents were industriously feeding the chicks, although the female was definitely putting in more effort.  The male would arrive with a mouthful of food, smash it down into the closest mouth or two, and then fly off.  When the female arrived, she would carefully make sure that all four chicks got an equal share.  She would also preen the chicks and clean and make repairs to the nest before setting off again for more food.  By June 22 all four chicks had successfully fledged and all that remained was an empty nest and a few Wood Thrush calls coming from within the forest.

CHA’s successfully nesting Wood Thrushes is the result of countless hours of dedicated work by CHA staff; Millburn’s Shade Tree department; and most importantly: volunteer board members, regular volunteers, teens, scouts, corporate volunteers, students, supporters, other environmental organizations, special needs groups, or just someone needing to spend a few hours doing something to help the environment.  This was truly a community effort and the Cora Hartshorn Arboretum would like to thank everyone who contributed to this wonderful occasion. 

Resources:

Cornell Lab of Ornithology - https://www.allaboutbirds.org/guide/

National Audubon Society - https://www.audubon.org/field-guide/

Postscript:

August 25, 2020

During the first week of July the pair made a new nest and started to raise a second brood.  By the end of July three more fledglings (bringing the known total of youngsters to seven) left the nest and started feeding within the CHA forest to prepare for their fall migration.