Urbanization has significantly altered the habitat structure and behavioral patterns of animals. In urban environments with frequent human disturbances, animals may undergo adaptive adjustments in their escape behaviors. This study focuses on Eurasian Tree Sparrows (Passer montanus) from five cities across different latitudes in China, comparing their flight initiation distance (FID) in urban and rural areas to assess the impact of urbanization on their escape behavior and the trend of ecological homogenization. The results confirm the established pattern at the geographic level: a reduction of FID in urban habitats and a decrease in FID with increasing latitude. That is, northern populations of the Eurasian Tree Sparrow in China are bolder than the southern populations. In addition, FID is positively correlated with flock size, with rural flocks exhibiting more pronounced group behavior (observed flocking frequency for urban and rural is 28.74% vs. 40.7%, flock size variance is 26.90 vs. 55.63). Despite the differences between northern and southern latitudes, the variability of FID in urban individuals consistently remains lower than that in rural areas, supporting the trend of reduced variability in escape behavior among birds in urban environments. Driven by urbanization, the escape behavior of Eurasian Tree Sparrows tends toward ecological homogenization, meaning that behavioral differences between urban areas in different cities are diminishing. This trend may obscure the behavioral variability brought about by latitude gradients, indicating that urbanization not only shapes the behavioral adaptations of birds but also potentially weakens their behavioral diversity.

Animal behavioral studies are often combined with research concerning cognitive abilities. Larger brains usually mean more complex neural networks and advanced cognitive functions. By measuring the brain size of different individual animals, we can explore differences in behavioral complexity between populations or species. However, obtaining accurate measurements of brain size is challenging both in field and laboratory environments, especially for rare and endangered species. Therefore, there is an urgent need to develop reliable methods for performing cranial brain mass. This study tests which external structures of the avian head can most accurately predict brain size. We selected five bird species from four orders, categorized external head measures into three types of parameters (direct, calculated and composite measurements), and analyzed these in relation to brain mass. The results showed that while head size can partially explain brain mass, the parameters of head height × head width were the most accurate predictors of brain mass in birds (90.4%). In addition, the positive correlation between endocranial volume and brain mass once again confirmed that avian endocranial volume can, to a certain extent, serve as a valid proxy for brain mass. Our study demonstrates that in the future we can more conveniently perform non-invasive measurements to better understand the relationship between bird brain size and behavior, ecology, and evolution.

With the ongoing acceleration of global urbanization, an increasingly larger proportion of natural habitats are being transformed and utilized by humans. Urbanization has a substantial impact on animal populations, exposing them to greater risks, but also presenting new opportunities and resources. Although the effects of urbanization and brood parasitism on the population dynamics and behavior of host birds have received considerable attention, there has been comparatively little research on how urbanization might affect the risk of brood parasitism on host birds. From April to August in 2023 and 2024, we investigated the risk of brood parasitism on Azure-winged Magpies (Cyanopica cyanus) breeding in the urban center, outskirt, and rural areas of Wuhan City, Hubei Province, China. The results revealed significant differences in the risk of brood parasitism among Azure-winged Magpies in the three regions, with the proportion of magpies parasitized by the Asian Koel (Eudynamys scolopaceus) being found to be significantly lower in the city center (0%) than that in outskirt (3.8%) and rural (11.4%) areas. Our results are consistent with the idea that cities can act as refuges that reduce the risk of brood parasitism experienced by host Azure-winged Magpies. Accordingly, breeding in cities enables Azure-winged Magpies to escape brood parasitism by the Asian Koel.

