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1-16 April 2017

Installment #653---Visitor #AmazingCounters.com

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BIRD EGGS (Part 1):

No one is quite sure when rabbits and eggs became associated with the Easter season, but it likely has something to do with being re-born in spring. That, in itself, seems a little odd because rabbits are viviparous mammals, birthing their young alive rather than laying eggs. Despite mammalian viviparity lots of other vertebrates do lay eggs, of course, and have external development of their young; these include fish and amphibians that deposit gelatinous eggs in water, reptiles that lay leathery eggs on land, and birds that frequently place hard-shelled eggs into nests in diverse terrestrial habitats. Satisfying and entertaining as the rabbit/egg myth may be, egg production in birds is in reality a fascinating process--as we were reminded this week at Hilton Pond Center.

All text, maps, charts & photos © Hilton Pond Center

During warm weather months at the Center we deploy mist nets to capture wild birds for banding; especially in winter we also operate traps baited with cracked corn, millet, sunflower seeds, and other bird attractants. Because neither nets nor traps are selective we never know what we might capture. This week, for example, we netted a passel of (i.e., 18) migrant White-throated Sparrows while trapping a slew of (224) American Goldfinches and a skosh of (six) Brown-headed Cowbirds (female above). It was one of the cowbirds that got us to thinking again about bird eggs and how they are formed.

All text, maps, charts & photos © Hilton Pond Center

Cowbirds are like other birds in that a male provides sperm when he copulates with a female; shortly thereafter fertilization occurs internally in the female's oviduct and eventually she lays the fertile egg in her nest. Cowbirds, however, are different from most of their avian relatives because they are brood parasites. Rather than building a nest herself the female cowbird places her fertilized egg in another bird's domicile, leaving the chosen set of foster parents to raise her young. (A single Red-eyed Vireo egg in a hanging nest, above, is somewhat dwarfed by TWO Brown-headed Cowbird eggs, probably from different cowbirds.)

Cowbirds have maximized this adaptation, with some reports stating an individual female lays up to 12 eggs to start. After taking a short break she can then recharge her reproductive tract and lay 12 more AND do this yet again for a phenomenal total of 36 eggs. That's 36 commandeered foster nests in just one breeding season. Even if only a documented 3% of these eggs result in adults, it's not surprising Brown-headed Cowbirds have had significant negative impact on breeding success of many species they parasitize--especially since host-family chicks often die when they are out-competed by more aggressive faux-sibling cowbirds.

All text, maps, charts & photos © Hilton Pond Center

Most songbirds are much less prolific than cowbirds in the first place. Through the years, a typical Eastern Bluebird female at Hilton Pond Center has laid 5 eggs (rarely six, above) before starting to incubate; if their first clutch is successful an established bluebird pair may try again with 3-4 eggs--or even a third time with 2-3. Our Carolina Chickadees typically have 6-7 eggs (sometimes eight) in a single clutch and do not double-brood--possibly worn out from feeding all those ravenous chick-a-dee-dees simultaneously. Local Wood Duck hens lay really big clutches of 16-20. We should mention passerine birds (and some other groups) are "determinate" layers producing a more or less specific number of eggs in a given clutch. (Ruby-throated Hummingbirds and Mourning Doves, for example, almost always make just two eggs at a time.) Alternatively, birds such as chickens--and, to an extent, Wood Ducks--are "indeterminate" layers and will continue to produce indefinitely if eggs are removed from a nest.

And one other note: To our knowledge all songbirds are "altricial," meaning their chicks are born blind, essentially naked (unable to thermoregulate), and totally dependent on one or both parents for warmth, protection, and food. Many non-passerines--including waterfowl--have "precocial" young that hatch covered by down and ready to run or swim. Fledgling Wood Ducklings (above left) don't require much care from their mother after she calls them from the nest and leads them to water; they get no attention from the drake.

All text, maps, charts & photos © Hilton Pond Center
Female bird reproductive tract diagram (above) courtesy W.H. Freeman and Co.

All the above is interesting, but if the bunny doesn't bring bird eggs just where DO they really come from? In explanation we refer you to the drawing above, representative of a female bird's reproductive tract. But first, a caveat: Many female birds can produce eggs without a male; if a female's hormones reach the right level, she may begin laying without stimulation from a partner. Those eggs would be infertile, of course, as happens in the hen house when no rooster is present.

Assuming there IS a male bird, he and the female copulate, albeit briefly. When the male mounts the female she turns her tail to the side and twists her cloaca upward (House Sparrows at right), making contact with his cloaca from which sperm are ejaculated. (The cloaca is a single opening in birds through which urine, feces, and sex cells are expelled.) Most male birds--waterfowl and ostrich-like species excepted--do not have an intromittent organ (penis), so this brief but oft-repeated copulation encounter is what we poetically refer to as a "cloacal kiss."

