Stephen I. Rothstein

Conserving migratory land birds in the New World: Do we know enough?

Migratory bird needs must be met during four phases of the year: breeding season, fall migration, wintering, and spring migration; thus, management may be needed during all four phases. The bulk of research and management has focused on the breeding season, although several issues remain unsettled, including the spatial extent of habitat influences on fitness and the importance of habitat on the breeding grounds used after breeding. Although detailed investigations have shed light on the ecology and population dynamics of a few avian species, knowledge is sketchy for most species. Replication of comprehensive studies is needed for multiple species across a range of areas, Information deficiencies are even greater during the wintering season, when birds require sites that provide security and food resources needed for survival and developing nutrient reserves for spring migration and, possibly, reproduction. Research is needed on many species simply to identify geographic distributions, wintering sites, habitat use, and basic ecology. Studies are complicated, however, by the mobility of birds and by sexual segregation during winter. Stable-isotope methodology has offered an opportunity to identify linkages between breeding and wintering sites, which facilitates understanding the complete annual cycle of birds. The twice-annual migrations are the poorest-understood events in a birds life. Migration has always been a risky undertaking, with such anthropogenic features as tall buildings, towers, and wind generators adding to the risk. Species such as woodland specialists migrating through eastern North America have numerous options for pausing during migration to replenish nutrients, but some species depend on limited stopover locations. Research needs for migration include identifying pathways and timetables of migration, quality and distribution of habitats, threats posed by towers and other tall structures, and any bottlenecks for migration. Issues such as human population growth, acid deposition, climate change, and exotic diseases are global concerns with uncertain consequences to migratory birds and even less-certain remedies. Despite enormous gaps in our understanding of these birds, research, much of it occurring in the past 30 years, has provided sufficient information to make intelligent conservation efforts but needs to expand to handle future challenges.

Recent advances in understanding migration systems of New World land birds

Our understanding of migratory birds year-round ecology and evolution remains patchy despite recent fundamental advances. Periodic reviews focus future research and inform conservation and management; here, we take advantage of our combined experiences working on Western Hemisphere avian migration systems to highlight recent lessons and critical gaps in knowledge. Among topics discussed are: (1) The pipeline from pure to applied researchers leaves room for improvement. (2) Population limitation and regulation includes both seasonal and between-season interactions. (3) The study of movements of small-bodied species remains a major research frontier. (4) We must increase our understanding of population connectivity. (5) With few exceptions, population regulation has barely been investigated. (6) We have increasingly integrated landscape configuration of habitats, large-scale habitat disturbances, and habitat quality impacts into models of seasonal and overall demographic success. (7) The post-breeding seas...

Mechanisms of avian egg recognition: Which egg parameters elicit responses by rejecter species?

SummarySome species of North American passerines nearly always reject nonmimetic eggs placed in their nests and have apparently evolved this behavior in response to brood parasitism. Experiments presented here examined the specific egg parameters to which ‘rejecter species’ respond, the relative tolerances rejecters show towards nonmimetic eggs and the degree to which rejection is limited to eggs of the brown-headed cowbird (Molothrus ater), the only parasitic bird widespread in North America.Relative to cowbird eggs, American robin (Turdus migratorius) eggs are larger, blue rather than white and immaculate rather than spotted. Experiments using 10 egg models at 137 nests showed that robins respond to each of these differences (Figs. 2 and 3) but do not usually reject an egg that deviates from their own by only one difference. Eggs that differ in ant two of the three parameters are usually rejected. This built-in tolerance reduces the likelihood that robins will reject their own eggs if these are atypical in size or coloration.Small egg size was the most important parameter eliciting rapid rejections (i.e. within 1 day), probably because differnces in size can be detected by both visual and tactile perception. By contrast, small egg size was the least important parameter determining whether eggs were eventually rejected (i.e. within 5 days, Tables 2 and 3). In terms of their eventual response, robins may be more sensitive to egg coloration than to size because the latter parameter is less reliable in distinguishing between robin and cowbird eggs.Experiments were also carried out at 37 nests of the gray catbird (Dumetella carolinensis), whose immaculate blue-green egg is only slightly larger than a cowbird egg. Catbirds are much more responsive to white ground color than to maculation (Table 4), perhaps because color is more reliable in distinguishing between catbird and cowbird eggs.Rejecter species exhibit degrees of tolerance towards foreign eggs that are proportional to the divergence between their eggs and those of the cowbird. Birds with eggs strongly divergent from cowbird eggs benefit from being relatively tolerant because they avoid rejecting their own eggs but still act against cowbird eggs. Species with cowbird-like eggs must be relatively intolerant to maximize the chances that cowbird eggs are rejected.Experiments show that rejection is not specific to cowbird eggs. Thus, birds have apparently responded evolutionarily to brood parasitism by developing recognition of their own eggs, rather than by developing recognition and rejection specific to parasitic eggs.

