Mixed effects of urbanization on density, nest survival, and nestling corticosterone of a generalist passerine

Abstract

Generalist species occupy a wide range of habitats and typically respond well to environmental disturbance. Highly disturbed urban areas may contain habitat that generalists can use and exploit; however, habitat use and quality of areas within urban landscapes are poorly understood. Here, we investigated habitat use of a generalist passerine, the red‐winged blackbird (Agelaius phoeniceus), in urban and rural grassland patches in greater metropolitan Chicago, USA. From 2012 to 2013, we conducted point counts on 30 grassland patches and searched for nests at a subset of these sites. We measured and weighed nestlings and collected fecal samples for corticosterone metabolite analysis. We found that population densities were greater in grassland patches in more rural areas, despite higher rates of nest predation and brood parasitism. We found that clutch size and number of young fledged did not differ between urban and rural areas. Average body mass and tarsus length of nestlings did not vary with urbanization, but nestlings in rural environments had significantly greater fecal corticosterone metabolites than urban nestlings. These results suggest that, even for widely successful generalist species, novel habitat can create a disconnection between habitat use and quality. The reason for this disconnection is unclear but may be related to perceived predation risk or other biotic or abiotic cues. Addressing the mechanisms behind undervaluation of habitat within urban landscapes will aid in wildlife conservation and management in novel environments.

Stress in the city: meta-analysis indicates no overall evidence for stress in urban vertebrates

“…the response to a sudden, temporarily stressful event such as a predatory attack) between urban and non-urban populations (table 1). It is generally assumed that urban populations will show higher baseline levels of glucocorticoids due to chronic stress, but the results are mixed: while some studies find higher baseline glucocorticoid levels in urban populations [26,37], others find no effect [24,36] or even the opposite effect [30,45]. Similarly, the stress-induced responses of chronically stressed urban dwellers is generally expected to be higher than that of their non-urban counterparts, but, in this case too, the evidence is mixed [14,25,36].…”

Feather, But Not Plasma, Glucocorticoid Response to Artificial Light at Night Differs between Urban and Forest Blue Tit Nestlings

“…Those studies that did measure CORT in nestlings show mixed effects of urbanization. For instance, while pCORT did not vary between suburban and rural American crow nestlings ( Heiss et al 2009 ), rural red-winged blackbird nestlings had higher fecal CORT than urban ones ( Buxton et al 2018 ).…”

Long-term, medium-term and acute stress response of urban populations of Eurasian red squirrels affected by different levels of human disturbance

“…Although it is generally assumed that urban populations should have higher levels of cortisol metabolite due to chronic stress [8], previous studies showed diverse results, i.e. higher levels of cortisol metabolites [68,69], lower levels [39,70,71] or no differences [42,72]. Variations among studies might be related to complex responses of neuroendocrine system to chronic stress or species/population/individual-dependent differences in the perception of stressors [8].…”