Avian brood parasitism is a unique reproductive behavior in which parasitic birds depend on other species to incubate their eggs and raise their offspring. In China, there are 20 species of cuckoos in the family Cuculidae, order Cuculiformes, of which 17 species are parasitic cuckoos. This makes China one of the countries with the largest number of parasitic cuckoo species worldwide. Understanding the host utilization of cuckoos provides fundamental data for studying the coevolution of cuckoos with their hosts. We collected information on cuckoo hosts from the literature, photographs provided by birdwatchers, and online short video platforms, combined these data with our field observations, and summarized the parasitic cuckoos and their host species in China. A total of 1155 parasitism events were counted, involving 12 parasitic cuckoo species and 87 bird host species. These hosts belonged to 26 families, among which Muscicapidae was the most diverse with 19 species, accounting for 21.8% of the total hosts, followed by the families Phylloscopidae and Leiothrichidae with 8 species each, accounting for 9.2% of the total hosts recorded. The Common Cuckoo (Cuculus canorus) had the largest number of host taxa with 38 species, accounting for 43.7% of the total host species. This study adds 14 host species that have not been reported in China. However, for five species, the Jacobin Cuckoo (Clamator jacobinus), Banded Bay Cuckoo (Cacomantis sonneratii), Violet Cuckoo (Chrysococcyx xanthorhynchus), Common Hawk-cuckoo (Hierococcyx varius), and Whistling Hawk-cuckoo (Hierococcyx nisicolor), information regarding host utilization is still lacking.

Brood parasitic birds lay eggs in the nests of other birds, and the parasitized hosts can reduce the cost of raising unrelated offspring through the recognition of parasitic eggs. Hosts can adopt vision-based cognitive mechanisms to recognize foreign eggs by comparing the colors of foreign and host eggs. However, there is currently no uniform conclusion as to whether this comparison involves the single or multiple threshold decision rules. In this study, we tested both hypotheses by adding model eggs of different colors to the nests of Barn Swallows (Hirundo rustica) of two geographical populations breeding in Hainan and Heilongjiang Provinces in China. Results showed that Barn Swallows rejected more white model eggs (moderate mimetic to their own eggs) and blue model eggs (highly non-mimetic eggs with shorter reflectance spectrum) than red model eggs (highly non-mimetic eggs with longer reflectance spectrum). There was no difference in the rejection rate of model eggs between the two populations of Barn Swallows, and clutch size was not a factor affecting egg recognition. Our results are consistent with the single rejection threshold model. This study provides strong experimental evidence that the color of model eggs can has an important effect on egg recognition in Barn Swallows, opening up new avenues to uncover the evolution of cuckoo egg mimicry and explore the cognitive mechanisms underlying the visual recognition of foreign eggs by hosts.

Common Cuckoos (Cuculus canorus) dependent on parental care for post-hatching demonstrate an intriguing ability to modify their begging vocalizations to ensure maximum care and resources from their interspecific foster parents. Here, we compared begging calls of the Common Cuckoo nestlings fed by four host species, the Grey Bushchat (Saxicola ferreus), Siberian Stonechat (Saxicola maurus), Daurian Redstart (Phoenicurus auroreus), and Oriental Magpie-robin (Copsychus saularis). Results showed that begging calls of the stonechat–, redstart–, and robin–cuckoo resemble those of host species' nestlings in various aspects like low frequency, high frequency, frequency bandwidth and peak frequency, while the bushchat–cuckoo chicks' begging calls were only comparable to their host species in terms of how long they lasted and their peak frequency. In addition, cuckoo nestlings raised in different host nests displayed significant variations in their begging calls in low and peak frequency. This study suggests that cuckoo nestlings do not mimic host species nestlings' begging calls throughout the nestling period, but may tune their begging calls according to host species, while begging calls vary with cuckoo and host species nestlings' ages. Future research should study the parents' reactions to these calls in different host species for a better understanding of the mechanisms underlying such adaptations.

