Female Warblers Research Articles

A Phenotypic Test of Haldane's Rule in an Avian Hybrid Zone

AbstractWe introduce a phenotypic method to test for excess mortality in hybrids of the heterogametic sex, as expected from Haldane's rule, and apply this method to the unusually narrow hybrid zones between Hermit Warblers (Dendroica occidentalis) and Townsend's Warblers (D. townsendi) in the Pacific Northwest. Our test requires establishing comparable hybrid indices for male and female warblers. The hybrid index that we developed for females produced age-corrected distributions for phenotypically pure reference samples that closely matched those used by Rohwer and Wood (1998) for males. The similarity in these distributions enabled us to compare the relative frequency of males and females in hybrids and parentals. We detected no deficiency of hybrid females and thus no inviability in the heterogametic sex. Our failure to find evidence of the inviability component of Haldane's rule is not unexpected given the close relationship between these taxa; nonetheless, our methods should be generally useful for studies of hybrid zones.

Influence of Niche Overlap and Territoriality on Hybridization Between Hermit Warblers and Townsend's Warblers

AbstractHybrid zones between Townsend's Warblers (Dendroica townsendi) and Hermit Warblers (D. occidentalis) in the Pacific Northwest are narrow relative to estimated dispersal distances and appear to be moving, with Townsend's replacing Hermits. We examined whether the habitat-transition and parental-fitness asymmetry models can explain why these zones are narrow and moving by comparing habitat variables associated with warbler territories in the Washington Cascades hybrid zone. Habitat variables did not differ among phenotypes, suggesting that the habitat-transition model cannot explain the narrow and dynamic nature of this hybrid zone. Habitat characteristics of Hermit Warbler territories did not differ inside versus outside the hybrid zone, also suggesting that this zone is not associated with a region of habitat transition. The lack of difference in habitat use could be the result of comparing variables that are not important to pairing success. However, warblers tended to select territories on west-southwest aspects. South aspects in the southern Washington Cascades are dominated by Douglas fir (Pseudotsuga menziesii) and true fir, which is a habitat selected by female warblers when choosing among territories and males. The parental-fitness asymmetry model does not necessarily make predictions about habitat use within the hybrid zone but predicts the superiority of one parental species over the other. However, if significant overlap occurs in habitat use or niche (as in these warblers), then competition between parental species is likely to occur. To determine whether these species compete, we mapped 12 warbler territories and monitored an additional 94 territories throughout the breeding season and found that all males with neighbors compete for and hold exclusive territories. Thus, the pattern of habitat use and territoriality is consistent with the parental-fitness asymmetry model.

A PHENOTYPIC TEST OF HALDANE'S RULE IN AN AVIAN HYBRID ZONE

Abstract We introduce a phenotypic method to test for excess mortality in hybrids of the heterogametic sex, as expected from Haldane's rule, and apply this method to the unusually narrow hybrid zones between Hermit Warblers (Dendroica occidentalis) and Townsend's Warblers (D. townsendi) in the Pacific Northwest. Our test requires establishing comparable hybrid indices for male and female warblers. The hybrid index that we developed for females produced age-corrected distributions for phenotypically pure reference samples that closely matched those used by Rohwer and Wood (1998) for males. The similarity in these distributions enabled us to compare the relative frequency of males and females in hybrids and parentals. We detected no deficiency of hybrid females and thus no inviability in the heterogametic sex. Our failure to find evidence of the inviability component of Haldane's rule is not unexpected given the close relationship between these taxa; nonetheless, our methods should be generally useful for s...

Predation risk affects trade-off between nest guarding and foraging in Seychelles warblers

