The Hatching, Growth, and Fledging of Nestling Purple Martins

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1 Modified from: Hill, III, J. R The growth of nestling Purple Martins. Purple Martin Update 5(3):1-9. The Hatching, Growth, and Fledging of Nestling Purple Martins Founder & Executive Director Emeritus Purple Martin Conservation Association 301 Peninsula Dr., Suite 6 Erie, PA martininfo@purplemartin.org Four 1-day-old nestling Purple Martins and two, yet-to-hatch eggs, nestled in a nest bowl of green leaves. This photo was obtained during a nest check in a removable nest tray from a wooden, crank-down T-14 martin house. Methods This study was conducted at the PMCA s main research colony located at Indianhead Landing Campground on the eastern shore of Edinboro Lake, in Edinboro, PA. Nestlings from a 41- nest subset of our 95 active nests were removed daily at midday and weighed on a portable Ohaus LS200 digital scale, accurate to one tenth of a gram. Using this method, the fates of 170 nestlings were followed from hatching day until death or fledging, 1 During the summer of 1994, under the auspices of the Purple Martin Conservation Association (hereafter referred to as the PMCA ), I conducted a study on the growth rates of nestling Purple Martins using daily weighings and studio photography. The graph on page 3, and the life-size nestling photos on pages 6-10, display the results. These nestling growth photos have become an extremely useful tool for the mar- tin landlord, especially when used in conjunction with the newly-developed Purple Martin Prognosticator (page 2). A landlord can compare the size and/or degree of feather development of any healthy martin nestling with the images on these pages to determine its exact age. This will help in estimating fledging dates, in replacing fallouts into the correct compartments, etc. whichever occurred first. Notice the declining sample sizes (shown above the upper axis of the graph on page 3), which demonstrate the normal attrition rate of nestling Purple Martins. One hundred and thirty-one nestlings fledged (77%), out of 170 hatched. This is a typical fledging rate and not bad considering the population was subjected to Great Horned Owl predation and a couple bouts of cool, rainy weather. Nest checks were done daily for 88 consecutive days, from 15 May 1994 through 10 August This daily disturbance did not cause nest abandonment, or premature fledging, nor did it impact nestling survival in any negative way that we could detect. Daily nest checks were begun before egg laying began and were continued all season long in order determine each nest s exact clutch-initiation date, the exact hatching 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 incubation period as well as well as the species typical age at fledging. Results: Incubation Period The incubation period of Purple Martins, defined here as the number of days elapsing between the laying of the last egg in

2 the clutch and the hatching of the last egg in the clutch, was determined in this study to be 16 days long in 33 (84.6%) of the 39 nests where the exact incubation period was measurable. One nest (2.5%) had an incubation period of 15 days, and the remaining five nests (12.9%) had an incubation period of 17 days. The statistical average was days. [Please note: On the Prognosticator, the Hatching Begins date arrow is set at 15 days after the date the last egg is laid, and indicates the date the first egg will hatch]. It was also learned that incubation begins before the last egg is laid, and in some cases, even before the penultimate egg is laid (i.e., the next-to-last egg). We know this is true because all the eggs within hatched over three calendar days. No nests took four days, or longer, to hatch. Such asynchronous hatching within a nest sets up a substantial size, age, and weight hierarchy among nest mates. In this study, fully 78% of the nests contained nestlings of different ages. A nestling Purple Martin being weighed on a portable field scale as part of this study. Notice that this 14-dayold weighs 60 grams, which is more than an adult martin. any given martin clutch do not hatch on the same calendar day. Of the 41 nests checked daily, only 9 nests (22%) had all the eggs hatch during the same calendar day; 21 nests (51%) had eggs that hatched over two calendar days; and the remaining 11 nests (27%) Patrick Kramer Results: Fledging Age The fledging age of Purple Martins is defined here as the age at which a nestling leaves it nest cavity and takes its first flight. Because of the differences in ages and/or development rates among nest mates in Purple Martins, fledging typically takes place over a period of several calendar days. Of the 40 nests that had young surviving to fledging age in this study, only 11 broods (27.5%) had all the siblings fledge on the same calendar day; 20 broods (50%) fledged over two calendar days; 8 broods (20%) fledged over three calendar days; and one brood (2.5%) fledged over four calendar days. The Purple Martin Prognosticator 2

