Combining Site Occupancy, Breeding Population Sizes and Reproductive Success to Calculate Time-Averaged Reproductive Output of Different Habitat Types: An Application to Tricolored Blackbirds

Abstract

In metapopulations in which habitat patches vary in quality and occupancy it can be complicated to calculate the net time-averaged contribution to reproduction of particular populations. Surprisingly, few indices have been proposed for this purpose. We combined occupancy, abundance, frequency of occurrence, and reproductive success to determine the net value of different sites through time and applied this method to a bird of conservation concern. The Tricolored Blackbird (Agelaius tricolor) has experienced large population declines, is the most colonial songbird in North America, is largely confined to California, and breeds itinerantly in multiple habitat types. It has had chronically low reproductive success in recent years. Although young produced per nest have previously been compared across habitats, no study has simultaneously considered site occupancy and reproductive success. Combining occupancy, abundance, frequency of occurrence, reproductive success and nest failure rate we found that that large colonies in grain fields fail frequently because of nest destruction due to harvest prior to fledging. Consequently, net time-averaged reproductive output is low compared to colonies in non-native Himalayan blackberry or thistles, and native stinging nettles. Cattail marshes have intermediate reproductive output, but their reproductive output might be improved by active management. Harvest of grain-field colonies necessitates either promoting delay of harvest or creating alternative, more secure nesting habitats. Stinging nettle and marsh colonies offer the main potential sources for restoration or native habitat creation. From 2005–2011 breeding site occupancy declined 3x faster than new breeding colonies were formed, indicating a rapid decline in occupancy. Total abundance showed a similar decline. Causes of variation in the value for reproduction of nesting substrates and factors behind continuing population declines merit urgent investigation. The method we employ should be useful in other metapopulation studies for calculating time-averaged reproductive output for different sites.

Using citizen science data in integrated population models to inform conservation decision-making

“…Carrasco et al (2017) showed that colonial nesting herons and egrets strive to strike a balance between nest site fidelity and habitat preferences, suggesting that a colony may not form in a site that had previously been occupied. This has also been observed in tricolored blackbirds, where most colony sites are not used in consecutive years (Beedy and Hamilton 1997;Holyoak et al 2014;Airola et al 2017). This variable site occupancy can make it challenging to re-capture banded birds and/or to collect yearly population count data (Marshall et al 2004).…”

Correcting for bias in distribution modelling for rare species using citizen science data

“…While the reproductive success of colonies that breed in alternative habitats have been shown to be similar to, or greater than, those in the traditional wetland habitats, the population continues to decline (Holyoak, Meese, & Graves, 2014;Weintraub, George, & Dinsmore, 2016). Breeding tricoloured blackbirds may have multiple breeding attempts in a single season, potentially at a different site within the same breeding season, and are also not likely to use the same breeding sites each year but seem to prefer the same habitat types (DeHaven, Crase, & Woronecki, 1975).…”

Low genetic diversity in a population of Tricolored Blackbird (Agelaius tricolor), a species pending Endangered status

“…It is not clear whether this movement is facilitated by habitat connectivity between colonies, or whether individuals are simply able to fly over unsuitable habitat en route to a different colony. However, evidence of dwindling site occupancy and recruitment due to habitat loss (Holyoak et al 2014) suggests migration may become more difficult with increasing or prohibitive distances between colonies. Thus, continuing habitat loss could lead (directly or indirectly) to population fragmentation across the Tricolored Blackbird's range, accelerating declines in genetic diversity due to absolute colony loss.…”

“…These declines are attributed to 2 primary causes: the widespread loss of the Tricolored Blackbird's native wetland habitat (estimated at a 96% loss over the last 150 years, Kreissman 1991) to agriculture and urbanization (Beedy 2008;Meese et al 2014), and the species' switch to nesting in active grain fields that are harvested as part of normal agricultural operations, which has resulted over several decades in complete losses of many of the largest colonies. Because of land-use changes and fragmentation of suitable habitat, breeding site occupancy of Tricolored Blackbirds has declined 3 times as quickly as sites have been recruited (Holyoak et al 2014).…”