by Dan and Diane True BLACK CHINNED HUMMINGBIRD NESTING (PART II)
(If you missed Part I of this series, just Click Here).
One of Dunnigan’s little hens was so determined to nest out of the wind that she built down low, 18” above the ground, and snuggled up in the shelter of a small hen house. It is probable this bird was not a first year mom, but rather a mom who had suffered the consequences of high winds in a previous nesting season. This suggests hummingbirds are capable of learning.
Molding the nest’s wall as the nest progresses upward was done by pressing the top edge between her wing and body, as a potter shapes soft clay on a spinning vase.
Rounding the nest’s inside was done by ramming her little bottom, with tail feathers straight up, against inner walls. She tamps the nest’s floor by hanging on with one foot and stomping rapidly with her free foot. Since her weight is about that of a penny, hanging on with one foot and stomping with the other to pack the nest’s floor allows her leg muscle power to compensate for her light weight. She stops work occasionally and simply sits in her nest, as if resting. The little hens are so focused they ignore photo equipment moved in increments to as near to them as five feet. One photo revealed a unique pattern in the structure of hummingbird nests, a pattern that was previously unknown.
Backlighting a nest revealed that the lower half was thick and dense while the upper half was thin enough to let some light pass. She probably incorporates this feature so that she can adjust air circulation to maintain an egg incubation temperature of 97 degrees F, which is 4 degrees less than her normal body temperature. On cold days, she maintains egg temperature by positioning her body below the thinner, upper half of the nests walls hold warmth inside her nest. On hot days, raising her body above the thinner portion of the nest’s wall would increase air circulation and allow excess egg incubation heat to escape. These smart little birds refine this construction feature even more.
For additional precision of egg temperature control, the windward side of the upper wall is thicker than its lee side. The “thickest” side of the upper wall invariably faces into prevailing wind patterns. That suggests she fashions the windward side of her nest to give her eggs protection against the probing fingers of a cool wind. Further, their first nests, those constructed in the relative cool of early spring, have thicker and therefore warmer upper walls than second nests built in summer. Another refinement in hummingbird nest building is that their spring nests are deeper than their summer nests. On cool days the hens snuggled down so deep inside their nests their beaks and tail aimed straight up. On warmer days they sat so high in the nest and were fully visible. An air temperature that remains above 97 degrees for several hours will cook her eggs and destroy the developing embryo. The eggs in one nest were “cooked” during a record heat wave that spawned eight straight days of high temperatures ranging between 100 and 103 degrees F. That phenomenon may explain why the majority of hummingbirds “disappear” from feeders in hotter climes during May and June. Apparently they had only paused at those feeders before moving on northward to nest in geographical areas where nesting season temperatures rarely rise above 100 degrees, rendering the area less of a threat to nesting success. (A tiny fraction of hummers do nest in geographical areas where nesting season temperatures commonly rise above 100 degrees.) Sometimes a nest was only half finished before the hen laid her first egg. Without exception, the hens skipped one day before laying their second egg. In proportion to body weight, hummingbird eggs are the largest in the bird world. If human babies were proportional to hummer eggs, we would give birth to 25 pound babies. A long handled mechanics mirror was used to check a nest when a hen flew off to feed. Activity was watched from a distance through binoculars and a telescope. Clues that a hen was “in labor” came when she settled on her nest and alternated between wiggling and shaking a few moments. In one case we knew within ten minutes when a hen’s first egg arrived.
Incubation time on each nest was 14 days with one exception...a 12 day period in Texas. (That hen was smaller, and her behavior different from other Black-chins we observed. The hen may have been a member of the smaller sub-species of Black-chinned Dr. Bill Baltosser believes exists.) When the nest held eggs, it appeared the hen flew into and out of the nest in a way that reduced the chance of downwash from her wings blasting an egg out of her nest. In the split second during either launch or landing, she seems to tilt her wings in a way that would direct her wing downwash away from the nest’s opening. In one nest it appeared that downwash from her hovering flight ejected one egg, which crashed on the ground. The hen abandoned that nest and its remaining egg.
Chick feeding intervals averaged twenty minutes. Without exception the moms brooded their chicks through eight nights. The ninth they spent somewhere other than on the nest. Chicks at that age were feathered enough to regulate their own temperatures. The two chicks were large enough by then that their little bodies stretched the nest and filled it side to side, with their backs almost flush with the nest’s rim.
When the chicks were 21 to 22 days old, the mother hummers began construction on a second nest while still feeding her first two nestlings. Fledge time for the chicks was commonly 23 days, however some where 24 or 25 days old before they left the nest. Individual chick fledge time is probably tied to its level of nourishment. First flights were usually no farther than the nearest branch.