Newly liberated sperm swim all the way up the oviduct to the ovary, where ripening ova (unfertilized eggs; singular is "ovum") receive yellow, fat- and protein-rich yolk material. Actual ovulation and subsequent fertilization occur in the infundibulum, a funnel-like structure that catches the fertile egg and directs it into the oviduct itself. The egg then passes through the magnum, where first layers of albumen (liquid egg white) form around the yolk. Albumen is about 90% water--which helps prevent desiccation--the remainder being essential vitamins, minerals, glucose, fats, and protein.

Further down the oviduct a structure called the isthmus produces more albumen, plus an inner membrane and the outer shell membrane. The last stop is the uterus, where still more albumen is made and a shell gland lays down calcium that may or may not be pigmented. (It's interesting pigments for various birds are sometimes placed in species-specific patterns, as at left.) At first rubbery, the shell hardens into a specific shape and eventually passes through the bird's vagina to be squeezed out through the cloaca--no easy task because the hardened egg is completely inflexible. In most birds this whole egg-making process only takes about 24 hours, and there is usually a fertilized egg starting down the "chute" as the previous one is being laid. The shell typically hardens overnight when the gravid female is inactive, with the egg being deposited very early in the morning.

Actual development of a bird embryo does not begin until heat is applied, i.e., when incubation starts. Songbirds don't typically incubate until the full clutch is laid, guaranteeing a more or less synchronous hatch and even-aged nestlings. Other species--some raptors, for example--begin incubating when the first egg is laid, meaning a second hatchling would be younger than its sibling. (In a raptor that lays an egg and skips a day before laying another, a five-day-younger third chick might eventually be known as "lunch.")

And one final note of interest: Careful review of the drawing above shows a "rudimentary" right oviduct. That's because in most female birds only the LEFT side of the bilateral reproductive system is developed and active. This is an obvious adaptation for flight; no sense having both ovaries functioning and adding weight when one will do just fine. (Amazingly, there are accounts of a bird's vestigial right oviduct springing into action after the left one had been damaged!)

All text, maps, charts & photos © Hilton Pond Center

We've always been fascinated by bird eggs and how they are produced, but what--you might ask--does all this have to do with the Brown-headed Cowbird we caught this week at Hilton Pond Center? Well, here's what actually happened. One morning we spotted this female cowbird in one of our automatic traps and went out to remove her for banding. As we reached in to gently grasp the bird, her excitement must have gotten the best of her because she laid an egg in the trap (above). This egg apparently was just beginning to develop its calcified shell and was still quite flexible, sort of like a water balloon. It likely would have taken several more hours before the shell was completely hardened, but for the moment we could see a yolk though the outer membrane and albumen. It was a ghostly, almost mystical egg, and somewhere inside was an ill-fated embryo waiting for foster parents that wouldn't have to raise this baby cowbird.

All text, maps, charts & photos © Hilton Pond Center

Don't forget to scroll down for Nature Notes & Photos,
plus lists of all birds banded or recaptured during the period.

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"This Week at Hilton Pond" is written and photographed by Bill Hilton Jr., executive director of Hilton Pond Center for Piedmont Natural History

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1-16 April 2017

Ruby-throated Hummingbird--
Ruby-crowned Kinglet--
Yellow-throated Warbler--1
American Goldfinch--224
Chipping Sparrow--39
Field Sparrow--3
Pine Warbler--1
Yellow-rumped Warbler--9
Eastern Phoebe--1
Northern Cardinal--5
Purple Finch--17
White-throated Sparrow--18
Eastern Tufted Titmouse--1
Brown-headed Cowbird--6

Carolina Wren--1
Eastern Towhee--1

Mourning Dove--3

* = new banded species for 2017

17 species
334 individuals

27 species (35-yr. avg. = 64.3)

1,030 individuals
(35-yr. avg. =
Ruby-throated Hummingbird = 3

(Banding began 28 June 1982; since then 171 species have been observed on or over the property.)
126 species banded
65,978 individuals banded

Ruby-throated Hummingbird = 5,676

(with original banding date, sex, and current age):
Ruby-throated Hummingbird (1)
08/06/16--2nd year male

Chipping Sparrow (3)
02/08/14--after 4th year male
02/15/14--after 4th year male
03/28/15--4th year male

American Goldfinch (9)
02/04/14--5th year female
02/15/14--5th year female
03/19/14--5th year male
02/08/15--4th year female
03/26/15--4th year male

