Karen L. Wiebe

NEST SITES AND NEST WEBS FOR CAVITY-NESTING COMMUNITIES IN INTERIOR BRITISH COLUMBIA, CANADA: NEST CHARACTERISTICS AND NICHE PARTITIONING

Abstract The mixed forests of interior British Columbia, Canada, support a rich community of cavity nesters, accounting for about one-third of forest vertebrate species. For 20 cavity-nesting bird and six cavity-nesting mammal species, representing excavators and secondary cavity nesters, we measured nest-cavity and nest-tree characteristics over 8 years in Interior Douglas-fir (Pseudotsuga menziesii) forest ecosystems. There was overwhelming selection for quaking aspen (Populus tremuloides); 95% of 1692 cavity nests were in aspen, which comprised only 15% of trees available. The full range of live and dead trees were used, but we observed a strong preference for live trees with decay (45% of nests) or dead trees (45% of nests). A cluster analysis based on tree and cavity characteristics divided the community into five groups, including large- and medium-sized woodpeckers and a group comprised mostly of weak excavators. A fourth group included Northern Flickers (Colaptes auratus), the most abundant excavator, and the larger secondary cavity nesters. The final group contained the most aggressive and most abundant secondary cavity nesters. European Starling (Sturnus vulgaris), the most aggressive secondary cavity nester, occupied a narrower nest niche (in less-decayed trees with smaller entrances) relative to their size. Less-competitive excavators and secondary cavity nesters occupied wider nest niches in terms of tree decay class and cavity size. We constructed a nest web for community structure that showed most cavity resource use flowed up the community through aspen trees and cavities excavated by Northern Flickers. Thus, aspen was the critical nesting tree and Northern Flickers were the keystone excavators in this community. Sitios de Nidificación y Redes de Nidos en Comunidades que Nidifican en Cavidades en el Interior de British Columbia, Canadá: Características de los Nidos y Separación de Nichos Resumen. Los bosques mixtos del interior de British Columbia, Canadá, albergan una rica comunidad de animales que nidifican en cavidades, los cuales representan aproximadamente un tercio de las especies de vertebrados de bosque. En este estudio medimos características de las cavidades y de los árboles de nidificación para 20 especies de aves y seis de mamíferos que nidifican en cavidades (incluyendo especies excavadoras y las que utilizan cavidades secundariamente) a lo largo de ocho años en ecosistemas de bosque interior de Pseudotsuga menziesii. Hubo una selección abrumadora de árboles de la especie Populus tremuloides; el 95% de 1692 cavidades de nidificación se encontraron en árboles de esta especie, la cual comprendía sólo el 15% de los árboles disponibles. Todo el espectro de árboles vivos y muertos fue utilizado, pero observamos una preferencia fuerte por árboles vivos con descomposición (45% de los nidos) o árboles muertos (45% de los nidos). Un análisis de agrupamiento basado en características de los árboles y las cavidades dividió la comunidad en cinco grupos, incluyendo carpinteros de tamaño grande y mediano, y un grupo formado principalmente por excavadores débiles. Un cuarto grupo incluyó al carpintero Colaptes auratus (el excavador más abundante) y a las especies de mayor tamaño que nidifican en cavidades secundarias. El último grupo incluyó a las especies más abundantes y agresivas que nidifican en cavidades secundarias. El estornino Sturnus vulgaris, la especie más agresiva que nidifica en cavidades secundarias, ocupó un nicho más estrecho (árboles menos descompuestos con entradas más pequeñas) con relación a su tamaño. Los excavadores menos competitivos y los usuarios de cavidades secundarias ocuparon nichos de nidificación más amplios en términos de la categoría de descomposición de los árboles y el tamaño de la cavidad. Construimos una red de nidos para estudiar la estructura de la comunidad, la cual mostró que la mayor parte del uso de las cavidades como recurso fluye en la comunidad a través de los árboles de P. tremuloides y las cavidades excavadas por C. auratus. Por lo tanto, P. tremuloides fue el árbol de nidificación crítico y C. auratus fue la especie de excavador clave en esta comunidad.

MICROCLIMATE OF TREE CAVITY NESTS: IS IT IMPORTANT FOR REPRODUCTIVE SUCCESS IN NORTHERN FLICKERS?

Abstract I measured structural characteristics of 160 Northern Flicker (Colaptes auratus) nests at Riske Creek, British Columbia, and placed electronic data-loggers in a subsample of 86 nests to record internal temperatures after the flickers completed nesting. Using multiple regression, I found that the best predictors of a variety of nest-cavity temperature variables were tree health, diameter of the tree at cavity height, and orientation of the cavity. Small and dead trees showed the most extreme (maximum and minimum) temperatures during the day, but, on average, were the coldest nests from the perspective of incubation. South-facing cavities reached the highest temperatures during the day, and the orientation of natural cavities was also biased towards the south. I predicted that cold nests would be energetically expensive for adults and nestlings, and found that clutch size was positively correlated with mean cavity temperature. However, there did not appear to be any relationship among nest temperature and hatching or fledging success.

