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 a bit 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.  
  • 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 swallow.  Note the CP
sticking out from the male's abdomen below.  Also note his area of bare skin
is very 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, 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?  Neither can we!









Aging Tree Swallows:
Can you age adult Tree Swallows by plumage?
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 to 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.

(For a further discussion of sexing and aging Tree Swallows see McGill Bird
Observatory's page on the subject.  Caution: the graphics are beautiful but
the download may be slow).

Here's a little sidenote:  Unlike humans, songbirds can see far into the
ultraviolet portion of the light spectrum, and many have markings visible only
in UV that are important in their identity and behavior but which are totally
invisible to humans!  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