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Some birds cover their eggs with nest material when they leave to forage. It has been suggested that such egg-covering aids thermoregulation or prevents predation but here we present a new hypothesis, that secondary cavity-nesting species cover their eggs to prevent nest usurpation by other birds. When the bottom of the cavity is dark, as when eggs are covered by nest material, it may be difficult for a prospecting competitor to see whether a defending nest owner or a predator is hiding inside the cavity. Competitors may therefore hesitate to enter dark cavities. We filmed 21 great tit (Parus major) nests during the egg-laying period and found that the female spent bouts of highly variable length outside the nest box (range 0.3–250 min, n = 51), so prospecting small passerines would have difficulty predicting whether an aggressive tit owner was in the box or would soon return. We presented prospecting male pied flycatchers (Ficedula hypoleuca) with a dyad of boxes (n = 93), each containing a great tit nest but only one with visible eggs. Flycatchers hesitated more to enter a nest box with no visible tit eggs than a box with exposed eggs. This was most evident for nest boxes with dark versus light interior paint, supporting the idea that better interior illumination makes prospecting birds more confident about entering an unfamiliar cavity. The usurpation and predation hypotheses are not mutually exclusive because both competitors and small predators may hesitate to enter dark, enclosed spaces if visibility is low.

We investigated the usurpation behavior of the hornet Vespa basalis Smith in nests of V. velutina Lepeletier. We collected 47 nests of the two species in Taiwan in their establishment and growth stages. Of these, 29 were mixed-species nests, and in at least 16 V. velutina was replaced by V. basalis . While most nests were initiated by V. velutina , V. basalis coexisted with it during the establishment stage, leading to intense conflict between the two. The multiplex PCR method gave evidence of the usurper’s eggs and larvae in the mixed-species nest. Over time, the number of adults and brood in usurped V. velutina nests decreased, while those of V. basalis increased, demonstrating behavior in V. basalis akin to facultative social parasitism. This study provides further evidence of nest usurpation in the genus Vespa , along with description of complex interactions among these hornets, their natural enemies, prey, and humans in Taiwan.

Harrison’s rule, that body size is positively correlated between parasites and hosts, has been reported in a range of taxa, but whether the rule is applicable to cleptoparasitic insects is poorly understood. Subfamily Nomadinae, the largest group of cleptoparasitic bees, usurp the nests of a variety of host bees. Within the subfamily, Nomada exploits the most diverse hosts, using at least ten genera from five families. Here, we reassess the phylogeny of Nomadinae, including the expanded sampling of the genus Nomada , to explore host shift fluctuations throughout their evolutionary history and test the applicability of Harrison’s rule for the subfamily. Our phylogenetic results are mostly congruent with previous investigations, but we infer the tribe Hexepeolini as a sister taxon to the tribe Nomadini. Additionally, the results reveal discrepancies with the traditional classifications of Nomada . Ancestral state reconstruction of host use indicates that, early in their evolution, parasites used closer relatives, before attacking less related groups later. Lastly, we confirm Harrison’s rule in Nomadinae, supporting that body size dynamics influence the host shifts of cleptoparasitic bees.

Some secondary cavity-nesting birds use nest material to cover their eggs when they are out foraging during the egg-laying stage. Egg covering presumably helps regulate nest temperature and/or prevents predation or parasitism. A recent study suggested that egg covering might prevent nest usurpation and therefore, we set up nest boxes with three different treatments to test this hypothesis: bottom of the nest box covered with moss containing two eggs covered with moss (type A); floor of the nest box covered with moss and two eggs exposed on top of the moss (Type B); and bare floor of the nest box, with two eggs added (Type C). The results showed the lowest egg predation rates in Type A nest boxes, which was significantly lower than those in type C boxes; the proportion of maritime striped squirrels (Tamiops maritimus), first appearing in the three types of nest box was not significantly different; however, the rate of first entry into Type A was significantly lower than that into Type C boxes. The rate of first entry and the time the squirrels spent in Type A boxes were significantly lower and shorter than those in Type C boxes. Our results suggest that the egg-covering behavior of secondary cavity-nesting birds delayed the entry of nest competitors and accelerated the departure of nest encroachers from the nest, thus provided experimental evidence for the usurpation hypothesis.Significance statementSeveral non-mutually exclusive hypotheses have been proposed to explain the function of egg covering by bird species. The present study showed that egg covering by secondary cavity-nesting birds delayed the entry of maritime striped squirrels and accelerated the departure of nest encroachers from the nest, suggesting that egg covering might reduce the cost of nest defense for birds and thus prevent nest encroachment by other species. We provided experimental evidence for the new hypothesis proposed by Slagsvold and Wiebe (2021a) that egg-covering behavior of birds deters nest encroachers from easily entering unfamiliar, poorly lit cavities due to fear, thereby effectively preventing nest usurpation by squirrels.

