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Hummingbird Nesting

Black Chinned Hummingbird Nesting
By
Dan & Diane True

Twenty-six Black-chinned hummingbird nests were observed over a three year period by my wife, Diane, and I in Texas and New Mexico. Some nests were in trees. Nesting platforms known as Hummingbird Houses served as sites for other nests. Hummingbird Houses were installed under eaves, porch ceilings, and covered patios.


Migrating female hummingbirds follow males in spring into the United States and Canada from wintering grounds in Mexico by three to ten days. The birds come north for one purpose: to raise young, and they waste no time in getting down to business. Nest construction generally begins the day they arrive. Distended abdomens on some of the hens indicated those little birds arrived impregnated.

This little hen is so heavy with eggs we can't see her feet because they have been absorbed by her feathers.

Top priority in a female hummingbird's mind for nest site selection is: nest in a geographical area where temperatures are likely to remain 96 degrees F or less throughout her nesting cycle. Her reason is, she aims for an egg incubation temperature of 96 degrees. If the temperature rises above 96 and remains above that level for several hours, (or days) her eggs will "cook", killing the embryo. This probably explains why areas where summer temperatures soar into the 100s see their arriving hummers "disappear" in May. The little birds "disappeared" because they were really only passing through on their way to cooler climate locations in our northern states and Canada. Invariably the little rascals "reappear" in late July and early August, and pause to show us their youngsters before humming on south to wintering grounds in Mexico and Central America. A few Ruby-throats don't follow the crowd and remain behind in hotter states. These birds are often found in states with mountains high enough in altitude to experience a summer with less harsh temperatures...the Ozark's and Appalachians, for example. (The air cools at a rate of 5.5 degrees F per 1,000 feet rise in altitude.) However, a few Ruby-throats remain at low altitudes in states that can sizzle in the summer. These hardy souls have learned a unique way to escape egg cooking temperatures.


The temperature within a stand of broadleaf trees will average 5 to 7 degrees cooler than open or urban areas. This is due to the tree's transpiration, which provides natural, evaporative air conditioning within the tree's umbrella. (An average oak transpires 50 gallons of water per day.) A very small population of Ruby-throats have discovered this natural phenomena and attempt to nest at altitudes where summer temperatures almost always exceed 100. In those cases nests are often found on the low branch of a broadleaf tree overhanging a body of water. Female Ruby-throats have apparently found the thin layer of cool air created by evaporation from the water's surface. That thin layer of cool air, combined with the tree's 5 to 7 degrees worth of cooling, creates a micro environment matching northern climates. Note that hummingbirds migrate to nesting areas where humans go to escape summer's heat...'way north, the mountains, or a lake.


The rule for eastern states is, if you have females in June, it is likely the birds are nesting with you. No females in June says they are nesting elsewhere, and elsewhere is probably somewhere up north. (In Arizona and southern California, Anna's hummingbirds avoid egg cooking temperatures by choosing to nest in January and February. Costa's hummingbirds avoid the desert heat by humming up to nest in those state's mountains.) The female's second priority for nest location is: Find a place out of the wind.


The importance of selecting a nest site that is protected from the wind was emphasized from the experience of Jay and Carrie Hollifield of Roswell, New Mexico. Winds catapulted ten hummingbird eggs out of five nests from elm branches in their ranch yard in 1999. In Amistad, New Mexico, broken hummingbird eggs were often found on the ground by Dave Dunnigan after strong winds raked his ranch yard. 8 to 10 Black-chinned hummingbirds nest around Dunnigan's place each year. One of his little hens was so determined to nest out of the wind she built down low, 18 inches above the ground, on a bush snuggled in the shelter of a hen house. It is probable this bird was not a first year mom, but rather an experienced mom who had suffered the consequences of high winds in a previous nesting season. This suggests hummingbirds ae capable of learning. In May of 2000, five of Dunnigan's hummers chose to nest on Hummingbird Houses placed under his porch eaves, out of the wind. In Roswell, Hummingbird Houses are installed at the Hollifield place, however a dozen pairs of cliff swallows dominate choice nesting sites under their eaves. One of those Hummingbird Houses was even taken over by swallows.