3 70 n=170 n=170 n=170 n=168 n=166 n=164 n=162 n=162 n=161 n=160 n=160 n=160 n=159 n=157 n=154 n=153 n=153 n=151 n=151 n=150 n=150 n=148 n=147 n=145 n=141 n=136 n=112 n=65 n=39 n=20 n=9 Weight in Grams Hatching Day 1-day old 2-day old 3-day old 4-day old 5-day old 6-day old 7-day old 8-day old 9-day old 10-day old 11-day old 12-day old 13-day old 14-day old 15-day old 16-day old 17-day old 18-day old 19-day old 20-day old 21-day old 22-day old 23-day old 24-day old 25-day old 26-day old 27-day old 28-day old 29-day old 30-day old Nestling Age R. III 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). 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 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 several 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. 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

4 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. The Life-size, Nestling, Growth-sequence Photos During this study, I attempted something that had never been done before with any other bird species anywhere in the world. I removed the same living nestling from its nest 29 days in a row and brought it into a photo studio to document its growth and feather development from hatching day through 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 color transparency film with a 35mm reflex camera mounted on a copy stand with two strobes to light the bird. In the beginning, the bird just wanted to run or flutter off the stand so it was a real 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 the bottom of the copy stand and this gave the bird something to A martin landlord doing his weekly nest check and using the laminated baby photos (available from the PMCA) to determine the age of a recently-hatched nestling (from the Super- Gourd in the background) by superimposing it on the various lifesize photos, looking for a match. In this case, the bird is 3-days-old. 4 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 dimensionality. The end result is quite stunning and these composite photo pages are a valuable tool that many martin hobbyists now use to accurately age their nestlings so they can more effectively manage their colony sites. They are available in waterproof, laminat- ed, life-size form from the Purple Martin Conservation Association. Pipping Egg: You can see the beak of the hatchling breaking through the shell of its egg; hatching birds break their eggs apart, symmetrically, around the equator. This results in two, nearly equal eggshell halves. Hatching Day: Once the nestling has removed the upper cap of its egg, it is ready to free itself from the bottom half. Note that a hatchling martin is totally featherless, its eyelids are grown

5 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. 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. 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. 10-day-old: At this age, the outer primary wing feathers begin to burst their sheaths, as do the tail feathers, exposing emerging, dark-gray feather tips. 13-day-old: Note the speed with which the wing and tail feathers burst their sheaths. The length of these emerging feathers is the best indicator of age in nestling martins day-old: As the feathers burst their disintegrating feather sheaths, the plumage of nestling martins becomes covered with a light-gray, powdery dandruff., is Founder and Executive Director Emeritus of the Purple Martin Conservation Association (PMCA). He has been hosting and studying Purple Martins continuously since For more information on martins, to purchase a Purple Martin Prognosticator, or a set of laminated baby photos for field use, please contact the PMCA at < 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. 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! 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 7-day-old: The folded wings. needlelike sheaths of the flight feathers begin to poke their A newly-fledged Purple Martin waiting to be fed by its parents. This fledgling is approximately 28-days-old and is still dependent 28-day-old: Nearly 75% of all martins fledge by way out the trailing on its parents for supplying all of its food. Notice the yellow beak this age. They are as large edge of the wing. flanges and stubby tail, characteristic of all young martins this age. and heavy as their parents, The feathers are but have a shorter, stubknown as pin feathers at this stage in their growth. bier appearance because their tail and wing feathers are still elongating. 8-day-old: Pin feathers begin to emerge from the fleshy tail. Photos continue on next 5 pages

6 Pipping Egg 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 6 9-day-old

7 10-day-old 11-day-old 12-day-old 13-day-old 14-day-old 7

8 15-day-old 16-day-old 17-day-old 18-day-old 19-day-old 8

9 20-day-old 21-day-old 22-day-old 23-day-old 24-day-old 9

10 25-day-old 26-day-old 27-day-old 28-day-old 10