Black-faced Cormorants (Phalacrocorax fuscescens) are endemic to Australia and only breed in a limited number of locations, often on offshore islands. Until recently, little was known about their ecology and, due to a lack of tracking data, nothing was known about their movements, habitat use, and foraging behaviour. To address this gap, we tracked Black-faced Cormorants breeding on Notch Island (northern Bass Strait, south-eastern Australia) using GPS-GSM loggers or GPS loggers with pressure sensors over four consecutive breeding seasons. This enabled the study of both movements and dive behaviour.
Black-faced Cormorant and chick. Image: Thomas Cansse
Analysis of their diving behaviour indicated that Black-faced Cormorants are predominantly benthic foragers. Additionally, the tracking data revealed that individuals repeatedly returned to the same foraging areas on successive foraging trips, although the population as a whole used a wide range of sites. Previous studies on several marine predators suggest that the distribution and availability of benthic prey are generally more predictable than for pelagic prey. Combined with the site fidelity observed in the tracking data, this sparked the question whether Black-faced Cormorants could potentially forage more efficiently by targeting familiar locations.
High levels of foraging site fidelity displayed by Black-faced Cormorants breeding at Notch Island (black dot). Each colour represents a different individual (N = 11). Figure 1 from Cansse et al. 2024
To investigate this, we applied a Hidden Markov Model to assign behaviours (foraging, commuting, or roosting) to GPS tracks and used the proportion of time spent foraging as a measure of efficiency. For individuals with dive data, we calculated dive rate (vertical m·h⁻¹) as an additional measure of foraging effort. Subsequently, we used the Bhattacharyya affinity index to quantify site fidelity and investigated how site fidelity affected our measures of foraging efficiency and effort.
Black-faced Cormorants in flight. Image: Thomas Cansse
Our results indicated that individuals with higher site fidelity had lower dive rates and spent less time foraging. However, there was also a within-individual effect, indicating that individuals have lower dive rates and spent less time foraging when returning to familiar sites. This suggests that site fidelity increases foraging efficiency and decreases foraging effort, which may be particularly beneficial during the chick-rearing period when birds need to collect food for themselves and their hungry chicks. Interestingly, site fidelity was not influenced by year or sex, suggesting that other factors such as age, individual experience or personality may affect this.
Model-predicted effects of foraging site fidelity on A) the proportion of time spent foraging in a trip and B) dive rate. Solid, dark grey lines represent within-individual effects, and dashed, light grey lines represent between-individual effects. Figure 2 from Cansse et al. 2024
These findings have important management implications, as disturbances to foraging sites, whether natural or anthropogenic, could reduce foraging efficiency and potentially impact reproductive success of affected individuals. However, due to the wide range of sites used on the population level, there is no straightforward way to protect all foraging sites. Importantly, our study investigated the effects of site fidelity on relatively short timescales. Therefore, for future studies it would be of interest to study how individual site fidelity changes over the course of the breeding season, or between years. This would provide information on the capability of individuals to cope with a changing environment and associated changes to prey distribution.
Data from this study are currently embargoed, but will be made available on the Seabird Tracking Database in the future.
Read the full paper:
Cansse, T., Lens, L., Sutton, G.J., Botha, J.A. and Arnould, J.P.Y. (2024), Spatial consistency affects foraging effort in a benthic diving seabird, the Black-faced Cormorant. Ibis. https://doi.org/10.1111/ibi.13371