With the exception of a few groups of birds, such as large raptors and colonial seabirds, direct counts of nests cannot be conducted over very large areas for most of the abundant and widely distributed species, and thus indirect methods are used to estimate their relative abundances and population sizes. However, many species of the Family Hirundinidae (swallows and martins) build their mud nests in discrete, predictable and accessible sites, which are reused across years. Therefore, the direct count of active nests could constitute a reliable method for estimating breeding population sizes and their changes at large spatial and temporal scales. We illustrate the feasibility of this monitoring approach through a single year survey of >2700 nests of three coexisting Old‐World species, the barn swallow (Hirundo rustica), the red‐rumped swallow (Cecropis daurica), and the crag martin (Ptyonoprogne rupestris), distributed across Portugal and Spain. Our results revealed changes in the use of nesting substrates and increases in interspecific nest usurpation rates over recent decades. While 56% of the nests of C. daurica were located in rocks five decades ago, almost 100% are nowadays located in anthropogenic substrates such as bridges, road culverts, and abandoned buildings, which could have favored the range expansion of this species. Nest occupation rates were surprisingly low (12% in C. daurica, 21% in H. rustica, and 37% in P. rupestris), and the proportion of abandoned nesting sites was very high (65% in C. daurica, 50% in H. rustica, and 27% in P. rupestris). Abandonment rates reflect the population decline reported for H. rustica. Notably, the usurpation of nests of C. daurica by house sparrows Passer domesticus, which is the main cause of breeding failure, has increased from 2.4% in 1976–1979 to 34.7% of the nests nowadays. The long‐term monitoring of nests may constitute a reliable and affordable method, with the help of citizen science, for assessing changes in breeding population sizes and conservation threats of these and other mud‐nest building hirundines worldwide. The long‐term monitoring of nests may constitute a reliable and affordable method, with the help of citizen science, for assessing changes in breeding population sizes and conservation threats of mud‐nest building hirundines worldwide. Breeding adults, and completely built and destroyed nests of red‐rumped swallows Cecropis daurica (a1–a5), barn swallows Hirundo rustica (b1–b5), and rock martins Ptyonoprogne rupestris (c1–c5), with scales for size comparisons. Photographs: José L. Tella.

Despite concerns about invasive Common Mynas (Acridotheres tristis), few studies have experimentally demonstrated their impact on native birds. Using a Before-After-Control-Impact (BACI) design with 142 nest boxes, we assessed how Myna presence affected House Sparrows (Passer domesticus) and Great Tits (Parus major) before (2009–2010) and after (2020–2021) their arrival in 2015. Half the boxes had large entrances accessible to all three species; half had small entrances accessible only to Great Tits. In a 2022 field experiment, we manipulated nest box entrances and used cameras to study Myna predation and nest usurpation. Following the establishment of Myna, House Sparrow breeding declined by 68.1%. Great Tits increased use of small entrance boxes by 59.9% and decreased use of large entrance boxes by 45.4%. Although overall Great Tit breeding increased, fledging success declined in large entrance boxes, but not small entrance ones. In the 2022 experiment, 46% (n = 26) of Great Tit nests were predated; Common Mynas were responsible for 50% of these cases and subsequently bred in the usurped nests. This study experimentally demonstrates that House Sparrow breeding numbers decrease and that Great Tits change their nesting behavior after the appearance of the Common Myna. Additionally, this research found that Common Mynas impact native species by predating and usurping nests. Installing small-entrance boxes can help manage Great Tit populations. Further research should explore nest box designs for House Sparrows and their impacts on other cavity-nesting and non-cavity-nesting bird species.

