Strong evidence supporting a relationship between colour pattern and apparent survival in common crossbills

Carotenoid staining has been repeatedly shown to serve as a sexually selected individual quality signal. In different species, individuals that show brighter carotenoid-based signals have been found to have superior feeding abilities, recover faster from disease, and generally enjoy better body condition. In the common crossbill (Loxia curvirostra), the colour has also been related to the different populations, with northern and central European populations being described as redder than those in the Mediterranean region. A study in the Pyrenees showed that long-winged individuals had lower apparent survival, and the proportion of red individuals was higher in long-winged birds, concluding that they could be nomadic birds (that travel long distances). A priori, if the red crossbills are more mobile than the yellow and orange ones, their apparent survival will be lower. However, in our study, red males showed a greater survival than males of other colours and almost double than that of the yellow ones. These results suggest that red coloration is linked to higher quality individuals regardless of their mobility.

Seasonal Patterns of Fat Deposits in Relation to Migratory Strategy in Facultative Migrants

Physiological preparations for migration generally reflect migratory strategy. Migrant birds fuel long-distance flight primarily with lipids, but carrying excess fuel is costly; thus, the amount of fat deposited prior to departure often reflects the anticipated flight duration or distance between refueling bouts. Seasonal pre-migratory deposition of fat is well documented in regular seasonal migrants, but is less described for more facultative species. We analyze fat deposits of free-living birds across several taxa of facultative migrants in the songbird subfamily Carduelinae, including house finches (Haemorhous mexicanus), American goldfinches (Spinus tristis), pine siskins (Spinus pinus) and four different North American ecotypes of red crossbills (Loxia curvirostra), to evaluate seasonal fat deposition during facultative migratory periods. Our data suggest that the extent of seasonal fat deposits corresponds with migratory tendency in these facultative taxa. Specifically, nomadic red crossbills with a seasonally predictable annual movement demonstrated relatively large seasonal fat deposits coincident with the migratory periods. In contrast, pine siskins, thought to be more variable in timing and initiation of nomadic movements, had smaller peaks in fat deposits during the migratory season, and the partial migrant American goldfinch and the resident house finch showed no peaks coincident with migratory periods. Within the red crossbills, those ecotypes that are closely associated with pine habitats showed larger peaks in fat deposits coincident with autumn migratory periods and had higher wing loading, whereas those ecotypes associated with spruces, Douglas-fir and hemlocks showed larger peaks coincident with spring migratory periods and lower wing loading. We conclude that population averages of fat deposits do reflect facultative migration strategies in these species, as well as the winter thermogenic challenges at the study locations. A difference in seasonal fattening and wing loading among red crossbill ecotypes is consistent with the possibility that they differ in their migratory biology, and we discuss these differences in light of crossbill reproductive schedules and phenologies of different conifer species.

A mitochondria-targeted antioxidant affects the carotenoid-based plumage of red crossbills

ABSTRACTThe mechanisms involved in the production of red carotenoid-based ornaments in vertebrates are still poorly understood. Those colours generated by red carotenoids often depend on the enzymatic production (ketolation) of these pigments from dietary yellow carotenoids. Recently, it has been proposed that this conversion takes place at the inner mitochondrial membrane (IMM). This implies that carotenoid ketolation and cell respiration could share the same biochemical pathways. Such a link would favour the evolution of red ketocarotenoid-based ornaments as reliable indices of individual quality under a sexual selection scenario. We exposed captive male red crossbills (Loxia curvirostra Linnaeus) to two different synthetic antioxidants designed to penetrate into the IMM: a synthetic ubiquinone (mitoQ) and a superoxide dismutase mimetic (mitoTEMPO). MitoQ decreased the blood levels of substrate yellow carotenoids and tocopherol. This could be attributed to the characteristics of the mitoQ molecule, which can distort the IMM structure, increasing free radical (superoxide) production and, potentially, antioxidant consumption. Contrarily, mitoTEMPO-treated birds increased the plasma levels of the second most abundant red ketocarotenoid of red crossbills (i.e. canthaxanthin). MitoTEMPO also increased plumage redness and total ketocarotenoid concentration in feathers among those birds exhibiting a redder plumage at the beginning of the study, rising the plasma values of the main red pigment (3-hydroxyechinenone) in paler birds. The results as a whole support the involvement of the mitochondrial antioxidant machinery in carotenoid biotransformation. The fact that the initial plumage redness determined the effect of mitoTEMPO suggests that the mitochondrial-based mechanism is intimately linked to individual quality.Summary statementAntioxidants designed to penetrate the mitochondrial membrane increased avian plumage redness but depending on pre-existing colouration. This supports mitochondrial involvement in the evolution of carotenoid-based ornaments as reliable quality signals.

