Introduction
Cowbirds are obligate brood parasites, meaning they only lay their eggs in the nests of other birds and do not rear their own young (Lowther 2020). Cowbirds can reduce nest success of host birds by removing the host's eggs prior to laying their own, destroying the host’s eggs, causing the host to abandon its nest, or creating competition for parental care in the host nest (Payne 1977). A meta-analysis by Lorenzana and Sealy (1999) demonstrated that 44 studies investigating the effects of cowbird nest parasitism on 25 bird species all reported some degree of reduced nest success.
Cowbirds were historically restricted to the Great Plains and associated with bison (Bison bison), likely due to bison creating favorable micro-habitats for insect prey, making insects more visible by flushing them, and by reducing vegetation height making it easier for female cowbirds to find host nests (Goguen and Mathews 1999). When bison were removed from the Great Plains by European settlers and replaced with millions of cattle, cowbirds seemingly adapted to this change and now associate with cattle likely for the same reasons (Morris and Thompson 1998). The opening of forest habitat through logging, agriculture, and grazing cattle has contributed to an overall increase in cowbird populations in North America (Goguen and Mathews 1999, 2001). Cowbirds are a forest ‘edge’ species, meaning they prefer transitional areas between forest and other habitat types and fragmented habitats from forestry and agriculture tend to have higher numbers of cowbirds (Howell et al. 2007).
Cowbirds can be a threat to bird conservation in cases where endangered species are exposed to excessive nest parasitism (Eckrich et al. 1999, Winfree 2004), especially in cases where the host species are naïve to cowbird parasitism. This can occur in areas previously not occupied by cowbirds that have been fragmented and used for grazing cattle. Species that have not evolved in the presence of cowbirds generally do not posses defense mechanisms to combat nest parasitism (I.e. ejecting the parasite’s eggs, or re-nesting) and are more susceptible to reduced nest success (Rothstein 1975). Cowbirds that occur in habitats where they did not historically occur are considered invasive and have higher offspring survival than cowbirds in their traditional range (Winfree 2004).
Fig 3. A male brown-headed cowbird (ebird.org/species/bnhcow)
Cattle grazing is an important part of Alberta’s rural economy, and ecologically necessary for the maintenance of native grassland biodiversity (Kraus et al. 2022). However, songbirds are in widespread decline across North America (Rosenberg et al. 2019) and cowbird parasitism on naïve bird species is a contributing factor to reduced nest success (Rothstein 1975). While cowbirds are known to associate with cattle, habitat specific associations remain unclear. This research aims to assess if the presence of cattle in areas outside of the traditional range of cowbirds increases the likelihood of cowbird occurrence.
Research objectives
This study uses bioacoustic and camera trap data collected across the province of Alberta (n = 2510 sites) to assess the relationship between cowbirds and cattle relative to the habitat suitability index for cowbirds. Objectives include:
Cowbirds were historically restricted to the Great Plains and associated with bison (Bison bison), likely due to bison creating favorable micro-habitats for insect prey, making insects more visible by flushing them, and by reducing vegetation height making it easier for female cowbirds to find host nests (Goguen and Mathews 1999). When bison were removed from the Great Plains by European settlers and replaced with millions of cattle, cowbirds seemingly adapted to this change and now associate with cattle likely for the same reasons (Morris and Thompson 1998). The opening of forest habitat through logging, agriculture, and grazing cattle has contributed to an overall increase in cowbird populations in North America (Goguen and Mathews 1999, 2001). Cowbirds are a forest ‘edge’ species, meaning they prefer transitional areas between forest and other habitat types and fragmented habitats from forestry and agriculture tend to have higher numbers of cowbirds (Howell et al. 2007).
Cowbirds can be a threat to bird conservation in cases where endangered species are exposed to excessive nest parasitism (Eckrich et al. 1999, Winfree 2004), especially in cases where the host species are naïve to cowbird parasitism. This can occur in areas previously not occupied by cowbirds that have been fragmented and used for grazing cattle. Species that have not evolved in the presence of cowbirds generally do not posses defense mechanisms to combat nest parasitism (I.e. ejecting the parasite’s eggs, or re-nesting) and are more susceptible to reduced nest success (Rothstein 1975). Cowbirds that occur in habitats where they did not historically occur are considered invasive and have higher offspring survival than cowbirds in their traditional range (Winfree 2004).
Fig 3. A male brown-headed cowbird (ebird.org/species/bnhcow)
Cattle grazing is an important part of Alberta’s rural economy, and ecologically necessary for the maintenance of native grassland biodiversity (Kraus et al. 2022). However, songbirds are in widespread decline across North America (Rosenberg et al. 2019) and cowbird parasitism on naïve bird species is a contributing factor to reduced nest success (Rothstein 1975). While cowbirds are known to associate with cattle, habitat specific associations remain unclear. This research aims to assess if the presence of cattle in areas outside of the traditional range of cowbirds increases the likelihood of cowbird occurrence.
Research objectives
This study uses bioacoustic and camera trap data collected across the province of Alberta (n = 2510 sites) to assess the relationship between cowbirds and cattle relative to the habitat suitability index for cowbirds. Objectives include:
- Model the probability of occupancy by cowbirds using a single species, single season, occupancy model with time of day as an detection covariate, and cattle presence/absence and habitat suitability as site covariates to determine if cattle presence increases the probability of cowbird occupancy.
- Model cowbird presence/absence against HSI and cattle presence/absence in a generalized linear model to see if results are similar to the occupancy model. This essentially removes the 'detection probability' element that may be confounded by differences in detection in different habitat types.