The fitness costs of egg loss for Seychelles warblers (Acrocephalus sechellensis) on Cousin Island are considerable because warblers have a single-egg clutch and no time to lay a successful replacement clutch. On the islands of Cousin and Cousine, with equal densities of Seychelles fodies (Foudia sechellarum), nearly 75% of artificial eggs placed in artificial nests were predated by fodies after 3 days. On Aride Island with no fodies present, loss of artificial eggs was not observed. Female warblers incubate the clutch, and male warblers guard the clutch when females are absent. Deterrence of fodies by male warblers is efficient: loss rate of eggs from unattended warbler nests was seven times as high as from attended nests, and the more nest guarding, the lower the egg loss and the higher the hatching success. Egg loss is independent of the amount of incubation by females. There is no trade-off between incubating and foraging by females. Nest guarding competes with foraging by males, and this trade-off has a more pronounced effect on egg loss when food availability is low The transfer of breeding pairs from Cousin to either Cousine with egg-predating fodies or to Aride without fodies allowed us ro experimentally investigate the presumed trade-off between nest guarding and foraging. On Cousine, individual males spent the same amount of time nest guarding and foraging as on Cousin, and egg loss was similar and inversely related to rime spent nest guarding as on Cousin. Males that guarded their clutch on Cousin did not guard the clutch on Aride bur allocated significantly more time to foraging and gained better body condition. Loss of warbler eggs on Aride was not observed. Time allocation to incubating and for-aging by individual females before and after both translocations remained the same.

Brood sex ratio is dependent on female mating status in polygynous great reed warblers

Females capable of adjusting the sex ratio of their offspring should be more fit than females lacking such an ability. In polygynous birds where breeding success in males is more strongly influenced by body size and/or attractiveness than in females, females might produce more sons when predicting good conditions or when mating with attractive males. Polygynous great reed warbler, Acrocephalusarundinaceus, males direct most of their feeding effort to the primary (first-hatching) nest and in these nests increase their feeding effort in relation to the brood sex ratio (proportion of sons). Therefore, with the expectation of well-nourished sons, we would predict that females which start breeding first within harems might produce more sons than those which start breeding later, and in anticipation of sons with good genes, that females mated to polygynous males might produce more sons than females mated to monogamous males. I took blood samples from hatchlings and determined the sex using DNA markers. The sex ratio of primary (monogamous and polygynous primary) broods is more male-biased (mean 0.58 males, n = 50) than that of secondary (polygynous secondary and tertiary) broods (mean 0.46, n = 25). Moreover, in the secondary broods with the largest clutch (five eggs), in which offspring are most likely to suffer food shortage, the sex ratio was distinctively female biased (mean 0.33, n = 10). In the primary broods, sex ratio was correlated to harem size. The results suggest that great reed warbler females modify the brood sex ratio to produce both well-nourished sons and sons with good genes, but the former effect is probably stronger than the latter factor.

HIGHER FITNESS FOR PHILOPATRIC THAN FOR IMMIGRANT MALES IN A SEMI-ISOLATED POPULATION OF GREAT REED WARBLERS.

To compare the fitness of philopatric and immigrant individuals we examined the lifetime reproductive success of 116 male and 137 female great reed warblers. The study was carried out in a semi-isolated population in Sweden and covered breeding adults hatched between 1985 and 1993. Lifetime fitness, measured as life time number of fledglings and offspring recruits, was lower for immigrant than for philopatric males. We found no such relationships for females. The difference in reproductive success could not be explained by immigrant males having lower phenotypic quality because they had similar life span, spring arrival date, and territory quality as philopatric males. The lower lifetime fitness among immigrant than philopatric males appeared to result from reduced mating success. This suggests that females are reluctant to mate with immigrant males despite their apparently similar phenotypic quality. Though it is not known whether females gain in fitness by avoiding matings with immigrant males, it is notable that immigrant males have smaller song repertoires than philopatric males. Large repertoires, previously shown to sexually arouse great reed warbler females, correlate with the occurrence of extrapair paternity and postfledging survival of offspring in our population.

Burial of cowbird eggs by parasitized yellow warblers: an empirical and experimental study

The responses of yellow warblers,Dendroica petechia, to naturally laid and experimentally introduced eggs of the brown-headed cowbird,Molothrus ater, were recorded. Some female warblers accepted naturally laid cowbird eggs. Rejected eggs were usually buried, sometimes along with warbler egg(s). Yellow warblers more frequently buried cowbird eggs that were laid in their nests before their own clutches were initiated through the day on which they laid their second eggs. Warblers accepted most cowbird eggs laid in their nests later in the laying cycle and during incubation. Warblers were more likely to reject parasitic eggs when they occurred during the first half of the clutch-initiation period. Cowbirds parasitized yearling and older female yellow warblers with equal frequency and rejection was not related to the age of the female warbler. Yellow warblers responded differently at nests parasitized experimentally with real cowbird eggs. At about 25% of the nests, warblers buried cowbird eggs added before clutch initiation. However, nearly all cowbird eggs added during the warbler's laying cycle were accepted, regardless of whether (1) a warbler egg was removed at the time of parasitism, (2) clutches were reduced, or (3) hosts were exposed to a stuffed female cowbird before being parasitized. The dichotomy of responses yellow warblers exhibit to cowbird parasitism may be because of the high cost of rejection. Selection for rejection/acceptance may oscillate over time as costs and benefits of each strategy vary with different environmental conditions.

