Seasonal shifts in sex ratios of fledgling american kestrels (Falco sparverius paulus): The early bird hypothesis

Peter D. Smallwood, John A. Smallwood

Research output: Contribution to journalArticle

55 Citations (Scopus)

Abstract

We document a seasonal shift in the sex ratios of broods produced by resident southeastern American kestrels (Falco sparverius paulus) breeding in nest boxes in Florida. Early in the breeding season, most biased broods were biased towards males, whereas later in the season, most biased broods were biased towards females. Computer-simulated broods subjected to sex-biased egg and/or nestling mortality demonstrate that it is possible that differential mortality produced the pattern of bias that we observed. However, these simulations do not exclude the possibility that female kestrels were manipulating the primary sex ratio of the broods. We present evidence that this sex ratio shift is adaptive: for males we detected breeding as yearlings, all had fledged early the previous season. No such relationship between season and the probability of breeding as a yearling was found for females. We propose the Early Bird Hypothesis as the ecological basis for the advantage of fledging early in males. We hypothesize that pre-emptive competition among post-fledging, dispersing males for breeding sites confers an advantage to males fledged early in the season. This hypothesis may explain why a non-migratory population of the Eurasian kestrel (F. tinnunculus) and non-migratory American kestrels breeding in Florida (F. s. paulus) exhibit this seasonal shift in sex ratios, whereas migratory American kestrels (F. s. sparverius) breeding in Saskatchewan, Canada, do not. We discuss the relevance of the Early Bird Hypothesis for other animal species.

Original languageEnglish
Pages (from-to)839-853
Number of pages15
JournalEvolutionary Ecology
Volume12
Issue number7
DOIs
StatePublished - 1 Jan 1998

Fingerprint

Falco sparverius
sex ratio
breeding
birds
fledging
yearlings
nest box
mortality
nest boxes
breeding site
Saskatchewan
nestling
breeding season
breeding sites
fledglings
Canada
egg
gender
simulation
animals

Keywords

  • Early Bird Hypothesis
  • Falco sparverius paulus
  • Intrasexual competition
  • Sex allocation
  • Sex ratio
  • Sex-biased mortality

Cite this

@article{055141154dd240b98350e84c2a78c43a,
title = "Seasonal shifts in sex ratios of fledgling american kestrels (Falco sparverius paulus): The early bird hypothesis",
abstract = "We document a seasonal shift in the sex ratios of broods produced by resident southeastern American kestrels (Falco sparverius paulus) breeding in nest boxes in Florida. Early in the breeding season, most biased broods were biased towards males, whereas later in the season, most biased broods were biased towards females. Computer-simulated broods subjected to sex-biased egg and/or nestling mortality demonstrate that it is possible that differential mortality produced the pattern of bias that we observed. However, these simulations do not exclude the possibility that female kestrels were manipulating the primary sex ratio of the broods. We present evidence that this sex ratio shift is adaptive: for males we detected breeding as yearlings, all had fledged early the previous season. No such relationship between season and the probability of breeding as a yearling was found for females. We propose the Early Bird Hypothesis as the ecological basis for the advantage of fledging early in males. We hypothesize that pre-emptive competition among post-fledging, dispersing males for breeding sites confers an advantage to males fledged early in the season. This hypothesis may explain why a non-migratory population of the Eurasian kestrel (F. tinnunculus) and non-migratory American kestrels breeding in Florida (F. s. paulus) exhibit this seasonal shift in sex ratios, whereas migratory American kestrels (F. s. sparverius) breeding in Saskatchewan, Canada, do not. We discuss the relevance of the Early Bird Hypothesis for other animal species.",
keywords = "Early Bird Hypothesis, Falco sparverius paulus, Intrasexual competition, Sex allocation, Sex ratio, Sex-biased mortality",
author = "Smallwood, {Peter D.} and Smallwood, {John A.}",
year = "1998",
month = "1",
day = "1",
doi = "10.1023/A:1006598600532",
language = "English",
volume = "12",
pages = "839--853",
journal = "Evolutionary Ecology",
issn = "0269-7653",
publisher = "Springer Netherlands",
number = "7",

}

Seasonal shifts in sex ratios of fledgling american kestrels (Falco sparverius paulus) : The early bird hypothesis. / Smallwood, Peter D.; Smallwood, John A.