Facultative sex ratio manipulation in American kestrels

SummaryFor animals that are sexually dimorphic in size, the larger sex is expected to be more costly to raise to independence. Manipulating offspring sex ratios may thus be one means by which parents can fine-tune their reproductive effort to resource availability. Parents in poor physical condition or during poor food years should produce more of the cheaper (smaller) sex. We examined the sex ratios of 259 broods of American kestrels (Falco sparverius) between 1988 and 1990 in relation to food abundance (small mammals) and various attributes to the parents. The proportion of males at hatching increased as the food supply declined, and both male and female parents in poor physical condition were more likely to have male-biased broods than those in good condition. The mortality of eggs and young did not appear to be responsible for the biased sex ratios. The sex ratio was independent of the laying date; however, it was correlated with female body size. Small females produced more sons, perhaps because small size is more detrimental for females than males.

NEST-SITE REUSE PATTERNS FOR A CAVITY-NESTING BIRD COMMUNITY IN INTERIOR BRITISH COLUMBIA

Abstract Most obligate cavity-nesting birds are considered to be nest-site limited, either by time or energy to excavate or to acquire suitable holes for nesting. We examined rates of nest-cavity reuse for a rich community of cavity-nesting birds in mixed forests in interior British Columbia. Using a sample of 402 cavity-reuse cases over five years, we measured cavity reuse for 20 cavity-nesting bird and mammal species (three guilds), and examined the relationship between nest-cavity reuse and features of cavities, nest trees, and forest stands. Eight percent of used cavities were destroyed between years. Reuse rates were 17% for the cavities of weak excavators such as nuthatches and chickadees, 28% for formerly active woodpecker nests, and 48% for cavities previously used by secondary cavity nesting birds, but there was considerable species variation within all guilds. Nest cavities in aspen that were deep with large entrances had the highest reuse. At the forest stand level, cavities in trees close to edges and in sites with more edge habitat had greater reuse. Reused cavities tended to be occupied in sequential years rather than being inactive for a year. With increasing amounts of managed landscapes, availability of suitable cavities for forest nesting vertebrates is decreasing. Reuse of existing cavities might help mitigate the problem of nest-site limitation.

Coping Mechanisms of Alpine and Arctic Breeding Birds: Extreme Weather and Limitations to Reproductive Resilience'

Abstract As ground nesting homeotherms, alpine and arctic birds must meet similar physiological requirements for breeding as other birds, but must do so in more extreme conditions. Annual spring snowfall and timing of snow melt can vary by up to 1 month and daily temperatures near the ground surface vary from below freezing to over 45°C in alpine and arctic habitats. Species breeding in these environments have various behavioral, physiological, and morphological adaptations to cope with energetically demanding conditions. We review the ways birds cope with harsh and variable weather, and present data from long term field studies of ptarmigan to examine effects of spring weather on reproduction. In variable but normal spring conditions, timing of breeding was not influenced by snow melt, snow depth or daily temperatures in the alpine, as breeding did not commence until conditions were generally favorable. Arctic ptarmigan tended to vary breeding onset in response to spring conditions. Generally, birds breeding in alpine and arctic habitats suffer a seasonal reproductive disadvantage compared to birds at lower latitudes or elevations because the breeding window is short and in late years, nest failure may be high with little opportunity for renesting. Coping mechanisms may only be effective below a threshold of climactic extremes. Despite strong resilience in fecundity parameters, when snowmelt is extremely delayed breeding success is greatly reduced. Alpine and arctic birds will be further challenged as they attempt to cope with anticipated increases in the frequency and severity of weather events (climate variability), as well as general climate warming.

Food Supply and Hatching Spans of Birds: Energy Constraints or Facultative Manipulation

Clutches of altricial birds generally hatch over a period of hours or days. Numerous hypotheses have been generated to explain hatching asynchrony, but most experiments have focused on the nestling stage to determine some post—hatch benefit such as increased reproductive success. We concentrated on proximate causes in the prelaying period, which have largely been ignored. In northern Saskatchewan, we censused prey species (small mammals) in early spring and measured the asynchrony of 273 clutches of the American Kestrel (Falco sparverius) over 4 yr. Hatching asynchrony was quantified in two ways: time for all the eggs within a clutch to hatch and mass hierarchies among nestlings. In good food years, the hatchling spans of clutches were more synchronous than in poor years. Similarly, parents on good territories or females in good physical condition had more synchronous broods than parents under food stress. In an experiment, we supplemented food to kestrels during the prelaying period. Parents that were supplemented laid larger eggs and hatched those eggs more synchronously, but control pairs were more successful at raising young to the fledging stage. Synchronous hatching when food is abundant is consistent with the hypothesis of facultative manipulation of hatching spans in relation to nestling food supply, but not with the energy constraint hypothesis.

