Modified from: Hill, III, J. R. 1994. The growth of nestling Purple Martins. Purple Martin Update 5(3):1-9.
The Hatching, Growth, and Fledging
of Nestling Purple Martins
James R. Hill, III
Founder & Executive Director Emeritus
Purple Martin Conservation Association
301 Peninsula Dr., Suite 6
Erie, PA 16505
[email protected]
James R. Hill, III
whichever
oc During the
curred first. Nosummer of 1994,
tice the declinunder the auspicing sample sizes
es of the Purple
(shown
above
Martin Conservathe upper axis
tion Association
of the graph on
(hereafter
repage 3), which
ferred to as “the
demonstrate the
PMCA”), I connormal attrition
ducted a study on
rate of nestling
the growth rates
Purple Martins.
of nestling Purple
One
hundred
Martins using daiand thirty-one
ly weighings and
nestlings fledged
studio photogra(77%), out of 170
phy. The graph
hatched. This is
on page 3, and
a typical fledgthe life-size nesting rate and not
ling photos on
bad considering
pages 6-10, disthe population
play the results.
was
subjected
These
nestling
Four 1-day-old nestling Purple Martins and two, yet-to-hatch eggs, nestled
growth photos
to Great Horned
in a nest bowl of green leaves. This photo was obtained during a nest check
Owl predation
have become an
in a removable nest tray from a wooden, crank-down T-14 martin house.
and a couple
extremely useful
bouts of cool,
tool for the martin landlord, especially when used in conjunction with the newrainy weather. Nest checks were done daily for 88 consecutive
ly-developed Purple Martin Prognosticator (page 2). A landlord
days, from 15 May 1994 through 10 August 1994. This daily discan compare the size and/or degree of feather development of
turbance did not cause nest abandonment, or premature fledgany healthy martin nestling with the images on these pages to
ing, nor did it impact nestling survival in any negative way that
determine its exact age. This will help in estimating fledging
we could detect. Daily nest checks were begun before egg laydates, in replacing fallouts into the correct compartments, etc.
ing began and were continued all season long in order determine each nest’s exact clutch-initiation date, the exact hatchMethods
ing date of each egg, and thus, the age of each nestling. These
daily checks enabled us to get an exact measure of the martin’s
This study was conducted at the PMCA’s main research colincubation period as well as well as the species’ typical age at
ony located at Indianhead Landing Campground on the eastern
fledging.
shore of Edinboro Lake, in Edinboro, PA. Nestlings from a 41nest subset of our 95 active nests were removed daily at midday
Results: Incubation Period
and weighed on a portable Ohaus LS200 digital scale, accurate
to one tenth of a gram. Using this method, the fates of 170 nestThe incubation period of Purple Martins, defined here as the
lings were followed from hatching day until death or fledging,
number of days elapsing between the laying of the last egg in
1
Patrick Kramer
the clutch and the hatching of the last egg in the clutch, was dehatched over three calendar days. No nests took four days, or lontermined in this study to be 16 days long in 33 (84.6%) of the 39
ger, to hatch. Such “asynchronous hatching” within a nest sets up
a substantial size, age, and
nests where the exact incuweight hierarchy among
bation period was measurnest mates. In this study,
able. One nest (2.5%) had
fully 78% of the nests conan incubation period of 15
tained nestlings of different
days, and the remaining five
ages.
nests (12.9%) had an incubation period of 17 days.
Results: Fledging Age
The statistical average was
16.10 days. [Please note:
The fledging age of PurOn the Prognosticator, the
ple Martins is defined here
“Hatching Begins” date aras the age at which a nestrow is set at 15 days after
ling leaves it nest cavity
the date the last egg is laid,
and takes its first flight. Beand indicates the date the
cause of the differences in
first egg will hatch]. It was
ages and/or development
also learned that incubation
rates among nest mates in
begins before the last egg is
Purple Martins, fledging
laid, and in some cases, even
typically takes place over a
before the penultimate egg
A nestling Purple Martin being weighed on a portable
period of several calendar
is laid (i.e., the next-to-last
field scale as part of this study. Notice that this 14-daydays. Of the 40 nests that
egg). We know this is true
old weighs 60 grams, which is more than an adult martin.
had young surviving to
because all the eggs within
any given martin clutch do not hatch on the same calendar day.
fledging age in this study, only 11 broods (27.5%) had all the sibOf the 41 nests checked daily, only 9 nests (22%) had all the eggs
lings fledge on the same calendar day; 20 broods (50%) fledged
hatch during the same calendar day; 21 nests (51%) had eggs that
over two calendar days; 8 broods (20%) fledged over three calendar days; and one brood (2.5%) fledged over four calendar days.
