At the project:
There are small nestlings in most boxes now, and the adult swallows are kept busy
foraging, feeding and brooding them.  

Ever since females started incubating there was a chance that when you checked a
box you might find an adult crouched on the nest.  You were advised to gently close
the door and leave the box alone until the adult left.  

However, since hatching has occurred and there are small young in the nests the
parents are much less apt to dessert.  This presents an opportunity to do something
different.  Now if you find an adult in a box with small nestlings grasp it, remove it,
and examine it.  Note its features close up: its beautiful plumage, its quick dark
eyes, its black lores, sword-shaped wings, and tiny legs.  

And you can try your hand at sexing and aging Tree Swallows.














Concepts:
How do you safely grasp and remove a swallow from a box?
  • Most adults you find in a box will simply crouch down as if trying to hide.  
  • Open the door just enough to get your hand in and grasp the whole bird from
    above gently but firmly.
  • Never grasp or hold a swallow by the wing, foot or tail.  It could flop around
    and injure itself.
  • If the bird eludes your hand and tries to escape through the door, let it go.  
    Don't ever try to grab a swallow trying to escape.
  • Swallows being removed from a box often clasp nest material or even nestlings
    in their feet.  Raise each bird straight up an inch or two and give it a chance
    to release whatever it's clasped.  You may need to give it an assist with your
    other hand.  
  • Be certain any displaced nestlings are returned gently back into the nest cup.

How do you hold a swallow for examination?
  • Gently but firmly, cradling the bird in your palm (see below).  
  • Loosely held birds are more apt to try to escape and be injured accidentally.
  • Never squeeze a swallow's body or throat.
  • If you notice your bird is gasping for breath, you are holding it to tightly.  
    Release a gasping bird at once.
  • Most Tree Swallows won't struggle much, and they never bite.
  • They are easy to rotate for viewing top or undersides.
  • When you are ready to release a swallow, don't put it back in the box.  Simply
    hold your hand out and open your fingers.  It should fly away, but if it doesn't,
    raise and then lower your hand slowly until it flies.  
  • Never toss a swallow into the air to release it.
















Sexing Tree Swallows:
How can you tell male Tree Swallows from females?
Sexual variations in plumage, size or behavior are often used to tell male from female
birds in the field, but some species can be very hard or impossible to sex.  Tree
Swallows are a species that can cause sex identification problems, since males and
females sometimes appear exactly the same from a distance.

But, now that you're holding a swallow in your hand you'll be able to tell its sex easily,
because during nesting Tree Swallows, like most birds, develop different physical
structures related to the sexes' different roles in breeding.

Breeding females have brood patches (BPs).  
Breeding males have cloacal protuberances (CPs).

To see which structure a swallow has:
  • Rotate the bird so its underside faces up.
  • Blow on its lower belly near its legs.
  • This separates its feathers so you can see its BP or CP.

What's a brood patch?
In Tree Swallows, as in most songbirds, only females incubate eggs.  Nesting females
shed feathers on a patch of their breast and belly skin.  This bare skin swells due to
retention of water and expansion of blood vessels and becomes an organ for heat
transfer.  Females press their "BP" against their eggs during incubation and on small
young during brooding, transferring their body heat to them.  A BP seen up close is
very obvious.  Note the extensive bare BP on the female below.  














What's a cloacal protuberance?
Male songbirds don't have a penis, but during nesting they develop a swelling of the
lower ducts that carry sperm from testes to "cloaca".  You'll remember that the cloaca
is the single chamber in birds that receives feces from the intestine, uric acid from
the kidneys, and eggs or sperm from the gonads.  

A male's cloacal protuberance serves as a storage chamber for sperm and also helps a
male make physical contact with a female's cloaca so his sperm can be passed to her.  
A cloacal protuberance or "CP" is not as obvious as a female's brood patch, but is still
easy to see in a hand-held male.  Note the CP sticking out from the male's abdomen
below.  Also note his area of bare skin is small compared to the female's above.














Can you tell male Tree Swallows from females by their plumage?
Sometimes yes, but mostly no, at least from a distance.

All adult males have identical definitive plumage, with iridescent blue-green upper
body and head feathers, dark flight feathers and white underbodies.

Females, on the other hand, have delayed plumage maturation.  Young females in
their second calendar year of life (their first nesting season) have mostly brown
upper surfaces with some blue-green feathers mixed in.  (The female below is a
brown extreme; some SY females show more green).

















But when they become older, after-second-year birds, females almost always acquire
the same total blue-green males have.  Most of these older females cannot be safely
told from males by plumage alone.  However, there is one exception: some after-
second-year females retain a few brown feathers just above their bills (see below),
that distinguish them from males.  















To summarize, a brownish adult is a female, but most females are not brownish!  The
swallows below are both females; SY at left, ASY at right.










Caution!  Field guides are often misleading.  They will picture a "drab female", (really
a second year female), and state "many are close to males" in color.  This is wrong.  
Many after-second-year females are identical to males in outward appearance when
viewed from a distance, and can't be told apart, except during the nesting season by
BP or CP when held in the hand.  Can you tell the sexes of the pair below?  

















Aging Tree Swallows:
Can you age adult Tree Swallows by plumage?
In the field adult males all look the same, no matter how old.  The most you can do
when aging males is say all adult males are "after-hatch-year" (AHY).

Adult females can be aged more precisely than males:
  • Females that have mostly brownish upper bodies and heads (less than 50% of
    their upper body and head blue) are in their second calendar year.  They are
    called "second year" (SY) birds, and are in their very first nesting season.  
  • Females with 90-100% of their upper body and head feathers blue-green are
    older, "after-second-year" (ASY) birds, and are in their second or later nesting
    season.
  • The bird below left is an SY female; the one on the right is an ASY female.  
    Within a few months the SY female will molt, acquiring ASY plumage, and she
    will reacquire ASY plumage at each molt for the rest of her life.











  • A very few females are mostly blue (50-90% of their upper body and head
    feathers blue), with the other 10-50% brown.  Recent evidence suggests these
    are almost all SY birds, but we can't be absolutely certain.  Like males, the most
    we can say is they are AHY, after-hatch-year birds.

These ageing guidelines were determined by researchers who examined thousands of
Tree Swallows whose true age was known because they were banded as nestlings.

Here's a little sidenote:  Using sophisticated instruments researchers have recently
found that there are measurable differences in both brightness and hue among male
and ASY females, and that older swallows tend to be bluer and brighter.  In addition
brighter males tend to pair with brighter females, and bluer females have better
success raising young.  It's probable these subtle (to us) plumage differences function
as signals of "quality" that Tree Swallows can use to identify individuals that are older
and therefore more experienced breeders, and who could make better potential
mates.  

Here's another little sidenote:  Unlike humans, most songbirds can see far into the
ultraviolet portion of the light spectrum, and many have markings visible only in UV
that are very important in their identity and behavior.  In other words in terms of
color how a bird looks to another bird may well be quite different than it looks to us!

One last thing; check each adult you handle for a band, and if one is present, note
the number and report it online at www.reportband.gov or by phone at 1-800-327-
BAND.

Questions for the next topic:  Nestling Growth and Development.
  • How are the nestlings changing in appearance and behavior?
  • Is the behavior of the adults changing also?
  • Have any nestlings died?  If so, any ideas why?









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Learn About Birds at Tree Swallow Nest Box Projects
Sexing and Aging