Archive for February 23rd, 2006

Incidence of brood parasitism per colony varied from 0 to 25% of nests in Cave Swallows and from 0 to 50% in Cliff Swallows. In neither species was the percentage of nests with brood parasitism related significantly to colony size (of conspecifics) in either the first or second wave of nesting (Fig. 1). However, in Cliff Swallows, correlation coefficients for both first and second waves were positive and much larger than in Cave Swallows, and that for the first wave in Cliff Swallows approached significance (Fig. 1).

In both species, there was considerable variation in frequency of brood parasitism, especially among colonies of 10 nests and smaller, with some of those sites having no brood parasitism and others showing the highest frequencies (Fig. 1). Because some of that apparent variation merely reflects the small number of nests in the smaller colonies (a single nest parasitized in a small colony affects the percentage more strongly than does a single nest in a large colony), we examined whether colony size affected frequency of brood parasitism for colonies of >10 nests. For neither species in either wave was the correlation between parasitism and colony size significant when analysis was restricted to colonies of >10 nests (P > 0.08 for all).

Brood parasitism in relation to nesting success.Frequency of brood parasitism was unrelated to nest success at a colony site. We found no significant correlation between the percentage of nests at a site with brood parasitism and the percentage of nests there that were successful in producing at least one young (Fig. 2). However, it was apparent that the colonies with the highest frequencies of brood parasitism tended to be among the more successful ones.

There was no relationship between whether a nest was parasitized and its eventual success at producing young to day 10. Of 23 Cave Swallow nests during the first wave that were parasitized and their success known, all 23 (100.0%) were successful. Of 400 Cave Swallow nests during the first wave for which no evidence of parasitism was detected, 381 (95.3%) were successful. The difference was not significant (χ^sup 2^ = 1.1, df = 1, P = 0.29). Of 25 Cave Swallow nests during the second wave that were parasitized, 22 (88.0%) were successful, compared with 92.6% (n = 378) of nests for which no evidence of parasitism was detected. That difference also was not significant (χ^sup 2^ = 0.7, df = 1, P = 0.40).

Egg transfer.-We detected a total of 7 Cave Swallow nests (1.1%) with egg transfer during the first wave (n = 651 nests) and 6 nests (1.0%) with egg transfer during the second wave (n = 598). For Cliff Swallows, there were 4 nests with egg transfer (2.0%) during the first wave (M = 202) and 1 (0.7%) during the second wave (n = 149). Half of the egg transfers to Cave Swallow nests occurred in the single largest colony during each wave, with the remaining ones distributed among colonies of all sizes (including one in a colony of five nests). The five transfers to Cliff Swallow nests occurred in colonies ranging in size from 16 to 82 nests.


The present study provides the first documentation of intraspecific brood parasitism (and egg transfer) in Cave Swallows. Overall frequency of brood parasitism in Cave Swallows, however, was less than half that seen in Cliff Swallows occupying the same colony sites at the same time. That result, together with the relatively high frequency of brood parasitism found in Cliff Swallows in Nebraska (Brown and Brown 1989, 1996), seems to indicate that Cliff Swallows are inherently more likely to parasitize nests than their closely related congener. The reason for that difference is unclear, but may be related in part to the Cliff Swallow’s greater degree of coloniality. Because nesting near conspecifics theoretically affords greater opportunities for brood parasitism, selection may have more strongly favored brood parasitism as a reproductive strategy in Cliff Swallows. The largest colony size known in Cliff Swallows (3,700 nests; Brown and Brown 1996) is >2× that reported for Cave Swallows (1,500 nests; Selander and Baker 1957). In addition, the much closer nest spacing in Cliff Swallows than in Cave Swallows may make parasitism easier, in part because it allows better assessment of neighbors for suitability as hosts and observation of when they leave their nest unattended.

However, in our Texas study area, we found no significant relationship between frequency of brood parasitism and colony size in either species. In Cliff Swallows, that may have resulted, in part, from the relatively small number of colonies (11); the pattern among Cliff Swallows resembled that seen in the Nebraska population (Brown and Brown 1996) and approached statistical significance. Lack of a colony-size effect in Cave Swallows may have primarily reflected that species’ lower overall incidence of parasitism. Although the colony-size range in our study area was smaller than that in Nebraska (our largest colony was only 243 total nests of both species combined), strong colony-size effects over the same range of colony sizes were detected in Nebraska for Cliff Swallows (Brown 1984, Brown and Brown 1989). Absence of very large colonies of either species in Texas, therefore, probably cannot explain our results.

In Cliff Swallows in Nebraska, brood parasitism is practiced by high-quality individuals that apparently accurately assess both individual host nests and overall certainty of reproduction at a site (Brown and Brown 1989, 1991, 1996, 1998). Colony sites that are ultimately more likely to have nest failure have higher frequencies of brood parasitism. That seems to be because individuals are more likely to parasitize nests in such situations, and when they do, they preferentially parasitize nests that are more likely to eventually fledge young, perhaps because those nests have lower infestations of blood-sucking cimicid bugs and fleas (Brown and Brown 1991). We could not determine if parasitic Cliff Swallows in south Texas show the same patterns, because we did not have data on reproductive success of Cliff Swallows. For Cave Swallows, however, we found that the frequency of brood parasitism was not related to overall colony reproductive success. That may be because Cave Swallows have not been as strongly selected to assess potential reproductive uncertainty at a site. Cave Swallows historically used caverns, which, because of their enclosed nature, may be safer and more secure nesting sites, on average, than the sides of cliffs and canyons where Cliff Swallows nest. Cliffs are subject to falling overhangs, crumbling dirt, and severe wind and rain storms that can destroy many nests (Brown and Brown 1996). Cave Swallows may also be less likely to parasitize nests because they suffer from fewer ectoparasites than Cliff Swallows, in which parasitism often seems to be an attempt to place eggs into other nests in a colony that are relatively uninfested (Brown and Brown 1991). We found that Cave Swallows, unlike in Cliff Swallows in Nebraska, did not preferentially parasitize nests that were more likely to fledge young-meaning that when they do parasitize nests, Cave Swallows seem unable to accurately assess the best conspecifics to parasitize.

We found evidence that both Petrochelidon swallows in the south Texas study area parasitized nests by physically transferring eggs between nests. Although apparently occurring occasionally in a few other species (Truslow 1967, Blomme 1983, Trost and Webb 1986), only in Cliff Swallows in Nebraska is that behavior known to be regular (Brown and Brown 1988). At least 6% of nests there were found to contain eggs transferred into them. Egg transfer was apparently less frequent in Texas than in Nebraska (though it is never a common behavior), which complicates quantitative comparison between the two areas. Perhaps the lower frequency in Texas (if real) may be brought about by the greater nest spacing there, especially among Cave Swallows. Cliff Swallows in Nebraska transfer eggs only to nests that are nearby, and larger distances between neighboring nests may discourage birds from engaging in such risky behavior (i.e. the egg may break or be dropped during flight).

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