hatched over two calendar days; and the remaining 11 nests (27%)
The Purple Martin
Prognosticator
2
n=20
n=9
n=39
n=65
n=112
n=136
n=141
n=145
n=147
n=148
n=150
n=153
n=151
16-day old
n=150
n=153
15-day old
n=151
n=154
n=157
13-day old
14-day old
n=159
12-day old
7-day old
n=160
n=162
n=161
6-day old
11-day old
n=162
5-day old
n=160
n=164
4-day old
10-day old
n=166
2-day old
n=160
n=170
n=170
n=168
1-day old
n=170
Hatching Day
Weight in Grams
70
60
50
40
30
20
10
30-day old
James R. Hill, III
James
R. Hill, III
Nestling Age
29-day old
28-day old
27-day old
26-day old
25-day old
24-day old
23-day old
22-day old
21-day old
20-day old
19-day old
18-day old
17-day old
9-day old
8-day old
3-day old
0
The growth curve of nestling Purple Martins at Edinboro, PA, during the 1994 nesting season. Nestlings from
41 nests were weighed daily, at midday, from hatching day until fledging day. The numbers across top of the
graph are the number of nestlings weighed at each age (i.e., the sample size). The red dots represent the average (mean) weight of young at each age. The thin vertical lines represent the range of weights encountered for
each age, connecting the heaviest and lightest individuals. And the thick, vertical lines represent one standard
deviation around the mean (i.e., 68% of all nestlings had a weight within this range of one standard deviation).
eral nests, especially in nests where young hatched over three calendar days. Not all runts died, however. Many were able to recover
and fledge, especially if ideal weather prevailed during which their
parents could supply adequate food to the entire brood. Many
nestlings broke the 60 gram barrier during their growth. The
heaviest reached its peak weight of 68 grams when just 17 days
old. On average, nestling martins reach peak weights at 20 days
of age and surpass the weight of their parents (about 50 grams)
when just 13 days old. Note that during the last week of nestling
life, young martins actual tend to lose weight before fledging at a
weight of about 50 grams.
Fledging in the study population didn’t begin until the young
reached at least the age of 26 days. Twenty-five nestlings out
of 131 (19.1%) fledged at age 26 days; 46 (35.1%) at 27 days; 23
(17.5%) at 28 days; 20 (15.3%) at 29 days; 11 (8.4%) at 30 days; and
five (3.8%) at 31 days of age. One debilitated nestling (0.8%) didn’t
fledge until it was 34 days old. As you can see, in this study, the
most common age at fledging was 27 days. The statistical average
fledging age of these 131 nestlings was 27.75 days of age. Not all
nestling martins grow and develop at the same rate. For instance,
a few 13-day olds looked like 12-day olds, whereas a few others
looked like 14-day olds. This is to be expected with different brood
sizes, different aged parents, differing food availability, and the genetics of individual variation. In a few nests with runts that couldn’t
compete for food, the runts became so extremely debilitated, and
their development so retarded, that they appeared to be a week or
more younger than their actual age, or the age of their sibs. On the
graph, note how much farther the statistical range extends below
the average weight at each age than it extends above. This is a
result of debilitated runts with low weights that were found in sev-
The Purple Martin Prognosticator
Based on the incubation periods, typical hatching patterns,
and earliest age-at-fledging determined by this study (plus rangewide clutch-initiation dates and average clutch sizes determined
by the PMCA in our other Purple Martin studies), one of the most
unique devices was created, the Purple Martin Prognosticator. This
3
8-inch plastic calculator wheel was conceived by martin landlord,
Ed Donath, and refined by me and a graphic artist using Adobe
Illustrator. It accurately tells landlords five important things about
each active nest under their care: 1). The clutch-initiation date of
each nest, 2). The hatching date of eggs in each
nest, 3). The age of the
young on any given date,
4). The earliest possible
fledging dates for the
young in any nest. Based
on these dates, landlord
can determine: 5). The
breeding success of each
nest.
The name of the
device is clever as well.
A “prognosticator” is a
person who can accurately predict what is
going to happen in the
future. That is just what
this wheel does, but with
nearly 100% accuracy,
unlike its human namesake. It’s name is also a
play on the taxonomic
genus name of martins,
Progne. Every martin
landlord who does weekly nest checks should
have one of these extremely-handy devices.
James R. Hill, III
grip and an apparent sense of security. After about a week or so,
the bird finally learned the routine and was relatively cooperative.
Each day when I placed the nestling on the copy stand for it daily
photo, I placed a 1” x 1” square next to it that had its age recorded
on it. The square was a
size guide that would
become useful in the
later stages of this project. To create the final
five composite growth
image sequences shown
at the end of this article,
each color transparency was scanned and
turned into a digital file.