We observed a Killdeer (Charadrius vociferus) incubating a nest containing 1 Piping Plover (Charadrius melodus) egg and 2 Killdeer eggs at a sand and gravel mine in West Point, Nebraska. Four days later, the nest contained 1 Piping Plover egg and 4 Killdeer eggs. Two days later, the Piping Plover egg was absent, and the 4 Killdeer eggs remained in the nest. Eight days later the nest was destroyed. We speculate the nest began as a Piping Plover nest and was usurped by a Killdeer pair. To our knowledge, this is the first documentation of a nest containing eggs from both species. Observamos a un chorlito Charadrius vociferus incubando un nido que contenía 1 huevo de chorlito Charadrius melodus y 2 huevos de chorlito C. vociferus en una mina de arena y grava en West Point, Nebraska. Cuatro días despúes, el nido contenía 1 huevo de chorlito C. melodus y 4 de chorlito C. vociferus. Dos días después, el huevo de chorlito C. melodus estaba ausente y los 5 huevos de chorlito C. vociferus permanecían en el nido. Ocho días después el nido fue destruido. Especulamos que el nido inició como nido de chorlito C. melodus y fue usurpado por una pareja de chorlito C. vociferus. Por lo que sabemos, este es el primer caso documentado de un nido que contenía huevos de ambas especies. Palabras clave: Charrán menor, minas de arena y grava, monitoreo de nidos de aves costeras, Platte River, Sternula antillarum, usurpación de nido.

Hunting other predators is dangerous, as the tables can turn and the hunter may become the hunted. Specialized araneophagic (spider eating) predators have evolved intriguing hunting strategies that allow them to invade spiders’ webs by adopting a stealthy approach or using aggressive mimicry. Here, we present a newly discovered, specialized hunting strategy of the araneophagic spider Poecilochroa senilis (Araneae: Gnaphosidae), which forces its way into the silk retreat of the potential spider prey and immobilizes it by swathing gluey silk onto its forelegs and mouthparts. Poecilochroa senilis has been reported from the nests of a several, often large, spider species in the Negev desert (Israel), suggesting specialization on spiders as prey. Nevertheless, in laboratory experiments, we found that P . senilis has a wider trophic niche, and fed readily on several small insect species. The specialized nest-invading attack was used more frequently with large spiders, and even small juvenile P . senilis were able to attack and subdue larger spiders. Our observations show that specific hunting tactics, like nest usurpation, allow specialized predators to overcome defences of dangerous prey.

During the summers of 2019–2022 on Fire Island, NY, we observed 3 instances of Charadrius melodus (Piping Plover) incubating Sternula antillarum (Least Tern) eggs within mixed clutches, including a nest where the incubating plovers hatched chicks of both species. While mixed clutches of plover–tern nests have been observed previously, this case appears to be one of the few documented instances of a shorebird (Order: Charadriiformes) hatching eggs of another species with different parental care requirements. Moreover, the mechanisms by which these mixed clutches occurred on Fire Island appear to differ from past observations. Previously, mixed plover–tern clutches involved nest usurpation, whereas our observations suggest egg dumping by terns. Future research should investigate the role of habitat limitation in shorebird nesting and offspring recognition.

Interspecific nest usurpation, a relatively common nesting strategy, is the act of one bird species taking over the nesting site of another species. Although used by secondary cavity-nesters, nest usurpation has never been reported for the Tufted Titmouse (Baeolophus bicolor). In this paper, we document a mixed brood successfully reared by Tufted Titmice after usurpation of an Eastern Bluebird (Sialia sialis) nest in northeastern Arkansas during the 2019 breeding season. We monitored the focal nest box every 3–4 d from the first sign of nest building on 23 March 2019 until chick fledging by 12 May 2019. The Tufted Titmice took over the nest at the time the Eastern Bluebird had laid its fourth egg in the nest. By the end of the nesting period, the Tufted Titmice fledged 2 of their own chicks and 3 Eastern Bluebird chicks. Such successful rearing of non-conspecific nestlings is rare and we discuss factors that may have allowed it.