Sievert Rohwer

THE SOCIAL SIGNIFICANCE OF AVIAN WINTER PLUMAGE VARIABILITY

The drive system includes a motor or engine whose output shaft is connected to the input shaft of a transmission, e.g. speed reducing gear, via an elastic coupling and a clutch, in that order. The input element of the coupling sits directly on the motor or engine shaft; the output element (ring) of the coupling is suspended in cardan fashion, possibly together with the input element of the clutch, by means of an additional elastic coupling which has one part secured to the clutch input and the other part is connected either to a bell shaped structure which is journalled in the transmission casing or to the output element of the motor coupled coupling. The two parts of the additional coupling are interconnected through annularly arranged elastic sleeves. The bell shaped structure can be used in either case, and extends from a hollow shaft being traversed by the transmission shaft and driving auxiliary equipment independantly from the operational state of the clutch.

Status signalling in harris sparrows: Experimental deceptions achieved

Harris sparrows (Zonotrichia querula) signal their social dominance by variations in the extent of black feathering on their throats and crowns. Earlier deception experiments showed sub-ordinates dyed blacker to be socially persecuted. Two explanations of this behaviour are here distinguished: (1) that cheating on the signalling system is socially controlled and (2) that the persecution resulted from an incongruence between signal and behaviour. The incongruence hypothesis is supported. Subordinates that were both dyed and administered testosterone show dramatic increases in social status; thus the evolutionary stability of the signalling system seems not to depend upon social control of cheating. A new hypothesis to explain the evolutionary stability of subordinance in this system is proposed: dominants and subordinates are adapted, respectively, to defensible and indefensible resource bases, and equal fitness for them is achieved through frequency-dependent selection.

Status Signaling in Harris Sparrows: Some Experiments in Deception

(1) Harris Sparrows signal their social dominance status by variations in the amount of black feathering on their crowns and throat. The potential ability of individuals to cheat on such a status signaling system was experimentally investigated by dyeing and bleaching some free-ranging individuals in a flock attracted to a bait station. As discussed in detail, cheating is a theoretically important problem because survival studies have shown subordinates to be much less likely to survive over winter than dominants. If true, this should result in strong directional selection toward the dominant plumage type. (2) Subordinates dyed to mimic the highest ranking birds of the winter hierarchy were, with but one exception, persecuted by the legitimate studlies. The single exception was apparently a bird which had underestimated his status during the fall molt when the appropriate plumage signal is produced. (3) Bleached birds were forced to fight much more for their status, as dramati. cally shown by what was likely the first encounter between a bleached bird and a normal individual of just slightly lower rank. Eventually the bleached birds behaved as though all others around them were perceived as disrespectful, and despotically attacked birds at an abnormally high rate. (4) While these data strongly suggest that cheating is socially controlled, such a social organization raises some complex evolutionary issues. An alternative explanation of the apparent social control of cheating is that the unusual behavior observed in response to my manipulations merely represented normal defense of winter resources. This explanation requires that the rate of winter dominance inter-actions be controlled by (1) the risk of attack, and (2) the importance of resource defense. If this is true, the dyed birds may have appeared sick since no hormonal manipulation accompanied their dyeing; thus they may have been attacked more because perceived risk was down and potential gain high (beat the toughy while he is down). Likewise, the bleached birds may have become unusually aggressive because the closer approach of low ranked birds suggested to them that a much more active defense of resources was required. Experiments distinguishing these hypothesis have not yet been performed.

Delayed Maturation in Passerine Plumages and the Deceptive Acquisition of Resources

We propose that the female-like plumage worn by some male birds in their first potential breeding season has evolved to facilitate breeding when 1-yr old through the deception of older males. By mimicking females 1-yr-old males exploit the tendency of old males not to attack females and, thus, are able to enter better quality habitats. Once subadults settle in such habitats, they hold territories in them by site dominance. An hypothesis invoking deception seems necessary because the cryptic hypothesis, which suggests that subadults are hiding from predators, cannot explain why subadult males of sexually dichromatic species always resemble females in every way by which they differ from older males. If the cryptic hypothesis were sufficient, subadult males should resemble juveniles in species for which the juvenile plumage is more cryptic than that of breeding females. Female mimicry should evolve (1) when competition for breeding resources is rigged against 1-yr-old males, (2) when obtaining a mate depends upon holding female-worthy resources, and (3) when females are limiting to males. Fitness set models suggest that individual variation in subadult appearance is evolutionarily stable only when broad gradients are established between high and low quality habitats. Using North American passerine birds, we have confirmed the following predictions from this theory. (1) Spring arrival times for 1-yr-old and older males should be nonoverlapping in species with subadult plumages. (2) Where a subadult plumage exists 1-yr-old males should more often be good mimics of females than intermediate in appearance. (3) Female limitation will favor the evolution of female mimicry in 1-yr-old males. (4) High variance in habitat quality favors subadult plumages; this holds both within and between species. (5) When a habitat gradient is abrupt and there is enough optimal breeding habitat, subadults must be better female mimics than when the gradient from good to poor habitat is gradual.

The Evolution of Conspicuous and Distinctive Coloration for Communication in Birds

Our goal is to elucidate the selective pressures that account for the origin and maintenance of conspicuous and distinctive coloration (hereafter referred to collectively as colorfulness) in birds. Our ideas are relevant to the study of other animals as well. We propose an “adaptationist program” (Gould and Lewontin, 1979) that features the use of the comparative method (Hailman, 1976; Ridley, 1983; Clutton-Brock and Harvey, 1984), color manipulations, and tests of a priori predictions. Our major effort is to compare and contrast the major hypotheses that have been proposed to account for colorfulness in birds. Progress in understanding bird coloration will come more quickly when researchers evaluate more than one hypothesis at a time. The availability of multiple hypotheses serves two major purposes. First, it frees the researcher from psychological dependence on the hypotheses being tested (Loehle, 1987). Second, with careful planning, it permits strong inference testing (Platt, 1964) among the most relevant hypotheses.