In Adobe Photoshop,
the bird in each photo
was properly sized using the 1-inch-square
size guides, then a clipping path was drawn
around each nestling
on the computer screen
to remove it from its felt
background, which also
contained harsh shadows. The final step was
to digitally place the
individual, life-size images on a fake, digital
gray background and
place uniform drop
shadows around them
to give them a three diThe Life-size, Nestling,
mensionality. The end
Growth-sequence
result is quite stunning
Photos
and these composite
photo pages are a valu During this study, I
able tool that many
attempted something
martin hobbyists now
that had never been
A martin landlord doing his weekly nest check and ususe to accurately age
done before with any
ing the laminated baby photos (available from the PMCA) to
their nestlings so they
other bird species anydetermine the age of a recently-hatched nestling (from the Supercan more effectively
where in the world. I
Gourd in the background) by superimposing it on the various lifemanage their colony
removed the same livsize photos, looking for a match. In this case, the bird is 3-days-old.
sites. They are available
ing nestling from its nest
in waterproof, laminat29 days in a row and brought it into a photo studio to document
ed, life-size form from the Purple Martin Conservation Association.
its growth and feather development from hatching day through
Pipping Egg: You can see the beak of the hatchling breaking
fledging day. The results are on pages 6-10 in five composite photos. This was done back in the pre-digital camera days, so I shot
through the shell of its egg; hatching birds break their eggs apart,
symmetrically, around the equator. This results in two, nearly equal
color transparency film with a 35mm reflex camera mounted on
a copy stand with two strobes to light the bird. In the beginning,
eggshell halves.
the bird just wanted to run or flutter off the stand so it was a real
Hatching Day: Once the nestling has removed the upper cap
challenge to photograph it without my hand in the photo holding
the nestling down. Then I discovered I could lay a piece of felt over
of its egg, it is ready to free itself from the bottom half. Note that
the bottom of the copy stand and this gave the bird something to
a hatchling martin is totally featherless, its eyelids are grown
4
shut, and its skin is a bright pink in color. At hatching, a baby
martin will only weigh about 3.0 to 3.5 grams.
15-day-old: At this age, the pure-white feathers that grow on the
upper-middle back become quite apparent.
3-day-old: Note the large, transparent, bellies these young
nestlings have. These
are their yolk sacs.
Although the nestlings are fed by their
parents from the
moment of hatching, they still acquire
nutrition from this
embryonic holdover.
16-day-old: As the feathers burst their disintegrating feather
sheaths, the plumage of
nestling martins becomes
covered with a light-gray,
powdery “dandruff.”
20-day-old: Martins reach
their peak weights at this
age, often several grams
heavier than their parents.
Note the gray neck collar all
nestling martins have, regardless of sex.
5-day-old: The nestlings of many cavitynesting species, such
as the Purple Martin,
have bright-yellow
beak flanges. These
function as a highlyvisible food target
for parent martins as
their young gape for
food in the darkness
of the nest cavity.
23-day-old:
Note how
conspicuously martins can
display their white flank
patches. All ages and sexes
of martins have these white
patches of feathers. Martins
keep them hidden most of
the time, but often expose
them during preening activities, and possibly at densely-packed nocturnal roosts
where it’s hypothesized they
use them to communicate
the message “Hey! I’m sitting
here! Don’t land on me!”
James R. Hill, III
6-day-old: Feather
tracts are beginning
to show as darkened
blotches under the
skin of the nestling,
causing the skin
to change from an
overall pink color
to a gray. The eyelid
slits begin to open.
24-day-old: Compare this
pose with the one the bird
had the day before. Today,
its white flank feathers are
completely covered by the
folded wings.
7-day-old:
The
needlelike sheaths
A newly-fledged Purple Martin waiting to be fed by its parents. 28-day-old: Nearly 75%
of the flight feathers
This fledgling is approximately 28-days-old and is still dependent of all martins fledge by
begin to poke their
on its parents for supplying all of its food. Notice the yellow beak this age. They are as large
way out the trailing
flanges and stubby tail, characteristic of all young martins this age. and heavy as their parents,
edge of the wing.
but have a shorter, stubThe feathers are
known as “pin feathers” at this stage in their growth.
bier appearance because their tail and wing feathers are still
elongating.
Photos continue on next 5 pages
8-day-old: Pin feathers begin to emerge from the fleshy tail.
James R. Hill, III, is Founder and Executive Director Emeri10-day-old: At this age, the outer primary wing feathers
tus of the Purple Martin Conservation Association (PMCA).
begin to burst their sheaths, as do the tail feathers, exposing
He has been hosting and studying Purple Martins continuemerging, dark-gray feather tips.
ously since 1981. For more information
on martins, to purchase a Purple Mar13-day-old: Note the speed with which the wing and tail
tin Prognosticator, or a set of laminated
feathers burst their sheaths. The length of these emerging
baby photos for field use, please contact
feathers is the best indicator of age in nestling martins.
the PMCA at <www.purplemartin.org>.
5
Hatching Day
1-day-old
2-day-old
3-day-old
4-day-old
5-day-old
7-day-old
6-day-old
8-day-old
9-day-old
6
James R. Hill, III
Pipping Egg
10-day-old
11-day-old
12-day-old
14-day-old
7
James R. Hill, III
13-day-old
16-day-old
17-day-old
18-day-old
8
19-day-old
James R. Hill, III
15-day-old
20-day-old
21-day-old
24-day-old
9
James R. Hill, III
23-day-old
22-day-old
25-day-old
26-day-old
27-day-old
10
James R. Hill, III
28-day-old