Martin Landlords Get To Know Their Birds
During one night in early July I banded all the Purple Martins on my property with numbered plastic color bands using traps that caught all the birds at once as they roosted inside their compartments. This is the story of my attempts to know who came back when. It was madness.
My colony in Severna Park, Maryland occupies 3 martin house ("Trio Castles") and is situated only 35 feet from my window. This allowed me to see the numbers on their colored bands and tally when each bird arrived at the colony by looking at their legs through a 60 power Bushnell telescope mounted on a tripod. There could be only three numbers that ranged from 001 to 500 on either white, red, blue, or yellow plastic bands. Easy, right?
If I were Dr. Suess it surely would have been the inspiration for The Foot Book, Dr. Seuss’s wacky book of opposites! I sat at my window on the second floor, at about the same height as the martins, from sunup until about 9:30 am trying to spot birds that had just arrived. Birdwatchers get ‘warbler neck’ looking up to see birds in the treetops. I got ‘martin neck’ from trying to mentally budge martin legs to reveal their hidden numbers. Stress is stress even when you’re having fun!
Naturally, the birds rarely showed me their band numbers right away. But I knew how many new birds had arrived each day so I could at least gauge how many new feet I needed to check for numbers. So, I persisted.
When martins first arrive from South America they spend the morning hours bickering and defending nest sites. By midday, the birds spend more and more time away, feeding. But in the evening, when darkening skies mean less food, the birds begin to return to the colony. They swirl around and make mad dashes to their future nest sites if anyone else lands. Counting the birds went like this: 14 in, 7 out, no, 3 just exited so that leaves 11 in and 10 out and so it went until they finally settled into the colony for roosting. From about 7:10 pm to 7:35 pm in early April, then later as the season progressed, I added birds that landed on the Trios and substracted birds leaving for that last snack before bedtime.
I got pretty good at adding and substracting and judged my accuracy on the count the next morning (I was ready for tax time!) I emphasize that this was not due to any natural talent but because I practised tallying arriving banded martins for 8 years during the springs of 1982 to 1989! I had become a certified martin accountant.
- Who arrives first, males or females?
- When do older and younger birds arrive?
- Do birds that nest together arrive together?
- And finally, do earlier arriving birds nest in different heights in the 4-tiered Trios. Are penthouses really better?
Let’s answer these questions in order. I age my birds by year so a first time breeder (born the previous summer) is Age 2, three year olds are Age 3, and so on. These I call year classes, like graduates.
To begin, it is true that males arrive earlier than females in all year classes except for Age 2. Instead, Age 2 females arrive 5.2 days earlier than their male counterparts. But in Year Classes 3 through 7, males average 4.8, 2.5, 7.1, 2.6 and 8.8 days earlier than females, respectively. I suspect the Age 2 girls are looking for old males as mates.
Birds 4 years or older arrive in early April, most are in by April 25, and there is no difference between any of the older year classes. It’s the Year 3 class that surprised me. Age 3 males may resemble the older males but they are not the same. They arrive after these older birds but before the Age 2’s. Year 3 males arrive April 25 and females on May 3 (on average).
The bickering begins to subside, the neighbors stop complaining and then it ratchets up again when the Age 2 birds arrive from May 15 into early June! Dawnsinging begins in mid May too, not surprisingly, if it functions to attract Age 2 hens.
Well, we have the arrival times down, but does this have anything to do with who pairs with whom? Indeed it does.
Let’s start with the old geezers and hens. None of the old hens Age 5 or older mate with Age 2 males. Three out of 4 (75%) pair with males older than Year 3.
Hens Age 3 and 4 are in between with 27% and 49% pairing with males greater than 3 years of age, respectively. Moving on to the Age 2 birds, 78% of the girls pair with Age 2 boys and less than 3% of them are fortunate enough to pair with males older than Year 3, leaving 20% lucky enough to latch onto Age 3 males. So, birds of a Year Class tend to pair with one another.
Now, this might happen because martins form lifetime pairbonds. I checked this out in 1984 and 1985, and found that the same individual martins rarely remate. For fifteen 1984 pairs where both pair members returned in 1985, only two pairs (13%) remated.
If you guessed that birds that arrive together get together you are wrong. There was zero correlation between the date a bird arrived back and that of the bird that became its mate, excluding Age 2 birds from consideration.
Finally to our last question about penthouse preference. Only a few (25%) of the Age 2 birds managed to nest in the top tier of the Trio Castles. The proportion of birds nesting in the penthouse tier increases with their age. The older you are the more penthouse space you get! Older birds prefer to nest in the top two tiers and keep many of the younger birds from using them. I think that predation explains this preference because certainly the compartments themselves are identical by tier in a Trio Castle!
In fact, before I found good predator guards, raccoons and black rat snakes preyed on 13 of 24 (54%) nests in the lowest tier, three of 15 (20%) nests in the second tier, one of 17 (6%) nests in the third tier and none of the 22 nests in the highest penthouse tier. So it looks like the early bird gets the penthouse and for good reason.
You might think that the earlier a female arrives the earlier she lays. I call the period between a females arrival and her first egg the delay period. The delay period is longer for earlier birds than for birds arriving closer to the time egg laying generally begins. Birds arriving in early April wait 40-50 days to lay while those returning in late April take only 20-30 days before laying. The delay to lay says birds wait for breeding until weather conditions are best for raising kids. So these guys and gals that arrive early do so in order to get the best places to nest, not a head start on nesting.
Martin arrivals are well orchestrated as well as coveted by landlords. The three arrival groups, old geezers and hens, prime of lifers (3 year olds), and Age 2 birds show that experience changes what works best. For the oldest, dangerous early arrival is balanced by a higher breeding success, for the 3 year olds, getting established means letting the oldest settle in first rather than compete with experienced birds, and for the Age 2 boys it works best to take little risk with the weather for, even if they do get to nest, their young are mostly fathered by the old geezers.
This article summarizes the information in this scientific paper:
Morton, Eugene S. and Derrickson, K. 1990. The biological significance of age-specific return schedules in breeding Purple Martins. The Condor, 92: 1040-1050.