In: Evolutionary Ecology, Vol. 12, No. 7, 01.01.1998, p. 839-853.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Seasonal shifts in sex ratios of fledgling american kestrels (Falco sparverius paulus)

T2 - The early bird hypothesis

AU - Smallwood, Peter D.

AU - Smallwood, John A.

PY - 1998/1/1

Y1 - 1998/1/1

N2 - We document a seasonal shift in the sex ratios of broods produced by resident southeastern American kestrels (Falco sparverius paulus) breeding in nest boxes in Florida. Early in the breeding season, most biased broods were biased towards males, whereas later in the season, most biased broods were biased towards females. Computer-simulated broods subjected to sex-biased egg and/or nestling mortality demonstrate that it is possible that differential mortality produced the pattern of bias that we observed. However, these simulations do not exclude the possibility that female kestrels were manipulating the primary sex ratio of the broods. We present evidence that this sex ratio shift is adaptive: for males we detected breeding as yearlings, all had fledged early the previous season. No such relationship between season and the probability of breeding as a yearling was found for females. We propose the Early Bird Hypothesis as the ecological basis for the advantage of fledging early in males. We hypothesize that pre-emptive competition among post-fledging, dispersing males for breeding sites confers an advantage to males fledged early in the season. This hypothesis may explain why a non-migratory population of the Eurasian kestrel (F. tinnunculus) and non-migratory American kestrels breeding in Florida (F. s. paulus) exhibit this seasonal shift in sex ratios, whereas migratory American kestrels (F. s. sparverius) breeding in Saskatchewan, Canada, do not. We discuss the relevance of the Early Bird Hypothesis for other animal species.

AB - We document a seasonal shift in the sex ratios of broods produced by resident southeastern American kestrels (Falco sparverius paulus) breeding in nest boxes in Florida. Early in the breeding season, most biased broods were biased towards males, whereas later in the season, most biased broods were biased towards females. Computer-simulated broods subjected to sex-biased egg and/or nestling mortality demonstrate that it is possible that differential mortality produced the pattern of bias that we observed. However, these simulations do not exclude the possibility that female kestrels were manipulating the primary sex ratio of the broods. We present evidence that this sex ratio shift is adaptive: for males we detected breeding as yearlings, all had fledged early the previous season. No such relationship between season and the probability of breeding as a yearling was found for females. We propose the Early Bird Hypothesis as the ecological basis for the advantage of fledging early in males. We hypothesize that pre-emptive competition among post-fledging, dispersing males for breeding sites confers an advantage to males fledged early in the season. This hypothesis may explain why a non-migratory population of the Eurasian kestrel (F. tinnunculus) and non-migratory American kestrels breeding in Florida (F. s. paulus) exhibit this seasonal shift in sex ratios, whereas migratory American kestrels (F. s. sparverius) breeding in Saskatchewan, Canada, do not. We discuss the relevance of the Early Bird Hypothesis for other animal species.

KW - Early Bird Hypothesis

KW - Falco sparverius paulus

KW - Intrasexual competition

KW - Sex allocation

KW - Sex ratio

KW - Sex-biased mortality

UR - http://www.scopus.com/inward/record.url?scp=0031788979&partnerID=8YFLogxK

U2 - 10.1023/A:1006598600532

DO - 10.1023/A:1006598600532

M3 - Article

AN - SCOPUS:0031788979

VL - 12

SP - 839

EP - 853

JO - Evolutionary Ecology

JF - Evolutionary Ecology

SN - 0269-7653

IS - 7

ER -