Nests we observed were established in places providing as much wind protection as was available. They were sheltered either by an outer perimeter of trees, or by buildings. Six to twelve feet above the ground in the first row of inner branches where protection is increased from weather elements were prevalent tree locations. Trees of choice, in order of preference, were sycamore, fruitless mulberry, maple, elm, and Russian olive. Note the larger the leaf, the higher the preference.


A fork in a branch about 18 inches from its end was a repeating tree nest location. The chosen branch averaged 1/4" in diameter...too small to support a cat, but dangerously whippy in high winds. A hen's search image includes the coincidence of either a large leaf or a cluster of leaves three inches or less directly above the fork. She utilizes this leafy "umbrella" to protect her nest against sun and rain, and to shield her eggs and chicks from prying predator eyes.


Nest construction averaged five days. She brings materials to her site at a rate of 34 trips per hour. The little hen's first load of material is spider webbing. She applies that material as a sticky foundation on the forked area of her nest site. Thereafter, her sequence is orderly. She airlifts plant down or other soft material in her beak and tucks it into the fork. After shaping and molding that material, she flies in another load of spider webbing. Most often she carries a glob of webbing clinging to the underside of her beak, under her throat, and down across her breast. Transfer of the webbing onto her nest is achieved by pressing her chin and breast against the nest and wiping the webbing onto her work. Stickiness of spider webbing appeared to be the only element binding the nest. Frame by frame scrutiny of video tapes revealed no sign that she used her spittle as glue. In that regard, for her little system to produce enough spittle to construct her nest seems beyond a hummingbird's physical capacity.


Bits of camouflage followed the spider webbing and were applied to her work-in-progress. Another load of plant down was followed by spider webbing followed by a bit of camouflage, and so on. Four hours straight was usually her work schedule before she quit for the day. Some of the little hens worked mornings, others were afternoon types. Since developing eggs burdens her with extra weight throughout nest building, it made sense that she work on the nest no more than four hours per day.


Concealing her work from its beginning is probably a reason the female hummingbird camouflages her nest as she builds. One hen was so picky about hiding her work that on the sun bleached side of a branch she chose light colored camouflage material to match. On the shaded, and therefore darker side of that same branch, she camouflaged that side of the nest darker to match that side's coloration. Such attention to detail created a nest that was camouflaged slightly differently on each side. Hummingbird House nests were camouflaged against the color of the eave, ceiling, or patio cover where the House was installed. Sometimes the hens gathered flakes of paint chips from the building and applied the chips to their nest. A male bird was never seen near a hummingbird nest. So, where is Daddy Bird during the female's flurry of nest building activity?


Flashy gorgets transform Daddy Birds into Mr. Neon. To protect her children from predators, the female would be foolish to tolerate a male spotlighting her work during nest building, or during chick raising. In whatever ways hummingbirds communicate, after she has been impregnated, a probable reason we don't see hummingbird males near hummingbird nests is that she has told Daddy Bird to take his brightly colored flashy suit and hum off.


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.

 

  Hummingbird hen shaping her nest's edge between her wing and body.

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 96 degrees F, which is 5 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.

 

  Nest with thinner, almost "see through" upper wall on downwind side. Note side facing camera, the side facing the prevailing wind, is thicker.

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. 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. Those eggs failed to hatch. Sometimes a nest was only half finished before the hen laid her first egg.

 

 First egg in a half finished nest.

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.

 

 New chick with hatching nestmate.


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-chinns 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 apeared 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. This age, nine days, is the earliest observation of youngsters humming their wings.


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.

 

 Chick making first flight.

While continuing to work on their second nests, 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. (Three hens returned and were nesting under County Line's porch ceiling as of May 24, 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 additional 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 & Diane True
Authors of "Hummingbirds of North America".

May 24, 2001 update: Observations gained so far during this 2001 nesting season have caused us to wonder:
Picture this: A hen heavy with eggs is looking for a place to build a nest. At the same time, winds are blowing hard, gyrating tree branch nest sites so violently as to make nest building difficult to impossible. Desperation, plus the urgency of eggs about to arrive forces her to build on a porch light fixture, or other device such as our Hummingbird House, neither of which is gyrating because they are protected from the wind. Picture the same situation on a day with steady rain. The urgency of eggs about to arrive forces her to search for a nest site while it is raining. Trees are wet and dripping. Again, she finds a porch light fixture, or our Hummingbird House, both of which are dry during the rain. Being smart, she builds a nest there and successfully raises her family.
   
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