Evolutionary lag versus bill-size constraints: a comparative study of the acceptance of cowbird eggs by old hosts

SummaryEvolutionary lag and resistance to puncture-ejection are two hypotheses explaining why hosts long exposed to brood parasitism by molothrine cowbirds accept their eggs. The former hypothesis assumes no physical constraints while the latter requires them. We show (1) that most acceptors of Brown-headed Cowbird eggs have small bills, (2) that the single species known to puncture cowbird eggs for ejection also has a small bill, and (3) that all grasp ejectors of Brown-headed Cowbird eggs have large bills. We propose that most acceptors cannot remove cowbird eggs by grasp ejection because their bills are too small. Small hosts probably cannot puncture cowbird eggs for ejection because their unusually thick shells make the costs of puncturing them higher than the benefits of ejection. This hypothesis is capable of explaining the success of cowbirds with scores of host species in the puzzling absence of egg mimicry.

PLUMAGE AND MITOCHONDRIAL DNA HAPLOTYPE VARIATION ACROSS A MOVING HYBRID ZONE

We analyze variation in phenotypes and mitochondrial DNA (mtDNA) haplotypes over the breeding ranges of hermit and Townsends warblers and across two of their three hybrid zones. Within these two hybrid zones, we demonstrate that the placement, shape, and width of transitions in seven plumage characters are remarkably similar, suggesting that a balance between dispersal and sexual selection keeps these hybrid zones narrow. A consistent asymmetry in these character transition curves suggests that Townsends warblers have a selective advantage over hermit warblers, which is presumably due to the aggressive superiority of Townsends over hermit males (Pearson and Rohwer 2000). An association between plumage and mtDNA haplotypes shows that pure Townsends warblers, but not pure hermit warblers, immigrate into these hybrid zones, further supporting the competitive superiority of Townsends warblers over hermit warblers. The mitochondrial haplotype transitions across these hybrid zones are much wider than the phenotypic transitions and provide no indication that the mtDNA haplotypes representing these two warblers are selectively maintained. More importantly, the phenotypically pure populations of Townsends warblers throughout a 2000‐km coastal strip north of the Washington hybrid zones contain a preponderance of hermit warbler mtDNA haplotypes. This result suggests massive movement of the hybrid zone between these warblers during the 5000 years since their most recent interglacial contact. We develop a model to explain the phenotypic and genetic divergence between these warblers and the evolution of their dramatic differences in aggressiveness; we also show how differences in male aggression, in combination with biased pairing patterns, can explain the haplotype footprint recording the historical movement of this hybrid zone.

A Comparative Study of Eggshell Thickness in Cowbirds and Other Passerines

We directly measured the eggshell thickness of the three brood parasitic Molothrus cowbirds, 17 other icterids, and 13 additional passerines. By correcting these shell thickness measurements for variation attributable to interspecific differences in egg volume, we show that the Molothrus cowbirds lay eggs with shells that are 30% thicker than expected for their size. Our samples of nonparasitic icterids and of other passerines do not differ significantly in shell thickness values corrected for differences in egg volume. We evaluate two hypotheses for the evolution of unusually thick-shelled eggs in cowbirds. The first, an old idea proposed for parasitic cuckoos, is that thick shells resist damage to the parasites egg at laying. From indirect tests we could find little or no evidence that the thin-shelled eggs of the hosts of cowbirds were damaged by impact when cowbird eggs were laid in their nests; thus, we tentatively conclude that resistance to laying damage has not been critical in favoring the evolution of thick shells in cowbird eggs. As an alternative hypothesis we propose that thick shells have evolved in cowbird eggs to resist puncture ejections by hosts that are too small to grasp whole cowbird eggs for ejection. We show that a puncture specialist, the Marsh Wren (Cistothorus palustris), has greater difficulty puncturing cowbird eggs than the thinner-shelled eggs of various other passerines. Implications of our hypothesis concerning resistance to puncture ejections are discussed.

Allometry of the duration of flight feather molt in birds.

Replacement of flight feathers takes disproportionately more time for large birds than it does for small birds, because feather length increases with body size almost twice as fast as feather growth rate increases.

Complex biogeographic history of a Holarctic passerine.

Our analysis of the ND2 sequences revealed six clades within winter wrens (Troglodytes troglodytes). These clades corresponded to six geographical regions: western Nearctic, eastern Nearctic, eastern Asia, Nepal, Caucasus and Europe, and differed by 3–8.8% of sequence divergence. Differences among regions explained 96% of the sequence variation in winter wren. Differences among individuals within localities explained 3% of the sequence variation, and differences among localities within regions explained 1%. Grouping sequences into subspecies instead of localities did not change these proportions. Proliferation of the six clades coincided with Early and Middle Pleistocene glaciations. The distribution of winter wren clades can be explained by a series of five consecutive vicariant events. Western Nearctic wrens diverged from the Holarctic ancestor 1.6 Myr before the present time (MYBP). Eastern Nearctic and Palaearctic wrens diverged 1 MYBP. Eastern and western Palaearctic birds diverged 0.83 MYBP. Nepalese and east Asian wrens diverged 0.67 MYBP, and Caucasian birds diverged from European wrens 0.54 MYBP. The winter wren has a much greater degree of inter– and intracontinental differentiation than the three other Holarctic birds studied to date—dunlin (Calidris alpina), common raven (Corvus corax) and three–toed woodpecker (Picoides trydactylus)—and represents an example of cryptic speciation that has been overlooked.