EFFECTS OF HABITAT DISTURBANCE ON REPRODUCTION IN BLACK-CAPPED CHICKADEES (POECILE ATRICAPILLUS) IN NORTHERN BRITISH COLUMBIA

Auk, The, Oct 2004 by Fort, Kevin T, Otter, Ken A

ABSTRACT. -

Avian species that persist in breeding in disturbed habitats are often thought to be less affected by disturbance than habitat specialists lost following disturbances, yet there is growing evidence that human-altered environments may negatively affect reproductive behavior and nest success of those generalists as well. We compared nest success of Black-capped Chickadees (Poecile atricapillus) in two adjacent habitats: a mature mixed-wood forest (undisturbed) and a forest regenerating after logging (disturbed). Despite similar breeding densities, proportion of nests that successfully fledged young was lower in the disturbed habitat than in the undisturbed habitat. Abandonment was the most common cause of nest failure. A within-habitat comparison of the social rank of birds revealed that low-ranking birds had lower nest success than high-ranking birds in the disturbed, but not in the undisturbed, habitat. Clutch size and brood size of nests that progressed to the point of hatch did not differ significantly between habitats. Average total number of fledglings produced per pair, though not significantly different, was suggestively lower in the disturbed habitat. Across habitats, nests situated in snags with lower amounts of internal decay were more successful. Successful nests were also located in sites with higher canopy height, low understory density below 1 m, and higher understory density between 2 and 3 m-all attributes generally associated with undisturbed, mature forests in the region. Our results provide evidence that disturbed habitats may represent poor-quality habitat for this forest generalist, and that habitat quality differentially affects individuals, depending on their dominance rank. Received 15 January 2003, accepted 12 May 2004.

RÉSUMÉ. -Les espèces aviennes qui persistent à se reproduire dans des habitats perturbés sont souvent considérées comme moins affectées que les espèces spécialistes en terme d'habitat. Néanmoins, il semble que les espèces généralistes puissent être également affectées dans leur comportement reproducteur et leur succès de nidification au sein d'un environnement perturbé par l'Homme. Nous avons comparé le succès de nidification de Mésanges à tête noire (Poecile atricapillus) dans deux habitats adjacents: une forêt mature mixte (non perturbée) et une forêt en régénération après coupe (perturbée). Malgré des densités de nidification semblables, la proportion de nids avec éclosion était plus faible dans les habitats perturbés que dans les habitats non perturbés. L'abandon du nid était la cause la plus courante pour expliquer les échecs de nidification. Une comparaison intra-habitat du rang social des oiseaux a révélé que les oiseaux de bas-rang avaient un succès de nidification plus faible que ceux de haut-rang dans les habitats perturbés. Ce n'était pas le cas dans les habitats non perturbés. Les tailles de ponte et de couvée des nids avec éclosion ne différaient pas de manière significative entre les habitats. Le nombre total moyen d'oisillons produit par couple, bien que non significativement différent, était plus faible dans les habitats perturbés. Au sein des différents habitats, les nids situés dans les chicots les moins décomposés étaient les plus productifs. Les nids avec éclosion étaient également situés dans les sites avec une plus grande hauteur de canopée, une plus faible densité de sous-couvert inférieure à 1 m et une plus grande densité de sous-couvert entre 2 et 3 m. Des caractéristiques qui sont généralement associées aux forêts matures et non perturbées de la région. Nos résultats apportent la preuve qvie les habitats perturbés peuvent représenter des habitats de faible qualité pour les espèces généralistes. Par ailleurs, les individus sont affectés de manière différentielle par la qualité de l'habitat selon leur rang de dominance.

EFFECTS OF HABITAT disturbance on reproductive success of forest songbirds are typically studied at the community level and determined by presence-absence census methods (Schmiegelow et al. 1997). Single-species studies tend to focus on habitat specialists (i.e. species with rigid habitat requirements), because they are most likely to respond negatively to habitat disturbance (Bayne and Hobson 2001). Conversely, habitat generalists (i.e. species with broad habitat requirements) are more likely to persist in habitats that are anthropogenically disturbed. Such habitats, however, may differ in availability of resources needed for successful reproduction; thus, disturbed areas may represent habitat of varying quality to birds (Zanette 2001, Chase 2002, Tarvin and Garvin 2002). Habitat quality can ultimately be measured in terms of reproductive output (Muller et al. 1997), but factors contributing to habitat quality are various and include degree of predation risk, availability of food resources, and access to suitable nest locations and microhabitats.

Assessing effects of habitat disturbance on a population by using presence-absence methods alone can be misleading. Specifically, if a disturbed environment is low-quality habitat for a breeding population, reproductive output may be diminished, despite breeding densities similar to those of populations breeding in undisturbed habitats. This reduced reproductive output might result from reproductive decisions by animals breeding under suboptimal and stressful conditions. A number of studies have shown that birds breeding in poor-quality territories will compensate for decreased resources by adjusting clutch size downward (Dhondt et al. 1990, 1992; Slagsvold and Lifjeld 1990). However, birds experiencing extremely stressful conditions may opt to forgo breeding altogether if the perceived survivorship risk is too high, as compared with the potential fitness benefit of a successful nest. How birds respond to variation in environmental quality may be condition-dependent; low-quality individuals may be forced to settle in poorer-quality habitats or, if settlement is random, may suffer disproportionately to high-quality individuals in those habitats (Fretwell and Lucas 1970). Regardless, such mechanisms may ultimately have population-level consequences if large portions of the population are breeding in low-quality, disturbed habitats.