The Oriental Reed Warbler (Acrocephalus orientalis) is one of the most commonly used hosts for the parasitic Common Cuckoo (Cuculus canorus). However, as hosts that feed unrelated parasitic nestlings may suffer extra reproductive costs, they may be less willing to care for nestlings that have prolonged nestling periods. To test this hypothesis, the duration of feeding by Oriental Reed Warblers under natural conditions for their own nestlings was compared with the duration of feeding under natural conditions for Common Cuckoo nestlings and for artificially prolonged cuckoo nestlings. The results showed that Oriental Reed Warblers did not starve, drive away, or desert any of the nestlings in the experiment, and neither parent was left alone. Our experimental study indicates that both Oriental Reed Warbler parents were willing to care for nestlings with a prolonged nestling period (up to 30 days, twice the average duration time that the Oriental Reed Warblers fed their own chicks in natural conditions). However, further experiments and observations are required in other host bird species to examine whether both parents or one of the parents may exhibit the behavior of abandoning nestlings with a prolonged nestling period.

Nests are important structures for birds to raise their offspring and for signaling. Many birds incorporate feathers into the nest, since feathers were traditionally thought to serve the function of insulation. Hypotheses in recent years have considered that some birds place feathers in conspicuous locations in the nest for decoration to trigger a fear response in the competitors. In this study, we investigated whether decorative feathers could deter nest usurpation by Crested Mynas (Acridotheres cristatellus) by manipulating nest box contents. The results revealed that Crested Mynas preferred black feathers to white feathers as decorations and occupied nest boxes decorated with black feathers significantly less than those decorated with white feathers, suggesting that black decorative feathers in the nest could be more effective in preventing nest usurpation by Crested Mynas and that white decorative feathers may have other functions. The black feathers in the nest are prominently placed at the edge of the nest to convey the message that "this nest is occupied" or "the owner of this nest has been preyed upon" to visitor Crested Mynas, thus effectively preventing them from usurping the nest at a later stage.

The interactions between avian brood parasites and their hosts provide an informative and easy-to-handle system for studying coevolution. Avian brood parasitism reduces the reproductive success of hosts, and thus, hosts have evolved anti-parasitic strategies, such as rejecting parasitic eggs and adopting aggressive nest defense strategies, to avoid the cost brought on by brood parasitism. To test whether host anti-parasitic strategies are adjusted with the risk of being parasitized when the breeding seasons of brood parasites and hosts are not synchronous, we conducted a field experiment assessing nest defense and egg recognition behaviors of the Isabelline Shrike (Lanius isabellinus), a host of the Common Cuckoo (Cuculus canorus). In the local area, the host Isabelline Shrike begins to breed in April, whereas the summer migratory Common Cuckoo migrates to the local area in May and begins to lay parasitic eggs. Results showed that nest defense behaviors of the Isabelline Shrike increases significantly after cuckoo arrival, showing higher aggressiveness to cuckoo dummies, with no significant difference in attack rates among cuckoo, sparrowhawk and dove dummies, but their egg rejection did not change significantly. These results imply that Isabelline Shrikes may adjust their nest defense behavior, but not egg rejection behavior, with seasonality.

Recognition and rejection of foreign eggs are effective defense of hosts against brood parasitism. However, brood parasitism can impose various selection pressures on different geographic populations of the same host species. In a multiple cuckoo system in China, Azure-winged Magpies (Cyanopica cyanus) are parasitized by both Indian Cuckoos (Cuculus micropterus) and Asian Koels (Eudynamys scolopaceus). In this study, egg recognition ability and recognition mechanism of the Azure-winged Magpie were investigated using a population in Fusong, southeastern Jilin, China. The results showed that 55.6% (20/36) of the Azure-winged Magpies correctly rejected quail (Coturnix japonica) eggs in their nests, while 13.9% (5/36) of the individuals experienced rejection costs by wrongly rejecting their own eggs. Azure-winged Magpies could accurately reject the experimental eggs when the number of such eggs in the nests was the same as that of the magpie eggs. However, Azure-winged Magpies do not recognize and reject conspecific eggs (0/28). The present study indicates that the Azure-winged Magpie has moderate egg recognition ability toward non-mimetic quail eggs and shows a true recognition mechanism with rejecting foreign eggs by accurately recognizing their own eggs. However, they cannot recognize conspecific eggs.