Nest desertion and cowbird parasitism: evidence for evolved responses and evolutionary lag ☆

Nest desertion with subsequent renesting is a frequently cited response to parasitism by the brown-headed cowbird, Molothrus ater, yet the role of desertion as an antiparasite defence is widely debated. To determine whether desertion represents an evolutionary response to brown-headed cowbird parasitism, we searched the primary literature, yielding data on the desertion frequencies of 60 host populations from 35 species. Species were categorized according to three habitat types (forest, intermediate and nonforest). Because cowbirds prefer open habitat and rarely penetrate deeply into forests, nonforest species have long been exposed to widespread cowbird parasitism, whereas forest species have not. However, due to increased forest fragmentation, forest species are being increasingly exposed to extensive parasitism. The frequency of desertion of parasitized nests was significantly higher in nonforest than forest species, suggesting that the latter experience evolutionary lag. We also considered whether desertion is affected by predation frequency, degree of current or recent sympatry with cowbirds, parasitism frequency, length of host laying season, phylogenetic relationships, and potential cost of cowbird parasitism. None of these variables created biases that could account for the observed difference in desertion frequencies of nonforest and forest species. However, species that incur large costs when parasitized had higher desertion rates among nonforest species but not among forest species. These results indicate that increased nest desertion is an evolved response to cowbird parasitism, as one would otherwise expect no relationship between desertion frequency and thezx costs and length of exposure to cowbird parasitism. Although nearly all hosts have eggs easily distinguished from cowbird eggs, few or none desert in response to cowbird eggs. Instead, desertion may be a response to adult cowbirds. The scarcity of species that desert in response to cowbird eggs suggests that egg recognition is more difficult to evolve than heightened desertion tendencies and that egg recognition quickly leads to ejection behaviour once it does develop. Copyright 2000 The Association for the Study of Animal Behaviour.

Social Dominance, Mating and Spacing Systems, Female Fecundity, and Vocal Dialects in Captive and Free-Ranging Brown-Headed Cowbirds

The expression of strong parental care is one of the reasons birds attract so much attention from both laymen and biologists. Given this attention, it is not surprising that the few bird species that exhibit little or no parental care have become especially noteworthy. In the case of the widespread and brood-parasitic Brown-headed Cowbird (Molothrus ater), much of the notoriety has been combined with loathing, because not only does the cowbird not care for its young but its breeding activities usually result in the death of some or all of the “innocent” young of its hosts. Dawson (1923, p. 77), for example, denounced the female cowbird as “destitute of all natural affection...” and as “... the unchaste mother of a race gone wrong, an enemy of bird society, [and] a blight upon the flower of Progress.”

Signals of status in wintering white-crowned sparrows, Zonotrichia leucophrys gambelii

The possibility of plumage status signalling within the social systems of wintering birds has been a controversial issue. Our results are the first to demonstrate conclusively the reality of such signalling. Data from eight groups of captive white-crowned sparrows (Zonotrichia leucophrys gambelii), each with 8 to 11 different individuals, show that immature and adult females with crowns painted to resemble more brightly coloured, dominant adult males consistently win encounters with control birds of their own age and sex. These experiments demonstrate that signals that correlate with age (adult versus immature) and sex (adult male versus adult female) are used by the birds as reliable indicators of relative dominance position. Our demonstration of status signalling draws attention to the need to explain how such a system can be evolutionarily stable and we discuss some suitable models.

Mechanisms of Avian Egg Recognition: Possible Learned and Innate Factors

THE belief that at least some passerine birds reject nonmimetic eggs placed in their nests was confirmed long ago by the experiments of Swynnerton (1916, 1918) and Rensch (1924). The latter worker (Rensch 1925) went on to question whether such birds actually recognize their own eggs (rejection by true egg recognition) or whether they simply reject any egg that differs from the majority (rejection by discordancy). Rensch was interested in egg recognition because he believed it to be vital to the evolutionary interactions between brood parasites and their hosts. Whether birds rejected eggs on the basis of true recognition or discordancy, the behavior would still function as an efficient antiparasite adaptation because brood parasites generally deposit only one egg in each host nest. Although Renschs (1925) experiments have been widely interpreted as demonstrating rejection by discordancy (e.g. Welty 1963), the bulk of his results actually indicate true egg recognition (Rothstein 1970). Numerous recent experiments and a literature review suggest that most or all passerines that reject foreign eggs practice true egg recognition (Rothstein 1974). Nearly all these recent experiments, though, were conducted on birds that had completed their clutch. The fact that birds that reject foreign eggs practice true egg recognition after completing their clutch prompts two significant questions: (1) Is this recognition as highly developed at earlier stages of the breeding cycle? (2) Which components of rejection by true egg recognition are primarily learned and which are primarily innate? This paper reports on experiments that were designed to deal with these two questions. The new experiments reported here were done on naturally breeding Gray Catbirds (Dumetella carolinensis). Previous experiments showed that catbirds are extremely intolerant of foreign eggs placed in their nests. Single artificial or real cowbird eggs experimentally added to 30 catbird nests in eastern North America (Connecticut, Maryland, and Michigan) were all removed by the catbirds. Catbirds in western North America (Manitoba and Nebraska) may be slightly more tolerant, with 22 of 25 nests yielding ejections of single cowbird eggs. In 17 additional catbird nests (from Connecticut and Maryland), where experiments resulted in clutches consisting of only cowbird eggs or of only one catbird egg and two or more cowbird or other type of foreign eggs, the catbirds ejected the foreign eggs in every instance (see Rothstein 1974 for details).

