Why do Some Martins Choose to Nest Alone?
Ken Kostka
Purple Martin Preservation Alliance
http://www.purple-martin.org
Pittsburgh, PA

    Many "would-be" landlords, especially in the northern part of the Purple Martin's range, have discovered that starting a new martin colony can be quite a challenge. Already reluctant to colonize new housing, martins are also scarce in many areas, making it difficult to attract the first breeding pair, especially in marginal habitat. When, after 20 years of attempting to establish a colony at a site north of Pittsburgh, PA, one breeding pair was finally attracted in 1999, the question of why a subadult pair of martins would ever choose to nest alone emerged.  Weeks later, when they carried those first few pieces of nesting material into the martin house, one could not help but ask the question "Why?"

    Purple Martins are colonial nesters, preferring the company of other breeding martins.  Subadult pairs, especially females, are reluctant to settle new sites, because it is believed that they prefer the proximity of older, genetically superior males from which they can solicit extra-pair copulations (EPC's) to increase the viability of their offspring.  DNA research has determined that subadult males living in typical colony sites sire only 27% of the nestlings they raise. The rest of "their" young are actually fathered by adult males of the colony that are already pair-bonded with another female. (Morton et al 1990)

        Why did this subadult pair decide to forego a colonial lifestyle when there were cavities available at an active colony only eight miles to the north with many breeding adult males?  I believe employing the social attraction method (dawnsong, decoys, etc...) played a significant role; this pair had at least, in part, been tricked, or so it seemed. But colonies have been established by one pair without the use of Social Attraction. Why would a female, who seems to have the final say in the choice of a nesting site, would ever choose to be a pioneer -- to strike out on her own rather than settle into an established site?

    While colonialism has benefits, it also has costs, and it's believed that Purple Martins, because of a scarcity of tightly clustered nesting cavities, were originally only semi-colonial anyways, making them less bound to a colonial lifestyle.  Evidence for a less colonial history includes asynchronous breeding, male porch domination, inability of parents to recognize their own nestlings until fledging age, and the martins extensive vocal repertoire that is more typical of a solitary nesting species.  Here are some of the possible reasons that a martin might forego a colonial lifestyle; most are advantages of solitary nesting and disadvantages of colonial nesting.

    1. Less competition for food.  It has been established that certain species of colonially nesting birds sometimes exhaust their local food supply and abandon their nests.  At large colonies, it's possible that hunting becomes more difficult due to the large number of hunters, forcing martins to forage further and further from the housing.  Might investigating or newly-arrived subadult martins experience this while temporarily residing at the site before they are committed, and choose to leave?  Perhaps they can sense that food-gathering for a hungry brood would become exponentially more difficult than simply hunting for themselves.  Perhaps more needs to be known about insect distribution and abundance in the atmosphere at different times during the season and in different habitats. The new site was only 1/3rd mile from the Allegheny River.  Perhaps it offered superior opportunities for hunting when compared to the closest active site.  Perhaps the "river site" was actually superior habitat, and the abundance of food was gauged to be more important than the presence of other breeding martins.

    2. Less sexual harassment from other martins.  It is not uncommon for as many as 8-10 adult male martins to attempt to copulate with an subadult female.  These females can be injured, exhausted, or even killed during these attempts, and so it is difficult to believe that these extra-pair copulations are always invited.  Perhaps some females have experienced this phenomenon and prefer to sacrifice the superior ASY genes in exchange for less harassment and physical stress, especially if their mate appears to be robust and of superior lineage.  It's also possible that females are are able to solicit extra-pair copulations (EPC's) from adult males at colony sites as far as 8-10 miles away, in which case they are not sacrificing EPC's at all.  Martins are known to fly long distances to visit or investigate other colony sites.  8-10 miles may seem like a long distance to ground-bound creatures like ourselves, but to strong fliers like martins, it is probably easily covered.  A colony that seems isolated to us may be considered proximate to martins.

    3. Less predator pressure.  Large colonies seem to attract more attention from hawks and owls, which are notorious for sometimes conducting daily and nightly raids respectively. Great-Horned Owls, raccoons, and snakes must certainly find it easier to detect large colonies because of the increased noise and activity levels.  Colonial nesters do have greater predator detection because of multiple pairs of eyes, but this would not help them to detect nocturnal predators like owls, which strike at night when the birds are sleeping. Resident but uncommitted subadults may experience or witness these raids and sense that their chances of reproductive success are better at a site less obvious to predators.

    4. Genetic predisposition towards new colony establishment. Perhaps some females simply have a "pioneering tendency" and prefer to nest at a new location.  For these females, perhaps the presence of other martins is not as important of a stimuli as for most. There is always some variation in behavior among individuals of the same species -- some differences in "personality". These pioneering females may simply have more confidence or be less colonial by nature.  Prior to the tradition shift, it was common for martins to nest solitarily because of a shortage of clustered nesting cavities.  Perhaps some martins - throwbacks, if you will - still prefer to nest alone.

    Whatever one chooses to believe, there is no clear explanation for why the pair of subadult martins that nested in my backyard in 1999 did not choose to nest at the colony site only eight miles away.  As I mentioned earlier, there were nesting cavities available at that site, and many adult nesting pairs.  The adult males at that site should have vigorously recruited this pair in order to increase their chances for EPC's. One can only conclude that there must be other factors other than a shortage of nest cavities that cause martins to strike out on their own.

Morton, E. S., L. Forman and M. Braun. 1990. Extrapair fertilizations and the evolution of colonial breeding in Purple Martins.  Auk 107: 275-283.