While continuing to work on her second nest, the busy mom hummers located and fed the newly flying chicks through two to three days before they were on their own. While building her second nest the first egg often appeared during the time she was feeding her two fledged chicks. The common view held by most ornithologists about hummingbirds reusing an old nest is that they don’t. We found a different answer.
One tree nest was reused by a different mom hummer almost before it had time to cool from previous use. Two Hummingbird House nesting sites were reused by different moms within a day or two after the first chicks fledged. Those three nests were home to a total of 12 hummingbird chicks during one nesting season. We think these nests were reused because they were still intact and in good condition. Few hummingbird nests survive the winter months, and those that do are so dilapidated they are not reusable. However, we found several cases where a new nest was built on top of an old nest. One site had a stack of four nests, probably covering four years. One of the little hens nested a third time. Her third set of chicks were only a month old before they pointed their little beaks south and hummed with her and their siblings toward wintering grounds in Mexico.
Some nests were exquisite, woven by hens that worked with great skill. Others were less than perfect. Differences in building skills likely resulted from first time nesters being less adept at nest building than experienced moms. Many times one female would hover a short distance from another female that was busy with nest building. The busy female invariably ran the intruder away. However, when the busy female left to gather more building material, the intruder would zip in and either steal nesting material, or hover all around the nest as if inspecting the work, possibly to gain building skills of her own. It also seemed as though an intruding female was considering a take over. In one instance, during her first day of nest building one female was building six nests simultaneously. Each was about 20 feet apart and all were on Hummingbird House platforms. During the second day she narrowed her building activity to three of the six. On the third day she cut her work down to two. On the fourth day she abandoned work on one and finished the other during her fifth day. In two other instances one female worked on two nests simultaneously before abandoning one and finishing the other.
A roadrunner raided one New Mexico tree nest, ants killed two day old chicks in another, and a Texas hailstorm destroyed another. The Hummingbird House nesting sites had no weather or predator problems.
Two nests became unattended when the mothers apparently met with unknown fates. One mom had collected fiberglass from somewhere and used the glass for lining her nest. We suspect she may have died from an overdose of fiberglass. In our determination to not disturb the nesting process, we waited two days after her disappearance to intervene. Her chicks were dead. After one full day of the other mom’s absence, we placed one of her starving orphans in an active Black-chinned nest built on a Hummingbird House, and the other orphan in an active Magnificent nest in a tree. What must have been two surprised hummingbird mothers, both accepted and fed the third chicks. The second day after these transfers, the Magnificent nest was empty and that mother not seen again. It is probable the nest was raided by a predator. Meanwhile, back at the Hummingbird House Black-chinned nest, three growing chicks soon created a space problem. The two largest chicks crowded the smaller chick outside the nest, where it hung by a tiny toenail hooked to spider webbing fourteen feet above the ground. The problem was solved by installing the orphaned chick’s “old” birth nest side by side with its new nest and putting the toe-nail-hanging chick alone back in its original nest. The mother continued to feed all three chicks and they all fledged.
Although we learned lessons from tree nests, we learned more from the Hummingbird House nests because they provided an ambiance that allowed intimate observations from the nest’s beginning to its end .
To see a hummingbird nest in action, check under the porch ceiling of County Line Barbeque in Albuquerque, New Mexico, between May 1 and August 30. Two moms nested there on Hummingbird Houses in the summer of 2000. Since hummers tend to return and breed in the area where they were raised, if both little hens survive the winter, they should return to the restaurant, along with surviving daughters in 2001. It is probable that between 3 and 6 nests will be built and occupied there in the summer of 2001. The total number of nesting hummers from year to year in the restaurant’s developing hummingbird colony will depend on how many moms and their daughters survive the winters. Extra Houses are in place for new moms. In 2002, there could be 6 to 10 hummingbirds raising chicks in nests under County Line’s porch.
Hummingbird banders have established the average life-span of female hummers to be 3 1/2 years. Average male life-span is 2 1/2. Black-chinned record longevity is 7 years. The longest Black-chinned migration on record is that of a male banded at Sonita, Arizona, in July of 1988. In April of 1991, this little guy was recovered a few miles NNW of Manzanillo, Mexico, 930 miles south of Sonita. This was the first documented hummingbird flight linking the US and Mexico. Here’s to many more.
Happy Humming.
Dan True is the author of Hummingbirds of North America, published by the University of New Mexico Press. He is an aviator and former weatherman for the Albuquerque and Amarillo, Texas markets. He and his wife, Diane, write extensively about hummingbirds. Visit their web site to learn more.
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