Mechanisms of avian egg-recognition: Do birds know their own eggs?

Abstract Egg-recognition in birds is generally thought to be weakly expressed since many birds accept a wide diversity of foreign eggs placed in their nests. It has long been believed that even those birds that generally reject foreign eggs (rejector-species) do not practise actual recognition of their own eggs but simply reject any egg-type in the minority (discordant). However, this belief is founded on conflicting evidence. The results from a large number of new experiments reveal that rejector-species do indeed recognize their own eggs. If rejector-species show intolerance toward any type of egg, they reject the foreign eggs whether these are as numerous as their own eggs, outnumber their own eggs, or are the only egg-type present. These and other results are analysed in the context of the evolutionary pressures exerted on birds by brood parasites.

Relic behaviours, coevolution and the retention versus loss of host defences after episodes of avian brood parasitism.

Most previous studies of brood parasitism have stressed that host defences, such as egg recognition, are lost in the absence of parasitism. Such losses could result in coevolutionary cycles in which parasites shift away from well-defended hosts only to switch back to them later at a time when these hosts have lost much or all of their defences and the parasites current hosts have built up effective defences. However, the alternative single trajectory model predicts that parasites rarely switch back to old hosts because ex-hosts retain egg recognition for long periods in the absence of parasitism. If true, egg recognition by the host may be a relic behaviour, because in the absence of parasitism its adaptive value is close to neutral. Using artificial nonmimetic eggs, I tested for egg recognition in two populations that are currently unparasitized but that are descended from lineages likely to have been parasitized in the past: the grey catbird, Dumetella carolinensis, on Bermuda and the loggerhead shrike, Lanius ludovicianus, in California. Both of these populations showed long-term retention, ejecting nonmimetic eggs at rates of nearly 100%. Because potential present-day selection pressures, such as conspecific parasitism, do not explain this egg recognition, Bermuda catbirds apparently retain recognition from North American conspecifics that were cowbird hosts before colonizing Bermuda and shrikes retain recognition from Old World congeners that were hosts of cuckoos. Retention is also indicated by passerines in California and the Caribbean that had high rejection rates of nonmimetic eggs before coming into contact with cowbirds. These new data suggest that both the coevolutionary cycles and single trajectory models have importance and that rejection behaviour can have insignificant costs, which is consistent with evolutionary lag explanations for the acceptance of parasitic eggs shown by some cuckoo and many cowbird hosts. Copyright 2001 The Association for the Study of Animal Behaviour.

The agonistic and sexual functions of vocalizations of male brown-headed cowbirds, Molothrus ater

Functional analyses of three vocalizations of male brown-headed cowbirds, Molothrus ater, are presented and it is shown that two of these function as songs. Perched song (PS) is given by stationary birds and is used over short distances in male-male aggression and female courtship. The ‘flight whistle’ (FW) is a multisyllabic, usually pure tone, vocalization and the ‘single syllable flight call’ (SS) a monosyllabic pure tone. FWs and SSs are given occasionally by perched males and repetitively by flying males. Field playbacks of FWs showed that male cowbirds responded in the agonistic way most passerines respond to playback of conspecific song. Playbacks of the females chatter call demonstrated that males of all three subspecies use FWs and SSs, but almost never PSs, to communicate with a female as they approach her. Thus FWs and SSs are used to communicate over long distances with both males and females. FWs are also used in one critical short-distance context as they are significantly more likely than PSs to be given just before copulations in nature, at least in our western study area. By contrast, studies of captive eastern cowbirds showed that PSs regularly precede copulations whereas FWs rarely do so. These different results for western and eastern birds may be an artefact of captivity and/or a result of geographical variation in behaviour.