Symposium: Brood parasitism

Brown-headed Cowbirds Molothrus ater are very social birds. Males have a rich vocal repertoire, and much of their social behavior is mediated by vocalizations. For example, subtle differences in song reflect dominance status and are used by females during mate choice. Differences in song result from feedback from social interactions, and, in the absence of normal social feedback, subordinate males may produce inappropriate dominant-like songs (1) and above-normal testosterone levels (2). Another vocalization, the flight whistle, is equally interesting. It occurs in dialects, and can be used in long distance communication. In some populations parts of the flight whistle are produced antiphonally, probably to designate specific recipients. While the importance of intraspecific vocalizations to cowbird biology is well-established, surprisingly little attention has been given to the possibility that cowbirds also may attend to host vocalizations, which could act as one cue in locating nests. Given that the most frequently parasitized cowbird hosts are highly vocal oscines, it is reasonable that cowbirds could use host vocalizations to discover nests (cf. 3). Preliminary data suggest that, in one host species, median vocal rates are higher in parasitized than in unparasitized pairs (UYEHaRA, unpubl.). Further, some birds produce vocalizations that are associated with nest-building, and cowbirds may use these to detect nests. For example, female Hooded Warblers Wilsonia citrina produce a particular vocalization during nest construction. Hooded Warbler nests were heavily (20/25 nests) and selectively parasitized in one songbird community, probably due to the parasite's use of the host's nest-building vocalization (STuTCHBUt~Y, unpubl.). Other females have nest departure calls (4) that could also be used to discover nests. Another host vocalization that could be used to locate nests are nestling distress calls. These are produced when nestlings are in grave and immediate danger, such as when in the grasp of a predator. Female M. ater quickly approach playback of tape-recordings of distress calls, and often are the only species to respond. Approaching cowbirds do not mob, nor do they vocalize. Instead, they peer quietly and intently in the direction of the playback. This may represent an attempt by female cowbirds to determine the general location of a nest that is about to be lost, and it may alert her to a territory where nest building activity (and a new opportunity for brood parasitism) may occur within a few days. Thus, it represents a new "function" for distress calls. Additional study of the cowbird use of host vocalizations is warranted.

Adaptive tolerance of cowbird parasitism by prothonotary warblers: A consequence of nest-site limitation?

In a study of prothonotary warbler, Protonotaria citrea, breeding ecology in which nest-site densities were manipulated, 21% of 172 warbler nests were parasitized by cowbirds, Molothrus ater; a relatively high frequency of parasitism compared to the majority of hole-nesting species. Cowbirds reduced warbler nest success by approximately 25%, primarily through egg removal by female cowbirds and lowered hatching success of warbler eggs. However, the impact of cowbird parasitism on warbler nesting success was relatively low compared to that reported for the majority of host species. Although female warblers accepted parasitism in 81% of cases, parasitized nests were abandoned significantly more frequently than were unparasitized nests, with the probability of nest desertion being contingent upon the numbers of nest-sites available on the territory. Similarly, the overall tendency for females to renest following either a successful or, especially, a failed nest was related to nest-site availablity. Timing of parasitism in relation to stage of the nest or time of the season did not significantly influence nest desertion by warblers. Thus, prothonotary warbler females may usually be forced to accept cowbird parasitism as a consequence of nest-site limitation. Relatively low costs associated with accepting cowbird eggs may be chosen by warblers only as an alternative to not breeding. These reuults support the hypothesis that acceptance of parasitism may be adaptive when the costs of rejection outweigh the costs of acceptance of parasite eggs.

Female Bay-Breasted Warbler in Male Plumage

Female Bay-Breasted Warbler in Male Plumage

A Male Golden-Winged Warbler (Helminthophila chrysoptera) Mated with a Female Blue-Winged Warbler (Helminthophila pinus) at Bethel, Conn

A Male Golden-Winged Warbler (Helminthophila chrysoptera) Mated with a Female Blue-Winged Warbler (Helminthophila pinus) at Bethel, Conn