Steroid hormone profiles in cowbirds and redwings

Avian brood parasites display enhanced annual fecundity compared to other passerine birds. Adult female Brown-headed Cowbirds (Molothrus ater), a brood parasite in the Icterid family, are estimated to lay 40- 50 eggs/year. Some studies suggest annual fecundity in various parasitic species can exceed even this upper estimate. However, boosting reproductive output to this extent may come at the cost of reduced or lost parental care. Steroids, such as testosterone, may act as a pivot point that balances the fitness consequences between reproduction and parental behavior. Moreover, steroids are targets of selection that shape life history strategies and can therefore potentially contribute to the evolution of novel behavioral phenotypes. Here, we examine how reproductive steroids may mediate a possible tradeoff between increased annual fecundity and parental care in female cowbirds. We compare seasonal fluctuations in steroid profiles and follicular development in cowbirds and Red-winged Bl..., Seasonal profiles of reproductive steroids
Female Brown-headed Cowbirds (n = 18) and Red-winged Blackbirds (n = 14) were captured using walk-in traps and mist nets in Ransom and Cass counties of North Dakota during the non-breeding and the peak breeding season in 2021 (May-June). Traps and mist-nets were continuously monitored to ensure the birds were removed within five minutes of capture. Birds were immediately euthanized via rapid decapitation for a separate study. This allowed us to measure the size of the largest follicle using calipers and collect blood to be stored on ice until centrifugation. Collections started May 1 (pre-breeding in North Dakota) and concluded June 25 (peak breeding). We categorized female cowbirds and Red-winged Blackbirds into three categories: pre-breeding, breeding onset, or peak breeding. These categories were defined by observing the breeding activity (or lack thereof) of female Red-winged Blackbirds, the most abundant preferred host available to cowbird..., , # Steroid hormone profiles in cowbirds and redwings

[https://doi.org/10.5061/dryad.9ghx3ffs3](https://doi.org/10.5061/dryad.9ghx3ffs3)

## Description of the data and file structure

### Files and variables

#### File: archived\_data\_sheet\_organized.xls

#### **Description:** 

BHCO = Brown-headed Cowbird

RWBL = Red-winged Blackbird

T = testosterone (pg/ml = picogram per milliliter)

E2 = 17 beta estradiol (pg/ml = picogram per milliliter)

T-p1-4 and E2-p1-4 = time point 1-4 for blood collection (p1=baseline or prior to GnRH injection and t2-4 = 30, 60 and 120 minutes after GnRH injection)

lrgst follicle = largest ovarian follicle as measured in millimeters (mm)

GnRH = gonadotropin releasing hormone (pg/ml = picogram per milliliter)

species (GnRH trt) = species with the GnRH treatment

## Code/software

NA

## Access information

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