Protective cover and the use of space by finches: is closer better?

The use of space by small birds is strongly influenced by dense vegetation which serves as protection from attacking predators. Current evidence suggests that birds dependent upon such cover for safety will venture away from it only if their energy intake rate is significantly enhanced in doing so. We found, however, that three species of finches (Emberizidae) regularly fed well away from cover without a corresponding increase in energy intake rate. Observations and experimental results strongly suggest that the birds perceived cover as both safety and a source of attacks. For instance, birds fed farther away from cover containing visual obstructions that might conceal predators lying in ambush. Their use of space appeared to reflect a trade-off between the perceived risk of feeding too close to cover vs that of feeding too far away. This trade-off was also influenced by a birds social environment; subordinate species fed farther from cover than dominant ones in an apparent effort to avoid direct aggression.

Learning the ecological niche

A cornerstone of ecological theory is the ecological niche. Yet little is known about how individuals come to adopt it: whether it is innate or learned. Here, we report a cross-fostering experiment in the wild where we transferred eggs of blue tits, Cyanistes caeruleus, to nests of great tits, Parus major, and vice versa, to quantify the consequences of being reared in a different social context, but in an environment otherwise natural to the birds. We show that early learning causes a shift in the feeding niche in the direction of the foster species and that this shift lasts for life (foraging conservatism). Both species changed their feeding niches, but the change was greater in the great tit with its less specialized feeding behaviour. The study shows that cultural transmission through early learning is fundamental to the realization of ecological niches, and suggests a mechanism to explain learned habitat preference and sympatric speciation in animals.

Social learning in birds and its role in shaping a foraging niche

We briefly review the literature on social learning in birds, concluding that strong evidence exists mainly for predator recognition, song, mate choice and foraging. The mechanism of local enhancement may be more important than imitation for birds learning to forage, but the former mechanism may be sufficient for faithful transmission depending on the ecological circumstances. To date, most insights have been gained from birds in captivity. We present a study of social learning of foraging in two passerine birds in the wild, where we cross-fostered eggs between nests of blue tits, Cyanistes caeruleus and great tits, Parus major. Early learning causes a shift in the foraging sites used by the tits in the direction of the foster species. The shift in foraging niches was consistent across seasons, as showed by an analysis of prey items, and the effect lasted for life. The fact that young birds learn from their foster parents, and use this experience later when subsequently feeding their own offspring, suggests that foraging behaviour can be culturally transmitted over generations in the wild. It may therefore have both ecological and evolutionary consequences, some of which are discussed.

Climate change, breeding date and nestling diet: how temperature differentially affects seasonal changes in pied flycatcher diet depending on habitat variation

1. Climate warming has led to shifts in the seasonal timing of species. These shifts can differ across trophic levels, and as a result, predator phenology can get out of synchrony with prey phenology. This can have major consequences for predators such as population declines owing to low reproductive success. However, such trophic interactions are likely to differ between habitats, resulting in differential susceptibility of populations to increases in spring temperatures. A mismatch between breeding phenology and food abundance might be mitigated by dietary changes, but few studies have investigated this phenomenon. Here, we present data on nestling diets of nine different populations of pied flycatchers Ficedula hypoleuca, across their breeding range. This species has been shown to adjust its breeding phenology to local climate change, but sometimes insufficiently relative to the phenology of their presumed major prey: Lepidoptera larvae. In spring, such larvae have a pronounced peak in oak habitats, but to a much lesser extent in coniferous and other deciduous habitats. 2. We found strong seasonal declines in the proportions of caterpillars in the diet only for oak habitats, and not for the other forest types. The seasonal decline in oak habitats was most strongly observed in warmer years, indicating that potential mismatches were stronger in warmer years. However, in coniferous and other habitats, no such effect of spring temperature was found. 3. Chicks reached somewhat higher weights in broods provided with higher proportions of caterpillars, supporting the notion that caterpillars are an important food source and that the temporal match with the caterpillar peak may represent an important component of reproductive success. 4. We suggest that pied flycatchers breeding in oak habitats have greater need to adjust timing of breeding to rising spring temperatures, because of the strong seasonality in their food. Such between-habitat differences can have important consequences for population dynamics and should be taken into account in studies on phenotypic plasticity and adaptation to climate change.