01/30/16--after 3rd year female
02/02/16--3rd year male
04/18/16--after 3rd year male
04/27/16--3rd year male

Carolina Chickadee (5)
05/06/13--after 5th year female
10/06/15--3rd year male
05/29/16--2nd year male
06/07/16--2nd year female
08/20/16--2nd year unknown

Field Sparrow (1)
12/13/16--2nd year unknown

Pine Warbler (1)
03/03/15--4th year female

Northern Cardinal (3)
03/20/14--after 4th year female
06/09/15--3rd year male
09/09/15--3rd year female

Purple Finch (1)
01/26/15--4th year male

Eastern Tufted Titmouse (4)
04/26/13--after 5th year male
09/15/14--4th year male

05/24/16--2nd year male
10/18/16--2nd year male e

Downy Woodpecker (5)
04/23/15--4th year female
04/24/15--after 3rd year male
08/21/16--2nd year male
10/31/16--2nd year male
10/31/16--2nd year female

Hermit Thrush (2)
12/31/13--5th year unknown
10/29/15--3rd year unknown

White-throated Sparrow (5)
12/20/14--after 3rd year unknown
02/08/16--3rd year unknown
02/13/16--3rd year unknown
03/19/16--after 2nd year unknown
03/25/16--3rd year unknown

Carolina Wren (1)
09/19/15--3rd year male

--American Goldfinches made their third incursion of the winter at Hilton Pond Center with 224 of them banded during the first half of April. (See complete list of 334 bandings above left.) We also recaptured nine AMGO banded locally as long ago as 2014 (see substantial list of returns and recaptures, below left). Curiously, we banded 16 White-throated Sparrows on just one day (16 Apr)--the first time we've ever had an apparent "migrational wave" for this species.

--The week's two oldest recaptures at Hilton Pond were after 5th year birds--a female Carolina Chickadee and a male Eastern Tufted Titmouse--both banded locally as adults in 2013. These are resident birds that are with us year-round; we recapture them frequently. More remarkable was a 5th year Hermit Thrush of unknown sex banded here on New Year's Eve in 2013 and not re-encountered until this year. This species is a winter resident at the Center and in years since must have made numerous migratory trips to and from its breeding grounds, likely in boreal Canada.

All text & photos © Hilton Pond Center

--An after 2nd year male Yellow-throated Warbler (above) banded this week was only the Center's 11th in 36 years. This is one of the few Wood Warblers that breeds in the southeastern U.S. but is a tree-top species that seldom comes to ground level where we deploy our mist nets.

--As of 16 Apr Hilton Pond Center's 2017 Yard List stood at 49--about 28% of the 171 avian species encountered locally since 1982. (If you're not keeping a yard list for your own property we encourage you to do so, and to report sightings via eBird.) New species during the first half of April: Fish Crow & Yellow-throated Warbler.

All text & photos © Hilton Pond Center

--This week at the Center Carolina Anole males (above) were displaying in the warm April sun. The one above was on his preferred Flowering Dogwood trunk outside our office window. The male's bright red dewlap is unveiled when he flips out a cartilaginous rib in his throat.

All text & photos © Hilton Pond Center

--This week we got word a young Barred Owl banded in Jul 2016 at Hilton Pond Center had been killed by a car back on 23 Dec about five miles to the southeast. We don't often get reports about our banded birds; this was only the 37th encounter within York County out of nearly 66,000 birds banded in 36 years!

All text & photos © Hilton Pond Center

--A Bullfrog, Lithobates catesbeianus, above (formerly Rana catesbeiana), shares a little water garden at the Center with a Green Frog, L. clamitans, of similar size, shape, and color, but it's easy to distinguish these two species. The Green Frog has a long, straight dorsolateral ridge that starts behind the tympanum and runs back toward the rump, while the Bullfrog's ridge curls down directly behind its eardrum. Mmwwonng!

--The immediate past installment of "This Week at Hilton Pond" was about unusual March weather and phenological happenings during the month. It is archived and always available on the Center's Web site as Installment #652.

All text & photos © Hilton Pond Center

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Hilton Pond Center for Piedmont Natural History is a non-profit research, conservation & education organization in York, South Carolina USA; phone (803) 684-5852. Directed by Dr. Bill Hilton Jr., aka "The Piedmont Naturalist," it is parent organization for Operation RubyThroat. Web site contents--including text and photos--may NOT be duplicated, modified, or used in any way except with express written permission of Hilton Pond Center. All rights reserved worldwide. To request permission for use or for further assistance, please contact Webmaster.