Character displacement of a learned behaviour and its implications for ecological speciation

Cultural evolution may accelerate population divergence and speciation, though most support for this hypothesis is restricted to scenarios of allopatric speciation driven by random cultural drift. By contrast, the role of cultural evolution in non-allopatric speciation (i.e. speciation with gene flow) has received much less attention. One clade in which cultural evolution may have figured prominently in speciation with gene flow includes the conifer-seed-eating finches in the red crossbill ( Loxia curvirostra ) complex. Here we focus on Cassia crossbills ( Loxia sinesciuris ; an ecotype recently split taxonomically from red crossbills) that learn social contact calls from their parents. Previous work found that individuals modify their calls throughout life such that they become increasingly divergent from a closely related, sympatric red crossbill ecotype. This open-ended modification of calls could lead to character displacement if it causes population-level divergence in call structure that, in turn, reduces (maladaptive) heterospecific flocking. Heterospecific flocking is maladaptive because crossbills use public information from flockmates to assess resource quality, and feeding rates are depressed when flockmates differ in their ability to exploit a shared resource (i.e. when flockmates are heterospecifics). We confirm the predictions of character displacement by documenting substantial population-level divergence in Cassia crossbill call structure over just two decades and by using field experiments to demonstrate that Cassia and red crossbills differentially respond to these evolved differences in call structure, reducing heterospecific flock formation. Moreover, because crossbills choose mates from within flocks, a reduction in heterospecific flocking should increase assortative mating and may have been critical for speciation of Cassia crossbills in the face of ongoing gene flow in as few as 5000 years. Our results provide evidence for a largely neglected yet potentially widespread mechanism by which reproductive isolation can evolve between sympatric lineages as a byproduct of adaptive cultural evolution.

Bird ringing in Slovenia in 2015 and the occurrence of Parrot Crossbills Loxia pytyopsittacus

In 2015, 170 bird species were recorded during bird ringing activities in Slovenia. We ringed 73,371 birds belonging to 162 species, there were 132 foreign recoveries of birds ringed in Slovenia, 120 recoveries of birds ringed abroad and found in Slovenia, as well as 1964 local recoveries. The most frequently ringed species were Blackcap Sylvia atricapilla and Great Tit Parus major. In ringed nestlings, Great Tits and Tree Sparrows Passer montanus predominated. In 2015, the first preliminary ringing of Scops Owls Otus scops during migration took place, resulting in the highest number of Scops Owls ringed so far. Concerning recoveries of birds ringed in Slovenia and later recorded abroad and birds ringed abroad and later recorded in Slovenia, the commonest were Mute Swans Cygnus olor and Black-headed Gulls Chroicocephalus ridibundus. The longest-distance recovery concerned a Reed Bunting Emberiza schoeniculus found in Sweden (2,144 km away). Among the interesting finds were also the first finds of ringed Pygmy Cormorants Microcarbo pygmeus so far from breeding sites in Hungary. Among rare species, Yellow-browed Warbler Phylloscopus inornatus, Spanish Sparrow Passer hispaniolensis and a pair of Parrot Crossbills Loxia pytyopsittacus were caught and ringed, the latter for the very first time in Slovenia after more than 100 years. An overview of records of the Parrot Crossbill is given herein, as well as analysis of irruptive years of the Red Crossbills Loxia curvirostra between 1980 and 2015, when the probability of boreal Crossbill species occurrence is the highest. According to the ringers’ data, the irruptive years of Red Crossbills in Slovenia were 1984, 1985, 2007, 2008, 2010, 2011 and 2012. The article points to the probability that Parrot Crossbills have been overlooked in the past, since larger specimens of Crossbills were ringed mostly in irruptive years, but no specific bill measurements important for distinguishing between Parrot and Red Crossbill had been taken.

Assortative flocking in crossbills and implications for ecological speciation

How reproductive isolation is related to divergent natural selection is a central question in speciation. Here, we focus on several ecologically specialized taxa or ‘call types’ of red crossbills ( Loxia curvirostra complex), one of the few groups of birds providing much evidence for ecological speciation. Call types differ in bill sizes and feeding capabilities, and also differ in vocalizations, such that contact calls provide information on crossbill phenotype. We found that two call types of red crossbills were more likely to approach playbacks of their own call type than those of heterotypics, and that their propensity to approach heterotypics decreased with increasing divergence in bill size. Although call similarity also decreased with increasing divergence in bill size, comparisons of responses to familiar versus unfamiliar call types indicate that the decrease in the propensity to approach heterotypics with increasing divergence in bill size was a learned response, and not a by-product of calls diverging pleiotropically as bill size diverged. Because crossbills choose mates while in flocks, assortative flocking could lead indirectly to assortative mating as a by-product. These patterns of association therefore provide a mechanism by which increasing divergent selection can lead to increasing reproductive isolation.

Coping with the extremes: stress physiology varies between winter and summer in breeding opportunists

Seasonal changes in stress steroid hormone secretions are thought to reflect investment in self-maintenance versus reproduction. The capricious conditions hypothesis (CCH) posits that reduced corticosterone (CORT) secretion during stress coincident with parental phases of breeding is necessary in harsh environments because a full response would otherwise trigger repeated nest abandonments. To test this hypothesis, we measured seasonal changes in stress physiology in free-living red crossbills ( Loxia curvirostra ), an opportunistically breeding songbird that regularly breeds in summer and winter. This species allows unique comparisons of breeding physiology under very different seasonal environmental conditions within locations. We found strong support for the CCH: red crossbills showed reduced CORT secretion only when in high reproductive condition in the winter, when compared with summer breeders and winter non-breeders. These data demonstrate that behavioural status and local environmental conditions interact to affect mechanisms underlying investment trade-offs, presumably in a way that maximizes lifetime reproductive success.