Reprinted from: Purple Martin Update 3(1): 26-27
Dr. Eugene S. Morton
National Zoological Park
Washington, D.C. 20008
|A photo taken in 1981 showing PMCA founder, James R. Hill, III, building his very first Purple Martin house. It was built from plans published by the Pennsylvania Game Commission. Shown is the bottom 10-room tier of a 3-tiered house. Notice how all ten compartments share a common porch. We now know such designs are detrimental to both nestlings and occupancy rates.|
Porches that connect adjacent nesting compartments on each level of many martin house designs cause the deaths of young martins, in fact many deaths. Here's the evidence:
I have three, 24-compartment Trio (Nature House) Castles at my colony site in Severna Park, Maryland, and have had a great success since I first erected one in 1976, then two more in 1980. These are typical of many designs, with the porch running around each of the four tiers. I band all of the nestlings in each compartment when they are about twelve to seventeen days old. Each year, as is typical in all colonies, the adult birds nest earlier than do the subadults. I generally band most young in adult nests by late June when the young of subadults are too young, or still unhatched. Later, in mid-July, I band the last of the nestlings of the subadult pairs. However, I make weekly nest checks just to see how things are going.
I noticed over the years that the nestlings in some subadult nests do poorly. They appear undernourished and weigh less than they should. I attributed this to poor subadult parenting, but now know this is not always correct. Many of these late nests failed, with the young either dying in the nest or the contents simply disappearing between nest checks, especially in early July when the older young in other nests are near fledging. The cause of these deaths, I determined this year, was food competition by older young that had run around the porch and settled in the nests containing small young, unable to compete with them for food.
A case history will show the point. House B, compartment 17, contained four eggs on 2 July, which hatched on 4 July. The adjacent compartment on the tier was then occupied by five, 12-day-old young. When I returned home after an eight-day trip on 16 July, I found four of the now 24-day-old young in compartment 17 with two 12-day-olds, whose weights were 25 grams, indicating that they would soon have starved had I not intervened by replacing the big youngsters back in their own compartment. Two other nests, that had young too small to band on 2 July, were completely empty on the 16th and both had broods sharing their tiers that had now fledged. There was no sign of predation and my houses are protected from climbing predators.
Going back through my records, I noticed many other cases of nest losses in the later nests of subadults that were probably, or certainly, due to nest infiltration by unfledged, but fully ambulatory, older broods (20-28 days of age). I often observed these broods scattering around the porches into compartments other than their own. Sometimes high heat causes this wandering, then the young make mistakes trying to return to their own compartments from the porch. Occasionally when nestlings venture out of their compartments onto the porch, tier-sharing adult martins attack them, knocking them to the ground. Adults will try to keep any foreign young out of their compartments. Adults often attack fledged young that attempt to return to the colony to become 'kleptoparasites.' Kleptoparasitism is the stealing of food or nesting material by one animal from another. Kleptoparasitism of food is a natural behavior in some martins that have already taken their first flights. When they return to the houses, they often go into neighboring compartments to steal incoming food, but rarely cause the starvation death of young there. This is because only one fledgling is usually involved and they are quickly identified and kicked out by the parents if they enter a compartment with small young. Kleptoparasitism by fledglings only succeeds if they blend in with fully-feathered nestlings and these are not as vulnerable to competitive starvation since they are nearly fledgling sized. But the roving bands of pre-fledged "porch invaders" are not easily removed from the compartments they enter. They cling tenaciously within their appropriated compartments since they are not yet capable of flight. Furthermore, their own parents will protect them against ouster by the rightful compartment owners if the new compartment is close to their home.
The end result is that porch wandering rapidly spells disaster for small nestlings through starvation since the interlopers grab food as soon as it arrives in the beak of the rightful owner adult. I lost four and a half of my subadult broods to porch invasion and five would have been lost in a matter of hours if I hadn't checked compartment 17. That amounts to about 17% of my nests! Most insidious, is the fact that colony sites that are highly successful in attracting breeding pairs, suffer the greatest nestling losses in subadult nests. The more adult pairs in the colony, the higher the likelihood that their young will become porch-invading kleptoparasites into the nests of subadults. Nest failure through porch invasion probably causes subadults to look elsewhere for breeding sites the following year, as does all types of complete nest failure.
All of the problems associated with porch-wandering nestlings are easily avoided, however. Martin landlords should insist that commercial houses come equipped with porch dividers. Homemade houses should be built with separated porches, no porches, or porch dividers. Gourds avoid the problem altogether.
|A current photo of the 30-compartment martin house shown on the previous page. This house is now in its 11th year of use at the PMCA's research grounds in Edinboro, PA. In ten years, this house has never had over a dozen pairs of breeding martins due to male porch domination (i.e., males claiming and defending several adjacent compartments). This year, however, homemade porch dividers were added and, as you can see, the occupancy rate has nearly doubled! This is the other benefit of housing with private porches.|
Dr. Morton is an authority on Purple Martins, having published several scientific papers on them. He serves on the scientific advisory board of the Purple Martin Conservation Association.
For information on how to put porch dividers on your martin housing, refer to the article: "Porch Dividers: A Way to Improve the Productivity of House-nesting Purple Martins" published in Update 2(1):6-7, (1989).
Copyright 1998 by Purple Martin Conservation Association. All Rights Reserved.
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