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, once hatching has occurred and there are small young in the nests, the
parents are much less apt to desert.  This presents an opportunity for you 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.

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.  
    Never 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
  • 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 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 at this stage have brood patches (BPs).  
  • Breeding males at this stage 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 female's brood patch?
  • In Tree Swallows, as in many 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 brood patch seen at close range is very obvious.  Note the extensive bare BP
    on the female below.  

What's a male's 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
  • 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 not from a distance.
  • All adult males have identical definitive plumage, with iridescent blue
    upper body and head feathers, dark flight feathers and white underbodies.
  • However, females have delayed plumage maturation.  Young females in
    the spring of their second calendar year of life (their first nesting season) have
    mostly brown upper surfaces with some blue-green feathers mixed in.  
  • Luis Villablanca took the photo of the pair below at a California nest box.  The
    male, at left, shows the blue upper body and head of all adult males.  The
    female, at right, illustrates a second-year female's brown with green highlights.

  • The amount of blue-green shown by second-year (SY) females varies, as you
    can see in Ed Craft's photo near the top of the Pairing Up page.  
  • However, the following year when they become after-second-year (ASY) birds,
    females like the one above will almost always acquire the same blue plumage
    males show.  From this time on many of these older females cannot be safely
    told from males by plumage alone.  
  • But, there is one exception: some after-second-year females show a few brown
    feathers just above their bills that distinguish them from males.
  • Compare the feathers above the bills of the male at left and the after-second-
    year female at right, in Euan Reid's photo below.

  • To summarize, a brownish adult in spring is a female, but most spring females
    aren't brownish!  The swallows below are both females; SY at left, ASY at right.

  • 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 not
    really accurate.  
  • 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.  
  • For example, 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 are older,
    "after-second-year" (ASY) birds, and are in their second or later nesting season.
  • Ed Post's photo below lets us review the three distinguishable adult Tree
    Swallow age classes.  
  • The bird on the left is an after-hatch-year (AHY) male, which like all adult males
    shows a uniform bright blue crown and forehead.  
  • The brown head with greenish highlights of the bird on the right identify it as a
    second-year (SY) female, in her very first nesting season.  
  • And the small brown patch above the bill of the otherwise blue-headed middle
    bird tell us she is an after-second-year (ASY) female, but remember not all ASY
    females' foreheads show this.    

  • The birds below are both females: at left is an SY, and at right is an ASY.  
    Within a few months the SY female will molt, acquiring the blue ASY plumage,
    and she will reacquire ASY plumage at every new 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.
  • The ageing guidelines we've discussed were determined by researchers who
    examined thousands of Tree Swallows whose true age was known because they
    were banded as nestlings.
  • The diagram below may help you understand the confusing Tree Swallow
    plumages.  The three color possibilities, gray of juveniles, brown and blue of
    adults, refer to a swallow's upper body and head, and its wing coverts.

  • And here's a little side note:  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
  • 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
  • And here's another little side note:  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.  So, in terms of
    color how a bird looks to another bird may be quite different than it looks to us.

  • One last thing; check each adult you handle for a band.  If one is present,
    note the number and report it online at or by phone at  

Questions for the next topic:  Tree Swallow Nestling Growth and
Development to 12 Days.
  • 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?

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